<figure><img src="image_1.png" /></figure>

# An SMA Survey of Chemistry in Disks Around Herbig AeBe Stars

Jamila Pegues<fnref n="1" />,<fnref n="2" />,<fnref n="8" /> ID, Karin I. Öberg<fnref n="2" /> ID, Chunhua Qi<fnref n="2" /> ID, Sean M. Andrews<fnref n="2" /> ID, Jane Huang<fnref n="3" />,<fnref n="9" /> ID, Charles J. Law<fnref n="2" /> ID, Romane Le Gal<fnref n="4" />,<fnref n="5" />,<fnref n="6" /> ID, Luca Matrà<fnref n="7" /> ID, and David J. Wilner<fnref n="2" /> ID

Received 2022 July 27; revised 2023 February 8; accepted 2023 February 24; published 2023 May 5

## Abstract

Protoplanetary disks around Herbig AeBe stars are exciting targets for studying the chemical environments where giant planets form. Save for a few disks, however, much of Herbig AeBe disk chemistry is an open frontier. We present a Submillimeter Array ~213–268 GHz pilot survey of millimeter continuum CO isotopologs and other small molecules in disks around five Herbig AeBe stars (HD 34282, HD 36112, HD 38120, HD 142666, and HD 144432). We detect or tentatively detect ¹²CO 2–1 and ¹³CO 2–1 from four disks, C¹⁸O 2–1 and HCO⁺ 3–2 from three disks, HCN 3–2, CS 5–4, and DCO⁺ 3–2 from two disks, and C₂H 3–2 and DCN 3–2 from one disk each. H₂CO 3–2 is undetected at the sensitivity of our observations. The millimeter continuum images of HD 34282 suggest a faint, unresolved source ~5''0 away, which could arise from a distant orbital companion or an extended spiral arm. We fold our sample into a compilation of T Tauri and Herbig AeBe/F disks from the literature. Altogether, most line fluxes generally increase with millimeter continuum flux. Line flux ratios between CO 2–1 isotopologs are nearest to unity for the Herbig AeBe/F disks. This may indicate emitting layers with relatively similar, warmer temperatures and more abundant CO relative to the disk dust mass. Lower HCO⁺ 3–2 flux ratios may reflect lower ionization in Herbig AeBe/F disks. Lower detection rates and flux ratios for DCO⁺ 3–2, DCN 3–2, and H₂CO 3–2 suggest smaller regimes of cold chemistry around the luminous Herbig AeBe/F stars.

*Unified Astronomy Thesaurus concepts: Astrochemistry (75); Protoplanetary disks (1300); Interstellar medium (847); Interferometry (808)*

## 1. Introduction

Protoplanetary disks are the birthplaces of planetary systems. Disks around pre-main-sequence A-type and B-type stars (Herbig AeBe stars) are typically larger and brighter observational targets in millimeter-wavelength (mm) continuum emission than disks around less massive stars (see, e.g., empirical relationships between disk size derived from the mm continuum, flux, and stellar mass in Figure 5 of Andrews 2020, and references therein). Herbig Ae stars are also expected to generally host more massive planets than less massive stars (e.g., Bowler et al. 2010; Johnson et al. 2010). Protoplanetary disks around Herbig AeBe stars (Herbig AeBe disks) are therefore ideal targets for studying the formation environments of giant planets (e.g., Quanz 2015).

Indeed, direct imaging has already detected gas giant planets around some A-type stars (e.g., Johnson et al. 2007; Marois et al. 2008; Carson et al. 2013; Rameau et al. 2013; Quanz et al. 2015). Depletions of heavy elements in the photospheres of Herbig AeBe stars with disks suggest that at least ~30% of their disks may host so far undetected planets of ~0.1–10 Jupiter masses (Kama et al. 2015). Spatially resolved observations from infrared to mm

wavelengths have also revealed complex structures in the gas and dust of many Herbig AeBe disks, including spiral arms, asymmetries, cavities, and rings (e.g., Lin et al. 2006; Isella et al. 2007; Fukagawa et al. 2010; Honda et al. 2012; Dong et al. 2018). Several studies have attributed these complex structures to nascent planets embedded within the disks (e.g., Grady et al. 2013; Dong et al. 2015; Bae et al. 2016; Matter et al. 2016; Huang et al. 2018; Isella et al. 2018; Pinte et al. 2018; Zhang et al. 2018).

The compositions of these giant planets are seeded by the gaseous and icy molecular distributions of their ancestral protoplanetary disks. It is currently unclear how similar or different Herbig AeBe disk compositions are to the better studied T Tauri disks. Disks around Herbig AeBe stars are larger, hotter, and more highly irradiated in the ultraviolet than disks around lower-mass stars. As density, temperature, and radiation are key drivers of protoplanetary disk chemistry (e.g., Aikawa & Herbst 1999), these environmental differences may foster different molecular compositions and distributions in Herbig AeBe disks compared to their lower-mass counterparts.

While Herbig AeBe disk studies are quite common, there are few such studies of their disk chemistry. The majority of Herbig disk studies have focused on CO gas and dust in order to characterize the surface density profiles and structures of the disks (e.g., Dent et al. 2005; Boissier et al. 2011; van der Plas et al. 2015; Gravity Collaboration et al. 2019; Stapper et al. 2022). Most existing chemistry surveys that do contain Herbig AeBe disks have targeted infrared wavelengths, which probe <10 au of these disks (e.g., Pontoppidan et al. 2010; Fedele et al. 2011; Meeus et al. 2012).

<figure><img src="image_2.png" /></figure>

Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹Space Telescope Science Institute, Baltimore, MD 21218, USA; jamila.pegues@stsci.edu</footnote>

<footnote n="2">²Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA 02138, USA</footnote>

<footnote n="3">³Department of Astronomy, University of Michigan, 323 West Hall, 1085 S. University Avenue, Ann Arbor, MI48109, USA</footnote>

<footnote n="4">⁴Univ. Grenoble Alpes, CNRS, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), F-38000 Grenoble, France</footnote>

<footnote n="5">⁵Institut de Radioastronomie Millimétrique (IRAM), F-38406 Saint-Martin d'Hères, France</footnote>

<footnote n="6">⁶Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA</footnote>

<footnote n="7">⁷Centre for Astronomy, School of Physics, National University of Ireland, Galway, University Road, Galway, Ireland</footnote>

<footnote n="8">⁸Space Telescope Science Institute (STScI) Postdoctoral Fellow.</footnote>

<footnote n="9">⁹NASA Hubble Fellowship Program Sagan Fellow.</footnote>
For species other than CO, few Herbig AeBe disks have been surveyed at mm wavelengths, which probe >10 au. Notably, Öberg et al. (2010, 2011) conducted a mm wavelength survey of a 12-disk sample containing one Herbig Ae disk (MWC 480) and three F-star disks. Their survey included lines of HCO+, CN, HCN, DCN, DCO+, and H₂CO. They found that small molecules (other than CO) were generally detected at lower rates for the Herbig AeBe/F disks than for the disks around lower-mass stars. They attributed the lower detection rates to a combination of (1) the brighter far-ultraviolet fields inherent to Herbig Ae stars and (2) the warmer disks themselves, which would cultivate less CO freeze-out and generally diminish abundances of molecules that form significantly in cold environments (e.g., deuterated molecules and H₂CO).

Beyond their survey, only a few Herbig AeBe disks have been well characterized in chemistry at mm wavelengths: AB Aur, HD 163296, and MWC 480 (e.g., Thi et al. 2004; Schreyer et al. 2008; Fuente et al. 2010; Henning et al. 2010; Chapillon et al. 2012a, 2012b; Qi et al. 2013a, 2013b; Guilloteau et al. 2013; Graninger et al. 2015; Guzmán et al. 2015; Huang et al. 2017; Bergner et al. 2018, 2019; Le Gal et al. 2019; Loomis et al. 2020; Pegues et al. 2020; Öberg et al. 2021a; Law et al. 2021a; Guzmán et al. 2021; Ilee et al. 2021; Le Gal et al. 2021). Otherwise, Oph IRS 48 has been detected in a mix of commonly and uncommonly targeted molecules, including H₂CO, CH₃OH, NO, SO, SO₂, and CH₃OCH₃ (van der Marel et al. 2014; Booth et al. 2021; van der Marel et al. 2021; Brunken et al. 2022). Other molecule-specific exceptions include DCO⁺ from HD 169142 (Macías et al. 2017), H₂CO, CH₃OH, and SO from HD 100546 (Booth et al. 2018, 2023), HC¹⁵N and H¹³CO⁺ from HD 97048 (Booth et al. 2019), and upper limits of CN, H₂CO, and SO from HD 36112 (also known as MWC 758; Guilloteau et al. 2013).

A subset of these studies (e.g., Guzmán et al. 2015; Le Gal et al. 2019; Loomis et al. 2020; Pegues et al. 2020) compared mm wavelength observations between the Herbig Ae disks and T Tauri disks in their samples. Altogether, they found tentative differences in the inventories of carbon-, nitrogen-, oxygen-, and sulfur-bearing molecules between the two disk types. The differences were attributed to differences in disk temperature, photochemical environment, and/or regions of CO freeze-out —all of which are properties derivative of the different luminosities and effective temperatures of the central stars. These chemistry studies, however, have each considered only one or two Herbig Ae disks in total, and have often called for more observations of Herbig Ae disks to test these tentative hypotheses over larger samples.

In this study, we present a molecular line survey of four Herbig Ae disks and one Herbig Be disk observed within ~213–268 GHz with the Submillimeter Array (SMA). We leverage the wide spectral range and high spectral resolution capabilities of the SMA to probe a suite of molecular lines in an efficient way, and to help enable follow-up of these lines at higher spatial resolution in future studies. In Section 2, we describe the target disks, SMA observations and data reduction, target molecular lines, and a comparative sample of disks from the literature. In Section 3, we overview the techniques used to analyze the molecular line emission. In Section 4, we present our results, including molecular line detections, velocity-integrated emission maps, spectra, and extracted fluxes. In Section 5, we interpret these results in the context of existing

disk chemistry observations across the pre-main-sequence stellar mass distribution. We summarize our findings in Section 6.

## 2. Sample

### 2.1. Target Disks

We target four protoplanetary disks around Herbig Ae stars and one disk around a Herbig Be star. Table 1 summarizes their stellar and disk characteristics. These star+disk systems were chosen because they have previously been detected in at least one CO emission line (e.g., CO J = 3–2 observations by Dent et al. 2005; van der Plas et al. 2017), but were otherwise not characterized in mm wavelength chemistry. The systems are associated with the Taurus-Auriga (HD 36112), Orion OB Ic (HD 38120), and Upper Scorpius (HD 142666 and HD 144432) star-forming regions, or are isolated (HD 34282; see the compilations by Sandell et al. 2011; Alecian et al. 2012). The stars were selected to span a range of spectral types, with four stars of types A3-A9 and the fifth star (HD 38120) of type B9 (Zuckerman et al. 1995; Meeus et al. 1998; Merín et al. 2004; Juhász et al. 2010). Three of the star+disk systems are reported to have orbital companions: HD 34282 (Wheelwright et al. 2010), HD 36112 (Thomas et al. 2007), and HD 144432 (Maheswar et al. 2002).

Altogether, these five Herbig AeBe disks have lower CO luminosities than the Herbig Ae disks that have been relatively well characterized in chemistry so far (i.e., AB Aur, HD 163296, and MWC 480, found to be brightest in CO 3–2 emission in the survey by Dent et al. 2005). Our observations of these disks serve as (1) an investigative probe into a larger and more representative sample of Herbig AeBe disk chemistry, and (2) a foundation for potential follow-up observations at higher spatial resolution in the future.

Studies have found complex morphologies in the dust disks of a subset of these systems, including a cavity within ~88 au, a bright point-like source, and a single spiral arm in HD 34282 (mm wavelength continuum and scattered-light observations of van der Plas et al. 2017; de Boer et al. 2021, respectively); bright dust clumps and at least three spiral arms in HD 36112 (mm wavelength continuum and scattered-light observations of Boehler et al. 2018; Dong et al. 2018; Reggiani et al. 2018, respectively); and an inner cavity with a radius of ~10 au, rings, and asymmetries in HD 142666 (mm wavelength continuum observations of Huang et al. 2018; Rubinstein et al. 2018). There has been a lack of detected scattered light from HD 144432 (Monnier et al. 2017), which may indicate that the disk was truncated by its stellar companions (see the discussion by Monnier et al. 2017), or that the disk has already lost much of its gas given the old age of its association (Upper Scorpius, >10 Myr; Pecaut et al. 2012).

### 2.2. SMA Observations and Data Reduction

All disks were observed with the SMA<fnref n="10" /> (Ho et al. 2004) during projects 2019B-S037, 2020A-S007, and 2020B-S007. Table 2 summarizes the observational details. Four of the disks were observed in the compact configuration with seven

<footnote n="10">¹⁰ The SMA is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica.</footnote>
<table><caption>**Table 1**

Star and Disk Characteristics</caption><thead><tr><th>Star+Disk System (1)</th><th>Spectral Type (2)</th><th>R.A.<sup>[1]</sup> (J2000) (3)</th><th>Decl.<sup>[1]</sup> (J2000) (4)</th><th>Dist.<sup>[1]</sup> (pc) (5)</th><th>t<sub>*</sub><sup>[2]</sup> (Myr) (6)</th><th>L<sub>*</sub><sup>[2]</sup> (L<sub>☉</sub>) (7)</th><th>M<sub>*</sub><sup>[2]</sup> (M<sub>☉</sub>) (8)</th><th>T<sub>eff</sub><sup>[2]</sup> (K) (9)</th><th>v<sub>LSR</sub><sup>a</sup> (km s<sup>-1</sup>) (10)</th><th>P.A.<sup>b</sup> (°) (11)</th><th>Incl.<sup>b</sup> (°) (12)</th><th>R<sub>mm,90</sub> (au) (13)</th></tr></thead><tbody><tr><td>HD 34282<sup>c,d</sup></td><td>A3<sup>[3]</sup></td><td>05:16:00.48</td><td>-09:48:35.41</td><td>309 ± 2</td><td>6.5<sup>+2.4</sup><sub>-0.6</sub></td><td>9.55<sup>+1.17</sup><sub>-0.84</sub></td><td>1.45<sup>+0.07</sup><sub>-0.07</sub></td><td>9500<sup>+250</sup><sub>-250</sub></td><td>-2.7</td><td>117<sup>[4]</sup></td><td>59<sup>[4]</sup></td><td>260<sup>[5]</sup></td></tr><tr><td>HD 36112<sup>c,d</sup></td><td>A5<sup>[6]</sup></td><td>05:30:27.53</td><td>+25:19:56.65</td><td>156 ± 1</td><td>8.3<sup>+0.4</sup><sub>-1.4</sub></td><td>11.0<sup>+3.49</sup><sub>-1.84</sub></td><td>1.56<sup>+0.11</sup><sub>-0.08</sub></td><td>7605<sup>+225</sup><sub>-297</sub></td><td>5.9</td><td>245<sup>[7]</sup></td><td>21<sup>[7]</sup></td><td>123<sup>[5]</sup></td></tr><tr><td>HD 38120</td><td>B9<sup>[8]</sup></td><td>05:43:11.89</td><td>-04:59:49.91</td><td>385 ± 5</td><td>3.5<sup>+13.6</sup><sub>-1.4</sub></td><td>52.5<sup>+54.7</sup><sub>-19.4</sub></td><td>2.37<sup>+0.43</sup><sub>-0.24</sub></td><td>10700<sup>+800</sup><sub>-900</sub></td><td>16.4</td><td>165<sup>[9]</sup></td><td>49<sup>[9]</sup></td><td>~100</td></tr><tr><td>HD 142666<sup>c</sup></td><td>A8<sup>[10]</sup></td><td>15:56:40.01</td><td>-22:01:40.36</td><td>146 ± 1</td><td>9.3<sup>+0.8</sup><sub>-0.5</sub></td><td>8.71<sup>+0.84</sup><sub>-0.95</sub></td><td>1.49<sup>+0.08</sup><sub>-0.08</sub></td><td>7500<sup>+250</sup><sub>-250</sub></td><td>4.0</td><td>162<sup>[11]</sup></td><td>62<sup>[11]</sup></td><td>52<sup>[5]</sup></td></tr><tr><td>HD 144432<sup>d</sup></td><td>A9<sup>[10]</sup></td><td>16:06:57.94</td><td>-27:43:10.15</td><td>155 ± 1</td><td>5.0<sup>+0.3</sup><sub>-0.6</sub></td><td>9.33<sup>+0.90</sup><sub>-0.21</sub></td><td>1.39<sup>+0.07</sup><sub>-0.07</sub></td><td>7500<sup>+250</sup><sub>-250</sub></td><td>5.8</td><td>76<sup>[9]</sup></td><td>30<sup>[9]</sup></td><td>~100</td></tr></tbody></table>

**Notes.** Column 1: Star+disk system. Column 2: Spectral type. Columns 3, 4, and 5: Right ascension, decl., and distance, respectively, from Gaia (Gaia Collaboration et al. 2016, 2018). Columns 6, 7, 8, and 9: Stellar age, luminosity, mass, and effective temperature, respectively, from the compilation of Vioque et al. (2018), who updated values from the literature as needed using parallaxes from the Gaia Data Release 2. Column 10: Systemic velocities in the local standard of rest frame, which were (1) converted from heliocentric values presented in Dent et al. (2005) and then (2) adjusted as needed to better match the center of the $^{12}$CO 2–1 emission spectra. We were unable to adjust the systemic velocity for HD 38120 (see Section 4.1). Columns 11 and 12: Position and inclination angles, respectively. The position angles are measured east of north. Column 13: Disk radii from the literature, derived from 90% of the ~1.3 mm continuum emission. No mm continuum radii are known for HD 38120 and HD 144432, so here we assume ~100 au.

<sup>a</sup> Adapted from Dent et al. (2005), as described in the main body of this caption.

<sup>b</sup> Estimated from near-infrared, submm, or mm continuum.

<sup>c</sup> HD 34282, HD 36112, and HD 142666 are also known as V1366 Ori, MWC 758, and V1026 Sco, respectively, in the literature.

<sup>d</sup> Orbital companions have been reported for HD 34282 (Wheelwright et al. 2010), HD 36112 (Thomas et al. 2007), and HD 144432 (Maheswar et al. 2002).

**References.** [1] Gaia Collaboration et al. (2016, 2018); [2] Vioque et al. (2018); [3] Merín et al. (2004); [4] van der Plas et al. (2017); [5] Stapper et al. (2022); [6] Zuckerman et al. (1995); [7] Isella et al. (2010); [8] Juhász et al. (2010); [9] Gravity Collaboration et al. (2019); [10] Meeus et al. (1998); [11] Huang et al. (2018).
<table><caption>**Table 2**
SMA Observations</caption><thead><tr><th>Project<br>Code<br>(1)</th><th>Date<br>(2)</th><th>Star+Disk<br>System<br>(3)</th><th>Config.<br>(4)</th><th>#<br>of Ant.<br>(5)</th><th>Time on<br>Source<br>(min)<br>(6)</th><th>Baselines<br>(m)<br>(7)</th><th>Bandpass<br>Calibrator<br>(8)</th><th>Flux<br>Calibrator<br>(9)</th><th>Gain<br>Calibrator(s)<br>(10)</th></tr></thead><tbody><tr><td rowspan="2">2020B-S007</td><td>2021 May 1<sup>a</sup></td><td>HD 142666</td><td>Compact</td><td>6</td><td>187</td><td>18–77</td><td>3c279</td><td>Titan</td><td>1517–243, 1626–298</td></tr><tr><td>2021 May 1<sup>a</sup></td><td>HD 144432</td><td>Compact</td><td>6</td><td>178</td><td>18–77</td><td>3c279</td><td>Titan</td><td>1517–243, 1626–298</td></tr><tr><td>2020A-S007</td><td>2020 Nov. 14</td><td>HD 38120</td><td>Subcompact</td><td>8</td><td>232</td><td>9–69</td><td>3c84</td><td>Uranus</td><td>0532+075, 0607–085</td></tr><tr><td rowspan="4">2019B-S037</td><td>2020 March 10</td><td>HD 36112</td><td>Compact</td><td>7</td><td>225</td><td>16–77</td><td>3c279</td><td>Uranus</td><td>0555+398</td></tr><tr><td>2020 March 9</td><td>HD 34282</td><td>Compact</td><td>7</td><td>200</td><td>16–77</td><td>3c279</td><td>Uranus</td><td>0541–056</td></tr><tr><td>2020 Feb. 27<sup>a</sup></td><td>HD 142666</td><td>Compact</td><td>7</td><td>178</td><td>16–77</td><td>3c279</td><td>Titan</td><td>1517–243, 1626–298</td></tr><tr><td>2020 Feb. 27<sup>a</sup></td><td>HD 144432</td><td>Compact</td><td>7</td><td>178</td><td>16–77</td><td>3c279</td><td>Titan</td><td>1517–243, 1626–298</td></tr></tbody></table>

**Note.** Column 1: SMA project code. Column 2: Date of observations. Column 3: Star+disk system observed. Column 4: SMA configuration. Column 5: Time spent per source. Column 6: Number of baselines. Columns 7, 8, and 9: Bandpass, flux, and gain calibrators, respectively.

antennas, while the fifth (HD 38120) was observed in the subcompact configuration with eight antennas. HD 142666 and HD 144432 were observed together during two shared tracks, while all other disks were observed during individual tracks.

The observations were conducted with the SMA SWARM correlator, which at the time of the initial observations consisted of 16~2 GHz bands, or ~32 GHz in total per source.<fnref n="11" /> For the 230 GHz receiver, we tuned band S1 in the upper spectral band to the ¹²CO J = 2–1 line (230.538 GHz). For HD 142666 and HD 144432, which were expected to be relatively inclined disks (based on the broad Keplerian profiles fitted to their ¹²CO 3–2 emission by Dent et al. 2005), we shifted the 230 GHz setup by –0.2 GHz to permit wider coverage of both the C¹⁸O J = 2–1 line (219.560 GHz) and the DCN J = 3–2 line (217.239 GHz). For the 240 GHz receiver, we tuned band S4 in the upper spectral band to the HCO⁺ J = 3–2 line (267.558 GHz).

This setup permitted coverage of molecular lines within four spectral bands: ~213–221 GHz, ~229–237 GHz, ~244–252 GHz, and ~260–268 GHz.<fnref n="12" /> The native resolution of the observations was ~140 kHz (~0.16–0.20 km s⁻¹ across the bands) for four disks. For the fifth and faintest disk (HD 38120), we binned the SMA data by 4, reducing the resolution from the native 140 kHz to ~559 kHz (~0.63–0.79 km s⁻¹ across the bands).

We calibrated the SMA data using the MIR package.<fnref n="13" /> We performed continuum subtraction for each SMA band with the Common Astronomy Software Applications package (CASA; McMullin et al. 2007; CASA Team et al. 2022) version 4.7.2. For CLEANing, we constructed a CLEAN mask that encompassed the mm continuum and ¹²CO 2–1 emission, and we used the mask to uniformly CLEAN all channels down to 3σ.

After the continuum subtraction, we imaged with spectral resolution coarser than the native SWARM resolution to

<table><caption>**Table 3**
Target Molecular Lines</caption><thead><tr><th>Molecular<br>Line<br>(1)</th><th>Frequency<br>(GHz)<br>(2)</th><th>A<sub>ul</sub><br>(s<sup>-1</sup>)<br>(3)</th><th>E<sub>u</sub><br>(K)<br>(4)</th></tr></thead><tbody><tr><td><sup>12</sup>CO J = 2–1</td><td>230.538</td><td>6.91 × 10<sup>-7</sup></td><td>16.6</td></tr><tr><td><sup>13</sup>CO J = 2–1</td><td>220.399</td><td>6.08 × 10<sup>-7</sup></td><td>15.9</td></tr><tr><td>C<sup>18</sup>O J = 2–1</td><td>219.560</td><td>6.01 × 10<sup>-7</sup></td><td>15.8</td></tr><tr><td>HCO<sup>+</sup> J = 3–2</td><td>267.558</td><td>1.48 × 10<sup>-3</sup></td><td>25.7</td></tr><tr><td>CS J = 5–4</td><td>244.936</td><td>3.49 × 10<sup>-4</sup></td><td>35.3</td></tr><tr><td>HCN J = 3–2<sup>a</sup></td><td>265.886</td><td>8.36 × 10<sup>-4</sup></td><td>25.5</td></tr><tr><td>C<sub>2</sub>H N = 3–2, J = 7/2–5/2<sup>a</sup></td><td>262.004</td><td>5.31 × 10<sup>-5</sup></td><td>25.1</td></tr><tr><td>DCN J = 3–2<sup>a</sup></td><td>217.239</td><td>4.59 × 10<sup>-4</sup></td><td>20.9</td></tr><tr><td>DCO<sup>+</sup> J = 3–2<sup>a</sup></td><td>216.113</td><td>7.66 × 10<sup>-4</sup></td><td>20.7</td></tr><tr><td>H<sub>2</sub>CO 3<sub>03</sub>–2<sub>02</sub></td><td>218.222</td><td>2.82 × 10<sup>-4</sup></td><td>21.0</td></tr></tbody></table>

**Note.** Column 1: Molecular emission line. Column 2: Frequency. Columns 3 and 4: Einstein coefficients and upper energy levels, respectively, which were taken from the CDMS database (Müller et al. 2001, 2005; Endres et al. 2016).

enhance the signal-to-noise ratio for any line emission. We used 0.5 km s⁻¹ velocity bins for HD 34282 and HD 36112 and 2 km s⁻¹ bins for the fainter, more highly inclined disks HD 142666 and HD 144432.

## 2.3. Target Molecular Lines

Table 3 describes the spectroscopic characteristics of the target molecular lines covered by the correlator setup.<fnref n="14" /> These lines have been observed in previous surveys that targeted disks around lower-mass pre-main-sequence M-type through G-type stars (collectively known as T Tauri stars; e.g., Öberg et al. 2010, 2011; Huang et al. 2017; Bergner et al. 2019; Le Gal et al. 2019; Miotello et al. 2019; Bergner et al. 2020; Pegues et al. 2020, 2021a), permitting direct comparison of our observations to T Tauri disk chemistry. Observations and

<footnote><sup>a</sup> HD 142666 and HD 144432 were observed during the same observing sessions.</footnote>

<footnote n="11">¹¹ The SWARM correlator has since been upgraded and now allows a total spectral bandwidth of ~48 GHz. HD 38120 and the second shared track targeting HD 142666 and HD 144432 were observed with the upgraded SWARM correlator, but we do not use the additional bandwidth in this study because the molecular line emission of the disks is faint.</footnote>

<footnote n="12">¹² Each ~8 GHz spectral band was observed in four ~2 GHz bands. We removed the ~0.1 GHz from the edges of each band during analysis to avoid spectral artifacts.</footnote>

<footnote n="13">¹³ See the MIR Cookbook (https://www.cfa.harvard.edu/~cqi/mircook.html) for standard calibration practices.</footnote>

<footnote><sup>a</sup> C<sub>2</sub>H N = 3–2, J = 7/2–5/2 includes the hyperfine transitions F = 4–3 and F = 3–2. HCN J = 3–2, DCN J = 3–2, and DCO<sup>+</sup> J = 3–2 only include the hyperfine transitions blended with their brightest transition (at 265.886 GHz, 217.239 GHz, and 216.113 GHz, respectively).</footnote>

<footnote n="14">¹⁴ We also checked the lines of CH₃CN, CH₃OH, c-C₃H₂, H¹³CN, HC¹⁵N, HC₃N, H₂CS, HNCO, H¹³CO⁺, and N₂D⁺ that fell within the total 213–268 GHz range of the correlator. These lines were either not detected or were at or beyond the edges of the individual SMA bands.</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Scaled mm continuum fluxes for Herbig AeBe disks from this work (dotted red squares) as a function of stellar mass (left) and stellar luminosity (right). The literature sample of Herbig AeBe/F disks (blank red squares) and T Tauri disks with stellar masses >1.0 $M_\odot$ (thick white diamonds), 0.5–1.0 $M_\odot$ (thin light gold diamonds), and $\le 0.5 M_\odot$ (blue circles) are shown for comparison (Section 2.4, Appendix A). All mm continuum fluxes have been scaled to (1) 265 GHz (~1.1 mm) using $F_\nu \propto \nu^{2.2}$ (see Andrews 2020), where $F_\nu$ is the mm continuum flux at frequency $\nu$, and to (2) 140 pc. Whenever the error in the mm continuum flux, stellar luminosity, or stellar mass is not reported in the literature, we assume that the error is 15%, 20%, or 20%, respectively, of the given value. In the left panel, detected disks from mm continuum surveys of the Chamaeleon I (Pascucci et al. 2016), Lupus (Ansdell et al. 2016), and Taurus (Andrews et al. 2013) star-forming regions are additionally shown in the background as pale gray circles. These star-forming regions were chosen to span the pre-main-sequence stellar mass distribution. For visual clarity, we do not show flux errors for the disks from the mm continuum surveys.</figcaption></figure>

models have shown that these molecules reflect the physical and chemical makeup of a disk, including the gas mass and surface density of the disk (the CO isotopologs $^{12}$CO J = 2–1, $^{13}$CO J = 2–1, and $C^{18}$O J = 2–1; e.g., Miotello et al. 2014; Zhang et al. 2021), distributions of small carbon-bearing, nitrogen-bearing, and sulfur-bearing organics ($C_2H N = 3–2$, J = 7/2–5/2, HCN J = 3–2, and CS J = 5–4; e.g., Bergner et al. 2019; Le Gal et al. 2019; Guzmán et al. 2021), cold and deuterated chemistry (DCN J = 3–2, DCO$^+$ J = 3–2, and $H_2CO$ 3$_{03}$–2$_{02}$; e.g., Huang et al. 2017; Pegues et al. 2020; Cataldi et al. 2021), and ionization (HCO$^+$ J = 3–2; e.g., Cleeves et al. 2014; Aikawa et al. 2021).<fnref n="15" />

## 2.4. Disk Sample from the Literature

To facilitate comparison of chemistry across the pre-main-sequence stellar mass distribution (i.e., from M-type through B-type stars), we have compiled a sample of protoplanetary disks from the literature that have interferometric mm wavelength observations of at least one of the target lines in Table 3.<fnref n="16" /> Appendix A describes the stellar characteristics of the literature sample and lists the total (i.e., disk-integrated and velocity-integrated as applicable) mm continuum and molecular line fluxes and upper limits.

We split the literature sample into four categories: (1) $M_* \le 0.5 M_\odot$, (2) $M_* = 0.5–1.0 M_\odot$, (3) T Tauri disks with $M_* > 1.0 M_\odot$, and (4) disks around F-type, B-type, and A-type pre-main-sequence stars (Herbig AeBe/F disks), where $M_*$ is the stellar mass of the disk. Figure 1 compares mm continuum fluxes for the five disks from this work to (1) the literature disk sample, as well as to (2) disks from mm continuum surveys.

We find that our five target disks and the literature sample are consistent with the generally positive correlation between mm continuum flux and the mass and luminosity of the host star.

Figure 1 also shows that although the Herbig AeBe/F disks host the most massive and most luminous stars in the combined sample, they do not all have the largest mm wavelength continuum fluxes. Assuming that the mm continuum flux is proportional to the disk size by a roughly consistent factor (see, e.g., empirical relationships in Andrews 2020), we note that the disks around less massive stars in our combined sample may have relatively large disks (which, indeed, makes them easier observing targets), and/or the Herbig AeBe/F disks have relatively small or truncated morphologies (possibly due to disk dust evolution over time, and/or to their preferentially massive orbital companions, as discussed in Section 1).

# 3. Analysis

## 3.1. Emission Extraction

To extract the mm continuum emission in each SMA spectral band, we used masks built from two-dimensional Gaussian fits to the mm continuum emission. We used the model fitted to the combined mm continuum emission (i.e., taken across all four SMA spectral bands) to estimate a characteristic center for each disk, and we used the mask derived from this fit to uniformly extract an unbiased spectrum across all bandwidths. The unbiased spectra are plotted in Appendix B, providing a complete view of the SMA bandwidth and the targeted molecular lines for each disk.

For the targeted molecular lines, we extracted emission with Keplerian masks (Pegues 2019). Keplerian masks use the expected Keplerian rotation of a disk to predict spatial distributions of emission within each velocity channel. These masks can reduce the noise contributed per channel relative to elliptical masks (e.g., Yen et al. 2016). The edges of these masks were set to 2.5× the radii in Table 1 (where ~2.5 is the

<footnote n="15">¹⁵ These lines are denoted as $^{12}$CO 2–1, $^{13}$CO 2–1, $C^{18}$O 2–1, $C_2H$ 3–2, HCN 3–2, CS 5–4, DCN 3–2, DCO$^+$ 3–2, $H_2CO$ 3–2, and HCO$^+$ 3–2 throughout the rest of this paper.</footnote>

<footnote n="16">¹⁶ For disks from the literature where $H_2CO$ 3–2 was not observed, we use $H_2CO$ 4$_{04}$–3$_{03}$ ($H_2CO$ 4–3) observations as a proxy when available. Both transitions are expected empirically to give similar fluxes (Pegues et al. 2020).</footnote>
<table><caption>**Table 4**
Millimeter-wavelength Continuum Emission</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>Repr. Freq.<br>(GHz)<br>(2)</th><th>Flux<br>(mJy)<br>(3)</th><th>Peak Em.<br>(mJy × beam<sup>-1</sup>)<br>(4)</th><th>rms<br>(mJy × beam<sup>-1</sup>)<br>(5)</th><th>Synthesized<br>Beam (P.A.)<br>(6)</th><th>Gaus. Param.<br>(σ<sub>x</sub>, σ<sub>y</sub>, θ)<br>(7)</th></tr></thead><tbody><tr><td rowspan="5">HD 34282</td><td>Comb.</td><td>130.5 ± 2.7</td><td>124.1 ± 1.3</td><td>2</td><td>4"0 × 2"6 (36"8)</td><td rowspan="5">1"7, 1"2, 127"9</td></tr><tr><td>218</td><td>100.8 ± 2.4</td><td>91.9 ± 1.0</td><td>1.5</td><td>4"4 × 2"9 (37"0)</td></tr><tr><td>234</td><td>128.4 ± 2.6</td><td>119.3 ± 0.9</td><td>1.6</td><td>4"0 × 2"5 (33"7)</td></tr><tr><td>249</td><td>162.8 ± 3.2</td><td>152.4 ± 1.4</td><td>2.4</td><td>4"0 × 2"7 (34"9)</td></tr><tr><td>265</td><td>205.2 ± 6.9</td><td>184.1 ± 3.0</td><td>3.6</td><td>3"7 × 2"3 (38"0)</td></tr><tr><td rowspan="5">HD 36112</td><td>Comb.</td><td>83.4 ± 2.3</td><td>75.1 ± 0.9</td><td>1.3</td><td>3"1 × 2"7 (53"1)</td><td rowspan="5">1"3, 1"4, 66"5</td></tr><tr><td>218</td><td>59.4 ± 1.0</td><td>53.7 ± 0.4</td><td>0.78</td><td>3"6 × 3"0 (38"9)</td></tr><tr><td>234</td><td>82.3 ± 1.6</td><td>74.9 ± 0.7</td><td>1.1</td><td>3"1 × 2"6 (33"0)</td></tr><tr><td>249</td><td>102.7 ± 2.1</td><td>90.3 ± 0.9</td><td>1.3</td><td>3"3 × 2"8 (68"2)</td></tr><tr><td>265</td><td>134.8 ± 3.9</td><td>115.0 ± 1.2</td><td>2.2</td><td>2"9 × 2"5 (63"9)</td></tr><tr><td rowspan="5">HD 38120</td><td>Comb.</td><td>38.7 ± 0.9</td><td>39.3 ± 0.3</td><td>0.59</td><td>3"6 × 2"8 (4"8)</td><td rowspan="5">1"5, 1"2, 84"6</td></tr><tr><td>218</td><td>28.6 ± 0.7</td><td>28.1 ± 0.3</td><td>0.49</td><td>4"3 × 3"2 (−0"4)</td></tr><tr><td>234</td><td>39.1 ± 1.3</td><td>39.8 ± 0.4</td><td>0.65</td><td>3"8 × 2"9 (2"8)</td></tr><tr><td>249</td><td>43.0 ± 1.6</td><td>43.8 ± 0.3</td><td>0.69</td><td>3"7 × 2"8 (4"4)</td></tr><tr><td>265</td><td>60.7 ± 2.5</td><td>55.8 ± 0.9</td><td>0.97</td><td>3"5 × 2"5 (9"7)</td></tr><tr><td rowspan="5">HD 142666</td><td>Comb.</td><td>135.5 ± 1.4</td><td>134.6 ± 0.9</td><td>1.1</td><td>3"3 × 2"8 (−24"8)</td><td rowspan="5">1"4, 1"2, 114"3</td></tr><tr><td>218</td><td>110.0 ± 1.1</td><td>106.9 ± 0.5</td><td>0.9</td><td>3"7 × 3"1 (−21"7)</td></tr><tr><td>234</td><td>137.3 ± 1.5</td><td>134.8 ± 1.0</td><td>1.2</td><td>3"4 × 2"8 (−21"2)</td></tr><tr><td>249</td><td>151.6 ± 1.5</td><td>143.8 ± 0.8</td><td>1.2</td><td>3"3 × 2"8 (−23"2)</td></tr><tr><td>265</td><td>183.1 ± 2.4</td><td>174.0 ± 1.2</td><td>1.9</td><td>3"1 × 2"5 (−21"4)</td></tr><tr><td rowspan="5">HD 144432</td><td>Comb.</td><td>54.6 ± 0.7</td><td>55.4 ± 0.3</td><td>0.51</td><td>3"5 × 2"7 (−16"7)</td><td rowspan="5">1"5, 1"2, 106"5</td></tr><tr><td>218</td><td>45.9 ± 0.9</td><td>44.7 ± 0.4</td><td>0.57</td><td>4"0 × 3"1 (−14"0)</td></tr><tr><td>234</td><td>56.4 ± 1.5</td><td>56.7 ± 0.6</td><td>0.84</td><td>3"6 × 2"8 (−13"0)</td></tr><tr><td>249</td><td>62.3 ± 1.3</td><td>59.5 ± 0.6</td><td>0.83</td><td>3"5 × 2"8 (−14"0)</td></tr><tr><td>265</td><td>70.2 ± 1.7</td><td>68.7 ± 0.7</td><td>1.1</td><td>3"3 × 2"5 (−16"5)</td></tr></tbody></table>

**Note.** Column 1: Star+disk system. Column 2: Representative frequency for the SMA spectral band from which the mm continuum emission was measured. Emission taken across all combined SMA spectral bands is labeled “Comb.”. Columns 3, 4, and 5: Total mm continuum flux, peak mm continuum intensity, and rms, respectively, along with the estimated errors (Section 3.2). Note that given the significant change in beam size across all SMA spectral bands, these values and errors were estimated from different elliptical masks (Section 3.1). Column 6: Dimensions of the synthesized beam. Column 7: Parameters for the Gaussian fits to the combined mm continuum emission (Section 3.1).
All errors do not include the 10% flux calibration uncertainties.

empirical dust-to-CO disk radius factor measured in Andrews 2020). We used the Keplerian-masked emission across channels to generate image products for each line, including spectra, velocity-integrated emission maps (i.e., zeroth-moment maps), and velocity-integrated fluxes.

## 3.2. Noise Estimation

We estimated the noise for each image product<fnref n="17" /> via bootstrapping over 1000 random samples of emission-free channels/regions. We used the average root mean square (rms; mJy beam⁻¹) as an estimate of (1) the noise for the mm continuum emission and (2) the noise per channel for the target line emission. In both cases, we extracted the random samples from within the corresponding elliptical masks (see Section 3.1).

For the mm continuum flux errors and the line flux errors, we took the standard deviation of the noise summed within the elliptical continuum mask (mJy) and within the Keplerian masks (mJy km s⁻¹), respectively. For the error in the velocity-integrated emission maps, we used the same method as Bergner et al. (2018): we (1) generated velocity-integrated noise maps

from samples of emission-free channels and (2) took the median of the resulting maps as a representation of the map error.

To avoid artifacts at the image edges caused by the primary beam corrections, all random samples were extracted from within a square region −18"0 to +18"0 around the disk center. We also excluded the inner square region −5"0 to +5"0 when estimating noise for the mm continuum.

# 4. Results

## 4.1. Millimeter-wavelength Continuum and Molecular Line Detections

Table 4 presents the mm continuum fluxes, synthesized beam sizes, and estimated error measured for the individual and combined SMA spectral bands for each disk. Figure 2 shows the mm continuum emission for the lowest frequency, highest frequency, and combined SMA spectral bands. We detect mm continuum emission at mm wavelengths above 3σ from all five disks in the sample.

For HD 34282, HD 36112, and HD 142666, ~1.3 mm continuum fluxes have been measured at higher spatial resolution in the literature (Stapper et al. 2022). While our

<footnote n="17">¹⁷ Our error estimates do not include the 10% flux calibration uncertainties.</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Millimeter-wavelength continuum emission maps for the faintest (~218 GHz, middle), brightest (~265 GHz, bottom), and combined ("Comb.;" ~213–268 GHz, top) SMA spectral bands. Outer edges of the elliptical masks (Section 3.1) are outlined in dashed gray. Contours are [3, 5, 10, 20, 40...] × σ, where σ is the rms (Section 3.2). Plus signs mark the disk centers estimated from the mm continuum emission (Section 3.1). All panels share the same color bar (bottom right), which is in log scale to emphasize faint emission. Synthesized beams are drawn in the lower right corners.</figcaption></figure>

234 GHz continuum fluxes are consistent with these literature values within 15% for HD 36112 and HD 142666, our 234 GHz continuum flux for HD 34282 (~128 mJy) exceeds the literature value by over 15% (99 mJy; Stapper et al. 2022). This excess could be due to differences in uv coverage between the observations, or to inclusion of an extended or external source of emission within the extraction masks (see Section 4.3).

We classified detections of the molecular line emission according to the following criteria (where the σ and error values are as described in Section 3.2):

1. Peak emission is $≥3×$ the noise estimated for the velocity-integrated emission map over a significant spatial area (at least 7%$^{18}$ of the beam).

2. The velocity-integrated line flux is $≥3σ$.

Molecular lines that fulfilled both criteria were classified as detections, while lines that fulfilled one criterion were classified as tentative detections. If neither criterion was met, then the lines were classified as nondetections.

Based on these criteria, we detect $^{12}$CO 2–1 from four disks. We detect $^{13}$CO 2–1, C$^{18}$O 2–1, and HCO$^{+}$ 3–2 from three disks, and we tentatively detect $^{13}$CO 2–1 from a fourth disk. We detect HCN 3–2 from one disk and tentatively detect the same line from another disk. Finally, we tentatively detect CS

5–4 and DCO$^{+}$ 3–2 from two disks each, and we tentatively detect C$_2$H 3–2 and DCN 3–2 from one disk each. All other line+disk pairs are not detected at the sensitivity of our observations.

We do not detect any molecular line emission, including $^{12}$CO 2–1, from HD 38120. This is in contrast to Dent et al. (2005), who observed a bright $^{12}$CO 3–2 spectrum from HD 38120 with the James Clerk Maxwell Telescope (JCMT; see their Figure 2(b)). We note that (1) Dent et al. (2005) reported a beam size of 1400 au at 100 pc, or an angular resolution of 14"/0, (2) Dent et al. (2005) fitted the $^{12}$CO 3–2 spectrum with a single rather than double-peaked Gaussian despite the disk’s high inclination angle (Table 1), and (3) there appears to be absorption affecting the $^{12}$CO 2–1 spectrum (see Appendix B). It is therefore possible that the $^{12}$CO 3–2 spectrum in Dent et al. (2005) was contaminated by CO emission from a surrounding cloud or envelope.

We do detect relatively faint mm continuum emission from HD 38120, suggesting that a dust disk does exist in this system. That being said, observing any molecular gas present in this disk would likely require greater sensitivity than initially expected in this survey. Since our line flux upper limits for this disk are nonconstraining, in that we are unable to distinguish any of the $^{12}$CO 2–1 emission of the disk from possible environmental contamination, we exclude HD 38120 from the subsequent line flux analysis and discussion.

All line fluxes and detection classifications (excluding HD 38120) are presented in Table 5. Appendix B also displays the spectral data across all SMA spectral bands, extracted using elliptical masks (see Section 3.1), where our 10 target molecular lines are highlighted in each panel.

<footnote>$^{18}$7% is based on a simple geometric argument for the beams, which are too large to fully resolve the observed emission: for a point-source image convolved with a Gaussian with a standard deviation σ, the ratio of the areas between a circle of diameter σ and a circle containing the bulk of the emission (taken to be ~95%, which is contained within a diameter of 4σ by the ~68-95-99 rule) is 1/16, or ~7%.</footnote>
<table><caption>**Table 5**
Molecular Line Emission</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>Molecular<br>Line<br>(2)</th><th>Int. Flux<br>(mJy<br>×km s<sup>-1</sup>)<br>(3)</th><th>Peak Flux<br>(mJy beam<sup>-1</sup><br>×km s<sup>-1</sup>)<br>(4)</th><th>Velocity<br>Range<br>(km s<sup>-1</sup>)<br>(5)</th><th>Channel<br>Spacing<br>(km s<sup>-1</sup>)<br>(6)</th><th>Channel rms<br>(mJy ×<br>beam<sup>-1</sup>)<br>(7)</th><th>Synthesized<br>Beam (P.A.)<br>(8)</th></tr></thead><tbody><tr><td>HD 34282</td><td rowspan="4"><sup>12</sup>CO 2-1</td><td>8149 ± 282</td><td>7721 ± 212</td><td>[-6.39, 1.11]</td><td>0.5</td><td>145</td><td>6.6 × 2.4 (24°3)</td></tr><tr><td>HD 36112</td><td>7370 ± 208</td><td>7142 ± 161</td><td>[3.02, 8.52]</td><td>0.5</td><td>125</td><td>4.8 × 2.6 (16°6)</td></tr><tr><td>HD 142666</td><td>3250 ± 205</td><td>3998 ± 168</td><td>[-1.64, 10.37]</td><td>2.0</td><td>42</td><td>3.5 × 2.9 (-19°9)</td></tr><tr><td>HD 144432</td><td>1034 ± 226</td><td>1221 ± 172</td><td>[-0.88, 11.13]</td><td>2.0</td><td>44</td><td>3.7 × 2.9 (-13°8)</td></tr><tr><td>HD 34282</td><td rowspan="4"><sup>13</sup>CO 2-1</td><td>2469 ± 249</td><td>2661 ± 184</td><td>[-6.69, 0.82]</td><td>0.5</td><td>126</td><td>7.0 × 2.5 (25°5)</td></tr><tr><td>HD 36112</td><td>2017 ± 176</td><td>2278 ± 132</td><td>[3.17, 8.68]</td><td>0.5</td><td>109</td><td>5.1 × 2.7 (17°8)</td></tr><tr><td>HD 142666</td><td>1206 ± 197</td><td>1293 ± 161</td><td>[-1.43, 10.59]</td><td>2.0</td><td>40</td><td>3.6 × 3.0 (-17°2)</td></tr><tr><td>HD 144432</td><td>≤646</td><td>712 ± 154</td><td>[-0.68, 11.33]</td><td>2.0</td><td>41</td><td>3.9 × 3.0 (-14°7)</td></tr><tr><td>HD 34282</td><td rowspan="4">C<sup>18</sup>O 2-1</td><td>912 ± 249</td><td>955 ± 187</td><td>[-6.22, 0.82]</td><td>0.5</td><td>133</td><td>7.0 × 2.5 (25°5)</td></tr><tr><td>HD 36112</td><td>732 ± 179</td><td>750 ± 135</td><td>[3.18, 8.71]</td><td>0.5</td><td>112</td><td>5.1 × 2.7 (17°8)</td></tr><tr><td>HD 142666</td><td>550 ± 167</td><td>761 ± 140</td><td>[-1.26, 10.70]</td><td>2.0</td><td>34</td><td>3.6 × 3.1 (-18°3)</td></tr><tr><td>HD 144432</td><td>&lt;552</td><td>&lt;402</td><td>[-0.51, 11.44]</td><td>2.0</td><td>35</td><td>3.9 × 3.0 (-13°4)</td></tr><tr><td>HD 34282</td><td rowspan="4">HCO<sup>+</sup> 3-2</td><td>1325 ± 428</td><td>1802 ± 308</td><td>[-6.29, 1.21]</td><td>0.5</td><td>190</td><td>3.3 × 2.3 (45°2)</td></tr><tr><td>HD 36112</td><td>1048 ± 333</td><td>981 ± 205</td><td>[3.11, 8.61]</td><td>0.5</td><td>172</td><td>2.8 × 2.3 (80°1)</td></tr><tr><td>HD 142666</td><td>1339 ± 280</td><td>1746 ± 245</td><td>[-2.24, 9.76]</td><td>2.0</td><td>55</td><td>3.1 × 2.5 (-18°9)</td></tr><tr><td>HD 144432</td><td>&lt;981</td><td>&lt;725</td><td>[0.51, 12.51]</td><td>2.0</td><td>59</td><td>3.3 × 2.5 (-14°1)</td></tr><tr><td>HD 34282</td><td rowspan="4">CS 5-4</td><td>≤847</td><td>750 ± 208</td><td>[-6.25, 1.27]</td><td>0.5</td><td>135</td><td>3.5 × 2.6 (43°4)</td></tr><tr><td>HD 36112</td><td>≤667</td><td>691 ± 182</td><td>[3.07, 8.58]</td><td>0.5</td><td>119</td><td>3.1 × 2.4 (84°4)</td></tr><tr><td>HD 142666</td><td>≤629</td><td>≤485</td><td>[-2.59, 9.44]</td><td>2.0</td><td>40</td><td>3.3 × 2.8 (-17°2)</td></tr><tr><td>HD 144432</td><td>≤677</td><td>≤491</td><td>[0.17, 12.20]</td><td>2.0</td><td>42</td><td>3.5 × 2.7 (-12°3)</td></tr><tr><td>HD 34282</td><td rowspan="4">HCN 3-2</td><td>≤1439</td><td>1719 ± 260</td><td>[-6.23, 1.26]</td><td>0.5</td><td>171</td><td>3.3 × 2.3 (43°5)</td></tr><tr><td>HD 36112</td><td>≤1313</td><td>≤568</td><td>[3.15, 8.64]</td><td>0.5</td><td>152</td><td>2.9 × 2.3 (80°4)</td></tr><tr><td>HD 142666</td><td>1770 ± 296</td><td>2226 ± 216</td><td>[-1.98, 10.00]</td><td>2.0</td><td>50</td><td>3.1 × 2.5 (-19°5)</td></tr><tr><td>HD 144432</td><td>≤1031</td><td>722 ± 196</td><td>[0.76, 12.75]</td><td>2.0</td><td>52</td><td>3.3 × 2.5 (-14°5)</td></tr><tr><td>HD 34282</td><td rowspan="4">C<sub>2</sub>H 3-2</td><td>≤1532</td><td>1228 ± 350</td><td>[-8.78, 1.20]</td><td>0.5</td><td>209</td><td>6.0 × 2.2 (27°3)</td></tr><tr><td>HD 36112</td><td>≤1281</td><td>≤802</td><td>[0.58, 8.56]</td><td>0.5</td><td>190</td><td>4.5 × 2.4 (25°4)</td></tr><tr><td>HD 142666</td><td>≤921</td><td>≤738</td><td>[-4.15, 9.83]</td><td>2.0</td><td>53</td><td>3.2 × 2.5 (-19°9)</td></tr><tr><td>HD 144432</td><td>≤1079</td><td>≤709</td><td>[-3.40, 12.57]</td><td>2.0</td><td>55</td><td>3.3 × 2.5 (-16°0)</td></tr><tr><td>HD 34282</td><td rowspan="4">DCN 3-2</td><td>≤581</td><td>≤409</td><td>[-6.36, 1.13]</td><td>0.5</td><td>96</td><td>3.9 × 2.8 (44°1)</td></tr><tr><td>HD 36112</td><td>≤437</td><td>≤288</td><td>[2.95, 8.43]</td><td>0.5</td><td>82</td><td>3.3 × 2.8 (61°9)</td></tr><tr><td>HD 142666</td><td>≤504</td><td>≤412</td><td>[-2.66, 9.32]</td><td>2.0</td><td>35</td><td>3.7 × 3.1 (-17°3)</td></tr><tr><td>HD 144432</td><td>≤566</td><td>446 ± 134</td><td>[0.09, 12.06]</td><td>2.0</td><td>36</td><td>3.9 × 3.1 (-11°2)</td></tr><tr><td>HD 34282</td><td rowspan="4">DCO<sup>+</sup> 3-2</td><td>≤529</td><td>567 ± 128</td><td>[-6.23, 1.29]</td><td>0.5</td><td>82</td><td>3.9 × 2.8 (44°1)</td></tr><tr><td>HD 36112</td><td>≤387</td><td>332 ± 89</td><td>[3.12, 8.64]</td><td>0.5</td><td>72</td><td>3.3 × 2.8 (61°9)</td></tr><tr><td>HD 142666</td><td>≤493</td><td>536 ± 136</td><td>[-2.01, 10.03]</td><td>2.0</td><td>33</td><td>3.7 × 3.1 (-17°3)</td></tr><tr><td>HD 144432</td><td>≤516</td><td>≤382</td><td>[0.75, 12.79]</td><td>2.0</td><td>34</td><td>3.9 × 3.1 (-11°2)</td></tr><tr><td>HD 34282</td><td rowspan="4">H<sub>2</sub>CO 3<sub>03</sub>-2<sub>02</sub></td><td>≤655</td><td>≤496</td><td>[-6.46, 1.06]</td><td>0.5</td><td>108</td><td>7.0 × 2.5 (25°4)</td></tr><tr><td>HD 36112</td><td>≤452</td><td>≤307</td><td>[3.40, 8.41]</td><td>0.5</td><td>92</td><td>5.2 × 2.7 (17°4)</td></tr><tr><td>HD 142666</td><td>≤509</td><td>≤413</td><td>[-1.19, 10.84]</td><td>2.0</td><td>34</td><td>3.6 × 3.1 (-18°3)</td></tr><tr><td>HD 144432</td><td>≤571</td><td>≤413</td><td>[-0.44, 11.59]</td><td>2.0</td><td>36</td><td>3.9 × 3.0 (-13°4)</td></tr></tbody></table>

**Note.** Column 1: Star+disk system. Column 2: Molecular line. Column 3: Velocity-integrated flux. “≤” and “<” are 3σ upper limits and mark tentative detections and nondetections, respectively. Column 4: Peak flux. Column 5: Total velocity range used to estimate Columns 3 and 4. The range is centered on the systemic velocity (Table 1). Column 6: Spacings between imaged channels. Column 7: Channel rms. Column 8: Dimensions of the synthesized beam. Columns 3, 4, and 7 were estimated within the Keplerian masks. rms and errors were estimated via bootstrapping (Section 3.2). Errors do not include the 10% absolute flux calibration uncertainties.

## 4.2. Gas and Dust Morphologies

Figures 3 and 4 display velocity-integrated emission maps and spectra, respectively, for molecular lines that are detected or tentatively detected from at least one disk in our sample. Channel maps for all detected and tentatively detected lines are given in Appendix C.

At the spatial resolution of our observations, all mm continuum emission (Figure 2) and detected molecular line emission (Figure 3) appear smooth, without any apparent cavities or gaps. However, we do find persistent asymmetries in the dust and/or bright molecular line emission for certain disks. Here we qualitatively describe these asymmetries in the context of complex morphologies known from the literature.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Velocity-integrated emission maps for the target molecular lines that were detected or tentatively detected from at least one disk in the sample. H₂CO 3–2 is not shown because it was undetected in all disks. The maps were extracted using Keplerian masks (Section 3.1), the edges of which are outlined in dashed gray. Contours are [3, 5, 10, 20, 40...] × σ (see Section 3.2 for σ). Panels are outlined in solid black, black, and dashed gray for detections, tentative detections, and nondetections respectively. Color bars start at 1σ. Plus signs mark the estimated disk centers (see Section 3.1). Synthesized beams are drawn in the lower right corners.</figcaption></figure>

4.2.1. HD 34282

HD 34282 is adjacent to what appears to be an extended or
external source of emission. This neighboring source is

centered eastward of the HD 34282 estimated disk center,
roughly ~5" away. The neighbor is seen above 3σ in the mm
continuum emission and is brightest in the 265 GHz band
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Keplerian-masked spectra are plotted with thick lines for the CO isotopologs (black), other oxygen carriers (red), sulfur carriers (yellow), hydrocarbons (green), and cyanides (blue) that were detected or tentatively detected from at least one disk in the sample. H₂CO 3–2 is not shown because it was undetected from all disks. Elliptical-masked spectra are plotted with thin gray lines for comparison. Panels are outlined in solid black, dashed black, and dashed gray for detections, tentative detections, and nondetections, respectively. Vertical dotted gray lines mark the systemic velocity (Table 1).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Emission maps (top) and spectra (bottom) for the HD 34282 neighboring source of emission. The brightest mm continuum emission (~265 GHz) is shown in the leftmost top panel. Velocity-integrated emission maps of the CO 2–1 isotopolog (top panels 2-4) and HCO$^+$ 3–2 (top panel 5) emission were extracted without using masks. Contours are [3, 5, 10, 20, 40...] × σ, where σ is the same as in Figure 3. Plus signs mark the disk centers estimated from the mm continuum emission (Section 3.1). Synthesized beams are drawn in the lower right corners. Note the difference in beam size across the different panels. The neighboring source is highlighted in each top panel with a custom mask drawn in dashed gray, which was used to extract the spectra shown in the bottom panels. The systemic velocity of HD 34282 is marked with a vertical dashed line for each spectrum.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Molecular line fluxes as a function of mm continuum fluxes for Herbig AeBe disks from this work (dotted red squares). The literature sample of Herbig AeBe/F disks (blank red squares) and T Tauri disks with stellar masses >1.0 $M_\odot$ (thick white diamonds), 0.5–1.0 $M_\odot$ (thin light gold diamonds), and $\le 0.5 M_\odot$ (blue circles) is shown for comparison (Section 2.4, Appendix A). 3σ upper limits are shown as faint downward-pointing triangles. All mm continuum fluxes have been scaled to 265 GHz using $F_\nu \propto \nu^{2.2}$ (Andrews 2020) for flux $F_\nu$ and frequency $\nu$. Both mm continuum and molecular line fluxes have been scaled to 140 pc. Whenever flux errors for a disk are not reported in the literature, we assume that the error is 15% (for the mm continuum) or 20% (for the molecular lines) of the given value.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Top and middle rows: CO 2–1 isotopolog line flux ratios as a function of stellar luminosity (top row) and mm continuum flux (middle row) for Herbig Ae disks from this work (dotted red squares). The literature sample of Herbig AeBe/F disks (blank red squares) and T Tauri disks with stellar masses >1.0 $M_{\odot}$ (thick white diamonds), 0.5–1.0 $M_{\odot}$ (thin light gold diamonds), and $\le 0.5 M_{\odot}$ (blue circles) is shown for comparison (Section 2.4, Appendix A). $3\sigma$ upper limits are shown as triangles and point in the direction of the limit. Points are not shown when both line fluxes are upper limits. Bottom row: Boxplot summaries of the CO 2–1 isotopolog line flux ratios for the combined samples, using the same color scheme as above. The boxplots contain detected fluxes only. Dashed boxes illustrate the span of any upper limits. The numbers of data points included in the boxplots are written along the x-axis. When $<4$, individual data points are drawn instead. The box portions of the boxplots illustrate the 25th, 50th, and 75th percentiles, while the whiskers span all data.</figcaption></figure>

(Figure 2). There also appears to be a faint stream of dust connecting HD 34282 and this neighboring source, although the stream is not detected above $3\sigma$.

This neighbor is not visible in the velocity-integrated emission maps of Figure 3 because these maps were extracted using Keplerian masks created for HD 34282. We have thus generated velocity-integrated emission maps for the brightest molecular lines from HD 34282 using no masks, which are displayed in Figure 5. Figure 5 shows that this neighbor is detected above $3\sigma$ in emission maps of $^{13}\text{CO}$ 2–1, $C^{18}\text{O}$ 2–1, and $\text{HCO}^+$ 3–2, and is seen in $^{12}\text{CO}$ 2–1 below $3\sigma$. Spectra extracted from a custom region containing the neighboring emission do not show Keplerian rotation, at least at the sensitivity of these observations.

HD 34282 has previously been observed in mm wavelength continuum and CO emission in the literature. Piétu et al. (2003)

mapped the Keplerian rotation of HD 34282 in $^{12}\text{CO}$ 2–1 emission using the Institut de Radioastronomie Millimétrique (IRAM) interferometer (the Plateau de Bure Interferometer, originally denoted PdBI, but now upgraded and renamed NOEMA). van der Plas et al. (2017) conducted an ALMA study of ~350 GHz continuum, $^{12}\text{CO}$ 3–2, and $\text{HCO}^+$ 4–3 emission (from ALMA Project 2013.1.00658.S). Stapper et al. (2022) presented an ALMA image of the ~230 GHz continuum emission for HD 34282 (their Figure 2; from the ALMA Project 2015.1.00192.S), while Law et al. (2022a) recently conducted an ALMA vertical structure study and presented new images of the ~230 GHz continuum, $^{12}\text{CO}$ 2–1, $^{13}\text{CO}$ 2–1, and $C^{18}\text{O}$ 2–1 emission (from ALMA Projects 2015.1.00192.S and 2017.1.01578.S). The latter three studies (van der Plas et al. 2017; Law et al. 2022a; Stapper et al. 2022) all presented images of HD 34282 within a field of view of $\lesssim 2''0 \times 2''0$.
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Top and middle rows: HCO$^+$ 3–2, DCO$^+$ 3–2, and CO 2–1 isotopolog line flux ratios as a function of stellar luminosity (top row) and mm continuum flux (middle row) for Herbig Ae disks from this work (dotted red squares). The literature sample of Herbig AeBe/F disks (blank red squares) and T Tauri disks with stellar masses >1.0 $M_\odot$ (thick white diamonds), 0.5–1.0 $M_\odot$ (thin light gold diamonds), and $\le 0.5 M_\odot$ (blue circles) is shown for comparison (Section 2.4, Appendix A). $3\sigma$ upper limits are shown as triangles and point in the direction of the limit. Points are not shown when both line fluxes are upper limits. Bottom row: Boxplot summaries of the HCO$^+$ 3–2, DCO$^+$ 3–2, and CO 2–1 isotopolog line flux ratios for the combined samples, using the same color scheme as above. The boxplots contain detected fluxes only. Dashed boxes illustrate the span of any upper limits. The numbers of data points included in the boxplots are written along the x-axis. When <4, individual data points are drawn instead. The box portions of the boxplots illustrate the 25th, 50th, and 75th percentiles, while the whiskers span all data.</figcaption></figure>

In terms of the dust, faint structure can be seen eastward of HD 34282 in the published ~230 GHz continuum images of Stapper et al. (2022) and Law et al. (2022a) within their $\lesssim 2''_0 \times 2''_0$ fields of view (see their Figures 2 and 1, respectively). However, the same structure is not apparent within the published $\lesssim 2''_0 \times 2''_0 \sim 350$ GHz continuum image of van der Plas et al. (2017; see, e.g., their Figure 4; see also Figure 1 of Law et al. 2022a). We expanded the field of view for the ~350 GHz continuum image from the same observing project as van der Plas et al. (2017; ALMA Project 2013.1.00658.S, extracted from the ALMA archive; Appendix D). The ~350 GHz continuum image observed in the compact configuration tentatively suggests what could be faint structure eastward of HD 34282, while the ~350 GHz

continuum image observed in the extended configuration shows no such structure.

In terms of the gas, model residuals of the $^{12}\text{CO}$ 2–1 emission in Piétu et al. (2003; see their Figure 5) suggest a neighboring emitting source, with a center that shifts from roughly ~3" away to ~5" away over the course of their channel maps. However, published ALMA images of the $^{12}\text{CO}$ 3–2, HCO$^+$ 4–3, $^{12}\text{CO}$ 2–1, $^{13}\text{CO}$ 2–1, and $^{C^{18}\text{O}}$ 2–1 emission in van der Plas et al. (2017) and Law et al. (2022a) show no emission structure eastward of the disk within $\lesssim 2''_0 \times 2''_0$. We expanded the field of view of the $^{12}\text{CO}$ 2–1, $^{13}\text{CO}$ 2–1, and $^{C^{18}\text{O}}$ 2–1 image cubes provided by Law et al. (2022a; derived from ALMA Projects 2015.1.00192.S and 2017.1.01578; not shown). We also expanded the archival ALMA images of the
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Top and middle rows: Line flux ratios involving DCO$^{+}$ 3–2, H$_{2}$CO 3–2, 4–3, and DCN 3–2 as a function of stellar luminosity (top row) and mm continuum flux (middle row) for Herbig Ae disks from this work (dotted red squares). The literature sample of Herbig AeBe/F disks (blank red squares) and T Tauri disks with stellar masses >1.0 $M_{\odot}$ (thick white diamonds), 0.5–1.0 $M_{\odot}$ (thin light gold diamonds), and $\le 0.5 M_{\odot}$ (blue circles) is shown for comparison (Section 2.4, Appendix A). $3\sigma$ upper limits are shown as triangles and point in the direction of the limit. Points are not shown when both line fluxes are upper limits. Bottom row: Boxplot summaries of line flux ratios involving DCO$^{+}$ 3–2, H$_{2}$CO 3–2, 4–3, and DCN 3–2 for the combined samples, using the same color scheme as above. The boxplots contain detected fluxes only. Dashed boxes illustrate the span of any upper limits. The numbers of data points included in the boxplots are written along the x-axis. When $<4$, individual data points are drawn instead. The box portions of the boxplots illustrate the 25th–50th–75th percentiles, while the whiskers span all data.</figcaption></figure>

$^{12}$CO 3–2 and HCO$^{+}$ 4–3 emission presented in van der Plas et al. (2017; ALMA Project 2013.1.00658.S, extracted from the ALMA archive; Appendix D). The $^{12}$CO 3–2 image observed in the compact configuration tentatively suggests faint structure eastward of HD 34282, spatial to the faint structure seen for the expanded ~350 GHz continuum image in the same configuration. However, no such structure is apparent in the other expanded images of line emission.

Altogether, we detect what appears to be a neighboring source eastward of HD 34282 in mm continuum emission, which is connected to HD 34282 by a faint stream of emission. Faint ~230 GHz continuum emission structure eastward of HD 34282 can also be seen in $\lesssim 2''0 \times 2''0$

images in the literature (Law et al. 2022a; Stapper et al. 2022). There is some very tentative suggestion of spatial $^{12}$CO emission from compact observations of this system (Piétu et al. 2003, Figure 5, Appendix D). However, the neighbor is not confidently detected in our brightest CO isotopolog ($^{12}$CO 2–1, Figure 5), and it is not detected in ALMA CO and HCO$^{+}$ 4–3 images observed in more extended configurations and at higher spatial resolution (van der Plas et al. 2017; Law et al. 2022a).

It is possible that this neighboring source is actually a persistent imaging artifact. That being said, based on mm continuum emission observations of HD 34282 in this work and in the literature, we do believe this neighbor to be real. If
this is so, the exact morphology of the neighbor is likely
distorted by the elongated beams of our observations.

We infer that the neighbor is distinct from the spiral arm and
point source previously identified within ~1''0 of the disk
center (van der Plas et al. 2017; de Boer et al. 2021). It is
also likely distinct from the orbital companion reported in
Wheelwright et al. (2010), which, based on their technique, is
within ~0.1–2'' from HD 34282 (Baines et al. 2006). We
speculate that this possible neighbor is either (1) an extended
spiral arm or (2) a distant orbital companion. It is possible that
any gas present for this neighboring source has been
dissociated or ionized, and so is not traceable by our target
molecules, or that the gas has dissipated or been disrupted
through dynamical interactions over time.

4.2.2. HD 142666

The mm continuum emission from HD 142666 shows what
appear to be asymmetries above 3σ to the northwest and
northeast of the disk center (Figure 2). However, Huang et al.
(2018) observed HD 142666 in 1.25 mm continuum emission
and found no indication of dust asymmetries near the outer disk
edge. It is possible that the asymmetries seen in this work are
imaging artifacts.

4.2.3. HD 144432

HD 144432 has the faintest mm continuum emission in
our sample (excluding HD 38120), and only CO emission
is detected or tentatively detected from the disk. The dust
disk has also previously been undetected in scattered
light (Monnier et al. 2017). Notably, HD 144432 has a known
stellar companion (Maheswar et al. 2002) at a distance of
~1''5 (Müller et al. 2011). Monnier et al. (2017) postulated that
interactions with this companion may have truncated the disk and
led to its nondetection in scattered light. Other possible
explanations include that the disk is old (~5 Myr; Section 2.1),
and so may have already lost much of its dust and gas over time.

4.3. Molecular Line Fluxes

Figure 6 plots scaled molecular line fluxes and upper limits as a function of mm continuum flux for our sample and for the literature sample (Section 2.4, Appendix A). Most molecular lines (e.g., the CO 2–1 isotopologs, C$_{2}$H 3–2, and HCN 3–2) generally increase in flux as mm continuum flux increases, with about one or more orders of magnitude of scatter in the flux. Potential exceptions are the CS 5–4 and DCO$^{+}$ 3–2 lines; for disks with stellar masses below 0.5 M$_{\odot}$, only few detections or upper limits are known for these lines.

5. Discussion

5.1. Disk Inventories across the Pre-main-sequence Stellar
Mass Distribution

We now compare molecular line flux ratios across the
combined sample of disks from this work and from the
literature sample (Section 2.4, Appendix A). We focus on flux
ratios here to help normalize against the different disk sizes
across the sample. We compare the flux ratios to stellar mass
and spectral type (via binning), stellar luminosity, and mm
continuum flux as representative properties of the disks and
their host stars.

We note that stellar age, and therefore the time span of
chemical evolution, is another important factor. So far,
however, previous mm wavelength chemistry surveys of
mostly T Tauri disks have not found any clear trends with
stellar age (e.g., Bergner et al. 2019; Le Gal et al. 2019; Pegues
et al. 2020). Noting this finding, and noting the large intrinsic
and explicit uncertainties in existing stellar age estimates for
pre-main-sequence stars (e.g., discussion by Hillenbrand &
White 2004), we do not consider stellar age in this work.

5.1.1. CO Isotopologs

CO is the second most abundant gas-phase molecule in
protoplanetary disks after H₂. Its isotopolog emission has a
complex dependence on disk structure: ¹²CO emission is
optically thick and traces the temperature and extent of the disk
surface layer; ¹³CO emission is less optically thick and traces
the temperature and extent of a vertically lower emission layer;
and finally, C¹⁸O emission is generally optically thin, emits
near the disk midplane, and can serve as a tracer of disk gas
mass (e.g., Miotello et al. 2014; Pinte et al. 2018; Law et al.
2021b; Zhang et al. 2021; Law et al. 2022a).

Chemically, astrochemical disk models exploring a range of
physical conditions have predicted that chemical processing, and
therefore depletion, of CO should be most efficient in colder
disks (Bosman et al. 2018). As a result, the warmer Herbig AeBe/
F disks should be most abundant in CO. The study by Zhang et al.
(2021) agreed with this prediction; they estimated higher CO gas
masses and higher CO-derived gas-to-dust ratios for the Herbig
Ae disks HD 163296 and MWC 480, relative to T Tauri disks, via
high-resolution CO observations.

Figure 7 plots CO isotopolog line flux ratios as a function of mm continuum flux and stellar luminosity for the combined disk sample. There is a potential decrease in the line flux ratios with respect to stellar luminosity, with about one order of magnitude in scatter. When considered with respect to the mm continuum flux, the line flux ratios appear scattered, but are consistent with a globally flat trend across both Herbig AeBe/F and T Tauri disks.

Notably, most of the Herbig AeBe/F disks appear in the lowest
regimes of the overall scatter for all three ratios. Let us assume
that the optically thick ¹²CO 2–1 and ¹³CO 2–1 emission trace the
temperatures of their emitting layers. Then the CO isotopolog flux
ratios for the Herbig AeBe/F disks, which are closest to unity in
value compared to the rest of the combined sample, may indicate
vertical emitting layers with overall warmer (and therefore more
numerically similar) temperatures. Warmer layers would be
consistent with the findings of Law et al. (2021b) and Law
et al. (2022b), who altogether derived generally warmer CO
temperature profiles from observations for Herbig AeBe disks
when compared to T Tauri disks.

Let us also assume that the C¹⁸O 2–1 emission is generally
optically thin over most of the outer disk (e.g., see models,
observations, and discussion by Miotello et al. 2014; Pinte et al.
2018; Law et al. 2021b, 2022a) and can trace the disk gas mass.
From Figure 6, we see that the Herbig AeBe/F disks have the
highest C¹⁸O 2–1 fluxes as a function of mm continuum flux.
Noting again from Bosman et al. (2018) that CO depletion is
expected to decrease in warmer layers, we can infer that the
Herbig AeBe/F disks are generally most abundant in CO relative
to disk mass, consistent with the results of Zhang et al. (2021).
### 5.1.2. Signposts of Ionization

Based on both theory and observations, HCO$^+$ 3–2 is one of the simplest molecular ions found in protoplanetary disks (e.g., Dutrey et al. 1997; Aikawa & Herbst 2001). Its formation depends on the degree of ionization within the disk, which in turn depends on the presence/absence of stellar X-ray radiation (e.g., Aikawa & Herbst 2001). Models have predicted that HCO$^+$ is the dominant molecular ion relative to H$^{+}_{3}$ in the warm CO-rich regions of the disk (specifically, where CO is at least a few orders of magnitude more abundant than free electrons; Aikawa et al. 2015). We thus expect HCO$^+$ to be most prominent in the warm upper layers of the disk, where (1) CO is most abundant and (2) the disk material is most exposed to X-ray radiation from the central star (Aikawa et al. 2021). Furthermore, because HCO$^+$ 3–2 emission is optically thick, we expect its emission to trace its emitting layer within the disk.

Figure 8 plots ratios of HCO$^+$ 3–2, DCO$^+$ 3–2, and CO isotopolog fluxes as a function of stellar luminosity and mm continuum flux for the combined disk sample. Here $^{12}$CO 2–1 and $^{13}$CO 2–1 serve as probes of the emitting surface layers, the optically thin C$^{18}$O 2–1 (e.g., see models, observations, and discussion by Miotello et al. 2014; Pinte et al. 2018; Law et al. 2021b, 2022a) as a normalizing metric for the HCO$^+$ 3–2 flux, and DCO$^+$ 3–2 as the deuterated counterpart of HCO$^+$ 3–2.

The existing data above >0.5 $M_{\odot}$ suggest a general decrease in the HCO$^+$ 3–2 flux relative to CO isotopolog fluxes as a function of both stellar luminosity and mm continuum flux. Furthermore, Figure 6 and the HCO$^+$ 3–2 / C$^{18}$O 2–1 flux ratios in Figure 8 show that the absolute HCO$^+$ 3–2 fluxes for the Herbig AeBe/F disks are distinctly offset from the T Tauri disks with existing data, and have lower HCO$^+$ 3–2 line fluxes when compared to T Tauri disks with the same mm continuum flux. Together, these trends suggest a general decrease in HCO$^+$ 3–2 for Herbig AeBe/F disks relative to T Tauri disks with $M_{*} > 0.5 M_{\odot}$.

One possible explanation for this trend is that the optically thick HCO$^+$ 3–2 emission is tracing a different emitting layer relative to CO for the Herbig AeBe/F disks than for the T Tauri disks. For T Tauri disks, models have predicted that HCO$^+$ is vertically cop spatial with CO (Aikawa et al. 2021). Observationally, Paneque-Carreño et al. (2023) estimated and compared vertical emission surfaces for CO 2–1 isotopologs and HCO$^+$ 1–0 in two Herbig Ae disks (HD 163296 and MWC 480). They found that the HCO$^+$ 1–0 emission surfaces were not cop spatial with $^{12}$CO 2–1, but were instead close to the midplane. For HD 163296, they also noted that the HCO$^+$ 1–0 emission was likely optically thin, based on a low brightness temperature profile. It is unclear if these results can be extrapolated to the HCO$^+$ 3–2 in this work, given the differences in excitation and optical depth between the two line transitions.

Another possible explanation is that Herbig AeBe/F disks are relatively less ionized than T Tauri disks, as predicted from some disk models in the literature (e.g., Walsh et al. 2015). Unlike convective T Tauri stars, Herbig AeBe stars are fully radiative and do not have a coronal layer. They are thus not strong X-ray emitters like their T Tauri counterparts (e.g., Cohen 1984), and so their disks should receive less stellar X-ray radiation. Less ionization could lead to less abundant HCO$^+$ in the Herbig AeBe/F disks. This is consistent with the conclusions of Aikawa et al. (2021), who found relatively low HCO$^+$ abundances for the Herbig Ae disk MWC 480, and attributed this result to relatively fewer high-energy X-rays incident on this disk.

Figure 8 also shows the DCO$^+$ 3–2/HCO$^+$ 3–2 line flux ratios. We note, however, that unlike DCO$^+$ 3–2 emission, HCO$^+$ 3–2 emission is expected to be optically thick. Therefore, the HCO$^+$ 3–2 emission is likely not tracing its molecular abundance. Keeping this in mind, we note that the sole existing Herbig AeBe/F disk DCO$^+$ 3–2/HCO$^+$ 3–2 detection is consistent with the median flux ratio for the Tauri disks. Intriguingly, this single flux ratio is from MWC 480, which was measured by Aikawa et al. (2021) to have a low HCO$^+$ abundance relative to the T Tauri disks in their sample. This could suggest that DCO$^+$ is similarly less ionized in Herbig AeBe/F disks, as discussed for HCO$^+$ in Section 5.1.2, although more observations are needed to further investigate this possibility.

### 5.1.3. Cold Chemistry

Three of our target molecules are expected to trace cold molecular gas: DCN, DCO$^+$, and H$_2$CO. Based on theoretical models, deuterated chemical pathways are expected to be most efficient at cold temperatures, although warm pathways exist as well. Production of the deuterated molecule DCO$^+$ is expected to peak within the cold disk midplane at temperatures $\lesssim 30$ K, while DCN production is expected to peak in the warm inner regions (e.g., results and discussion by Millar et al. 1989; Aikawa & Herbst 1999, 2001; Huang et al. 2017; Öberg et al. 2021b; Muñoz-Romero et al. 2023). The simple oxygen carrier H$_2$CO is predicted to form through both gas-phase pathways in the warm inner disk and through CO ice hydrogenation on grain surfaces beyond the CO snowline (e.g., Hiraoka et al. 1994; Fockenberg & Preses 2002; Hiraoka et al. 2002; Watanabe & Kouchi 2002). Notably, Pegues et al. (2020) and Guzmán et al. (2021) estimated lower H$_2$CO column densities for the two Herbig Ae disks in their samples (HD 163296 and MWC 480) relative to colder T Tauri disks, possibly due (at least in part) to relatively small reservoirs of CO ice around the warmer stars.

Figure 9 plots DCN 3–2, DCO$^+$ 3–2, and H$_2$CO 3–2, 4–3 line fluxes relative to C$^{18}$O 2–1 line fluxes as a function of stellar luminosity and mm continuum flux for the combined disk sample. We also show DCN 3–2 / HCN 3–2, although we note that HCN 3–2 emission is expected to be optically thick. For the Herbig AeBe/F disks, only a few disks have been detected in some or all of these three lines (see also Figure 6). The rest of the existing Herbig AeBe/F disk data are upper limits from tentative and nondetections.

The few existing detected DCO$^+$ 3–2, DCN 3–2, and H$_2$CO 3–2, 4–3 line fluxes relative to C$^{18}$O 2–1 for the Herbig AeBe/F disks are below the median values for the T Tauri disks. When considered as a function of mm continuum flux, these few detections occupy the lowest regimes of the overall scatter across all observations. These results are consistent with the hypothesis that these molecules with dominant cold pathways are less abundant in the warmer Herbig AeBe/F disks. The few detected, relatively low line flux ratios may also be a byproduct of weaker CO depletion (and therefore more C$^{18}$O 2–1 emission) in the Herbig AeBe/F disks (see Section 5.1.1).

We emphasize, however, that only a few Herbig AeBe/F disk detections exist for these lines. Upper limits for the Herbig AeBe/F disks are either consistent with T Tauri disk fluxes and upper limits, or are lower, and are not all constraining. Surveys of these lines toward Herbig AeBe/F disks at higher sensitivity are needed to further investigate these tentative trends.
## 6. Summary

Using the SMA, we have surveyed mm wavelength (~213–268 GHz) molecular line emission from protoplanetary disks around four Herbig Ae stars (HD 34282, HD 36112, HD 142666, and HD 144432) and one Herbig Be star (HD 38120), and we have compared our results to disk chemistry around Herbig AeBe/F stars and T Tauri stars from the literature. We summarize our main findings below.

1. All five disks are detected in mm continuum emission, while four are detected in molecular line emission (Section 4.1). The fifth disk, HD 38120, appears to be cloud contaminated near its systemic velocity and likely has fainter line emission than was previously expected from single-dish observations in the literature.

2. Focusing on the other four disks in the sample, <sup>12</sup>CO 2–1 is detected from all four disks (Section 4.1). <sup>13</sup>CO 2–1, C<sup>18</sup>O 2–1, and HCO<sup>+</sup> 3–2 are detected from three disks, and <sup>13</sup>CO 2–1 is tentatively detected from a fourth disk. HCN 3–2 is detected from one disk and is tentatively detected from another. Finally, CS 5–4 and DCO<sup>+</sup> 3–2 are tentatively detected from two disks each, and C<sub>2</sub>H 3–2 and DCN 3–2 are tentatively detected from one disk each. H<sub>2</sub>CO 3–2 is not detected at the sensitivity of our observations.

3. Based on gas and dust morphologies from this work and from Piétu et al. (2003), van der Plas et al. (2017), Law et al. (2022a), Stapper et al. (2022), and the ALMA archive, HD 34282 appears to be adjacent to a faint source of emission. This source is detected in mm continuum emission and is either tentatively or not detected in molecular line emission (Section 4.2.1). This neighbor is located roughly ~5" eastward of HD 34282. We speculate that the neighbor could be an extended, clumpy spiral arm or a distant orbital companion, for which the gas is either not traceable in our target molecular lines or has dispersed.

4. HD 144432 has previously been undetected in scattered light (i.e., emission from about micron-sized dust grains; Monnier et al. 2017). Here we detect HD 144432 in emission from about mm sized dust grains (Section 4.2.3). It is possible that this disk is compact, truncated (see the discussion by Monnier et al. 2017), or has lost material with age.

5. We have compiled a database of known fluxes and upper limits for protoplanetary disks from the literature that have been detected in at least one of our 10 target molecular lines (Section 2.4, Appendix A).

(a) Across the combined sample, CO 2–1 isotopolog flux ratios appear closest to unity for Herbig AeBe/F disks relative to T Tauri disks (Section 5.1.1). The CO line flux ratios may indicate that their vertical emitting layers have relatively similar, warmer temperatures in Herbig AeBe/F disks, and more abundant CO relative to the disk dust mass overall. Relatively diminished HCO<sup>+</sup> 3–2 flux ratios may result from less ionization in Herbig AeBe/F disks, as Herbig AeBe stars are fainter X-ray emitters than their T Tauri counterparts (Section 5.1.2).

(b) DCN 3–2, DCO<sup>+</sup> 3–2, and H<sub>2</sub>CO 3–2 have been detected in only a few Herbig AeBe/F disks so far. Observations from this work for these lines yielded only upper limits (Section 5.1.3). These low detection rates and flux ratios for the few existing detections are

consistent with studies from the literature, which have predicted smaller regimes of cold chemistry in disks around the hotter Herbig AeBe/F stars.

We stress that these conclusions are based on the small sample of disk chemistry surveyed so far (Appendix A). We look forward to any future follow-up or new observations of protoplanetary disks, particularly Herbig AeBe/F disks, that could test these hypotheses over a larger sample of the pre-main-sequence stellar mass distribution.

We thank the anonymous referee for their helpful comments that greatly improved the structure and clarity of this manuscript.

C.J.L. acknowledges funding from the National Science Foundation Graduate Research Fellowship under Grant DGE1745303.

Support for J.H. was provided by NASA through the NASA Hubble Fellowship grant #HST-HF2-51460.001- A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555.

The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica.

This paper makes use of the following ALMA data: ADS/JAO. ALMA#2013.1.00658.S, ADS/JAO.ALMA#2015.1.00192.S, ADS/JAO.ALMA#2017.1.01578.S.

ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

*Software:* All data reduction scripts and computer code used for this research were written in Python (version 3.8). Data calibration and imaging were performed using the MIR package (https://www.cfa.harvard.edu/~cqi/mircook.html) and CASA (McMullin et al. 2007; CASA Team et al. 2022). All plots were generated using Python's Matplotlib package~(Hunter 2007). Colormaps were generated using the cmasher package~(van der Velden 2020). This research also made use of Astropy (http://www.astropy.org), a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013, 2018), as well as the NumPy (Oliphant 2006) Python package.

## Appendix A

### The Literature Sample of Protoplanetary Disks

#### A.1. Stellar Characteristics for the Literature Sample

Table 6 lists the stellar characteristics of the literature sample (Section 2.4).
<table><caption>**Table 6**

Stellar Characteristics of the Literature Sample</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>Distance<br>(pc)<br>(2)</th><th>Spectral<br>Type<br>(3)</th><th>L<sub>*</sub><br>(L<sub>☉</sub>)<br>(4)</th><th>M<sub>*</sub><br>(M<sub>☉</sub>)<br>(5)</th><th>F<sub>ν</sub><br>(mJy)<br>(6)</th><th>ν<br>(GHz)<br>(7)</th><th>References<br>(Distance; Spectral Type; L<sub>*</sub>; M<sub>*</sub>; F<sub>ν</sub>)<br>(8)</th></tr></thead><tbody><tr><td>AA Tau</td><td>135</td><td>K5</td><td>0.56<sup>+0.20</sup><sub>-0.20</sub></td><td>0.84<sup>+0.04</sup><sub>-0.04</sub></td><td>113 ± 11</td><td>267</td><td>1; 2; 3; 4; 5</td></tr><tr><td>AK Sco</td><td>140</td><td>F5</td><td>~4.70</td><td>2.49<sup>+0.10</sup><sub>-0.10</sub></td><td>33 ± 0.07</td><td>231</td><td>1; 6; 7; 8; 8</td></tr><tr><td>AS 205</td><td>142</td><td>K5</td><td>2.14<sup>+0.88</sup><sub>-0.62</sub></td><td>0.87<sup>+0.15</sup><sub>-0.09</sub></td><td>528 ± 53</td><td>267</td><td>1; 9; 10; 10; 11</td></tr><tr><td>AS 209</td><td>121</td><td>K5</td><td>1.41<sup>+0.83</sup><sub>-0.52</sub></td><td>0.83<sup>+0.24</sup><sub>-0.23</sub></td><td>229 ± 23</td><td>267</td><td>1; 12; 10; 10; 11</td></tr><tr><td>CI Tau</td><td>160</td><td>K5.5</td><td>~0.81</td><td>0.90<sup>+0.02</sup><sub>-0.02</sub></td><td>~161</td><td>231</td><td>1; 13; 14; 4; 15</td></tr><tr><td>CM Cha</td><td>192</td><td>K7</td><td>0.86<sup>+0.11</sup><sub>-0.09</sub></td><td>0.81<sup>+0.17</sup><sub>-0.18</sub></td><td>39 ± 0.20</td><td>231</td><td>1; 16; 17; 18; 18</td></tr><tr><td>CQ Tau</td><td>149</td><td>F5</td><td>~10.70</td><td>1.51<sup>+0.07</sup><sub>-0.07</sub></td><td>258 ± 26</td><td>267</td><td>1; 19; 7; 4; 5</td></tr><tr><td>CW Tau</td><td>132</td><td>K3.0</td><td>2.40<sup>+0.69</sup><sub>-0.66</sub></td><td>0.64<sup>+0.01</sup><sub>-0.01</sub></td><td>160 ± 27</td><td>337</td><td>1; 13; 20; 4; 21</td></tr><tr><td>CX Tau</td><td>127</td><td>M2.5</td><td>0.37<sup>+0.11</sup><sub>-0.11</sub></td><td>0.38<sup>+0.02</sup><sub>-0.02</sub></td><td>25 ± 7.6</td><td>337</td><td>1; 13; 20; 4; 21</td></tr><tr><td>DE Tau</td><td>128</td><td>M2.3</td><td>0.39<sup>+0.12</sup><sub>-0.11</sub></td><td>0.41<sup>+0.03</sup><sub>-0.03</sub></td><td>84 ± 10</td><td>337</td><td>1; 13; 20; 4; 21</td></tr><tr><td>DM Tau</td><td>144</td><td>M1</td><td>0.20<sup>+0.03</sup><sub>-0.02</sub></td><td>0.55<sup>+0.02</sup><sub>-0.02</sub></td><td>143 ± 14</td><td>267</td><td>1; 2; 3; 4; 5</td></tr><tr><td>DO Tau</td><td>139</td><td>M0.3</td><td>0.72<sup>+0.17</sup><sub>-0.21</sub></td><td>0.54<sup>+0.08</sup><sub>-0.12</sub></td><td>~89</td><td>300</td><td>1; 13; 20; 20; 15</td></tr><tr><td>DS Tau</td><td>158</td><td>M0.4</td><td>~0.25</td><td>0.83<sup>+0.02</sup><sub>-0.02</sub></td><td>40 ± 6.9</td><td>337</td><td>1; 13; 14; 4; 21</td></tr><tr><td>EX Lup</td><td>155</td><td>M0</td><td>~0.76</td><td>0.56<sup>+0.13</sup><sub>-0.13</sub></td><td>20 ± 0.30</td><td>225</td><td>1; 22; 23; 23; 24</td></tr><tr><td>FP Tau</td><td>127</td><td>M2.6</td><td>0.36<sup>+0.10</sup><sub>-0.12</sub></td><td>0.40<sup>+0.01</sup><sub>-0.01</sub></td><td>11 ± 0.02</td><td>273</td><td>1; 13; 20; 25; 26</td></tr><tr><td>GM Aur</td><td>158</td><td>K3</td><td>~1.20</td><td>1.14<sup>+0.02</sup><sub>-0.02</sub></td><td>251 ± 25</td><td>267</td><td>1; 21; 7; 4; 5</td></tr><tr><td>HD 100453</td><td>104</td><td>A9</td><td>~6.00</td><td>~1.54</td><td>149 ± 3.0</td><td>234</td><td>1; 27; 7; 7; 28</td></tr><tr><td>HD 100546</td><td>108</td><td>B9</td><td>~24.70</td><td>~2.30</td><td>~492</td><td>231</td><td>1; 29; 7; 7; 30</td></tr><tr><td>HD 142527</td><td>159</td><td>F6</td><td>~9.90</td><td>~1.70</td><td>1030 ± 100</td><td>267</td><td>1; 31; 7; 7; 11</td></tr><tr><td>HD 143006</td><td>167</td><td>G7</td><td>3.80<sup>+1.57</sup><sub>-1.11</sub></td><td>1.78<sup>+0.22</sup><sub>-0.30</sub></td><td>~127</td><td>231</td><td>1; 9; 10; 10; 15</td></tr><tr><td>HD 163296</td><td>101</td><td>A1</td><td>16.98<sup>+16.90</sup><sub>-8.47</sub></td><td>2.04<sup>+0.25</sup><sub>-0.14</sub></td><td>~255</td><td>300</td><td>1; 31; 10; 10; 15</td></tr><tr><td>HD 169142</td><td>115</td><td>F3</td><td>~6.10</td><td>~1.51</td><td>226 ± 23</td><td>233</td><td>1; 32; 7; 7; 33</td></tr><tr><td>Hn 24 A</td><td>197</td><td>M0</td><td>1.28<sup>+0.19</sup><sub>-0.16</sub></td><td>0.57<sup>+0.15</sup><sub>-0.09</sub></td><td>10 ± 0.20</td><td>231</td><td>34; 35; 17; 18; 18</td></tr><tr><td>IM Lup</td><td>156</td><td>K5</td><td>2.57<sup>+1.50</sup><sub>-0.95</sub></td><td>0.89<sup>+0.21</sup><sub>-0.23</sub></td><td>331 ± 33</td><td>267</td><td>1; 36; 10; 10; 11</td></tr><tr><td>J0432+1827</td><td>146</td><td>M5.0</td><td>0.09<sup>+0.12</sup><sub>-0.06</sub></td><td>0.19<sup>+0.01</sup><sub>-0.01</sub></td><td>29 ± 0.08</td><td>273</td><td>1; 13; 20; 25; 26</td></tr><tr><td>J1100-7619</td><td>193</td><td>M4</td><td>0.10<sup>+0.01</sup><sub>-0.01</sub></td><td>0.46<sup>+0.06</sup><sub>-0.06</sub></td><td>35 ± 0.08</td><td>273</td><td>1; 37; 38; 25; 26</td></tr><tr><td>J1545-3417</td><td>155</td><td>M5.5</td><td>0.06<sup>+0.01</sup><sub>-0.01</sub></td><td>0.15<sup>+0.01</sup><sub>-0.01</sub></td><td>24 ± 0.28</td><td>273</td><td>1; 37; 38; 38; 26</td></tr><tr><td>J15560210-3655282</td><td>158</td><td>M1</td><td>~0.26</td><td>0.50<sup>+0.14</sup><sub>-0.14</sub></td><td>24 ± 0.20</td><td>225</td><td>1; 22; 23; 23; 24</td></tr><tr><td>J16000236-4222145</td><td>160</td><td>M4</td><td>0.17<sup>+0.04</sup><sub>-0.03</sub></td><td>0.19<sup>+0.01</sup><sub>-0.05</sub></td><td>50 ± 0.38</td><td>225</td><td>1; 36; 38; 38; 24</td></tr><tr><td>J1604-2130</td><td>145</td><td>K2</td><td>~0.60</td><td>~1.10</td><td>~956</td><td>300</td><td>1; 39; 7; 7; 15</td></tr><tr><td>J16070384-3911113</td><td>152</td><td>M4.5</td><td>0.10<sup>+0.02</sup><sub>-0.01</sub></td><td>0.22<sup>+0.03</sup><sub>-0.01</sub></td><td>1.0 ± 0.3</td><td>225</td><td>1; 36; 38; 38; 24</td></tr><tr><td>J16070854-3914075</td><td>175</td><td>M</td><td>0.14<sup>+0.03</sup><sub>-0.02</sub></td><td>0.22<sup>+0.02</sup><sub>-0.01</sub></td><td>92 ± 1.5</td><td>337</td><td>1; 40; 38; 38; 41</td></tr><tr><td>J16085324-3914401</td><td>163</td><td>M3</td><td>~0.21</td><td>~0.31</td><td>8.0 ± 0.1</td><td>225</td><td>1; 42; 42; 42; 24</td></tr><tr><td>J1609-1908</td><td>137</td><td>K9</td><td>0.35<sup>+0.22</sup><sub>-0.14</sub></td><td>0.65<sup>+0.31</sup><sub>-0.18</sub></td><td>~237</td><td>300</td><td>1; 43; 38; 38; 15</td></tr><tr><td>J16090141-3925119</td><td>159</td><td>M4</td><td>0.09<sup>+0.01</sup><sub>-0.01</sub></td><td>0.26<sup>+0.03</sup><sub>-0.02</sub></td><td>5.5 ± 0.4</td><td>225</td><td>1; 36; 38; 38; 24</td></tr><tr><td>J16102955-3922144</td><td>160</td><td>M4.5</td><td>0.10<sup>+0.02</sup><sub>-0.01</sub></td><td>0.22<sup>+0.03</sup><sub>-0.01</sub></td><td>2.9 ± 0.2</td><td>225</td><td>1; 36; 38; 38; 24</td></tr><tr><td>J1612-1859</td><td>135</td><td>M0.5</td><td>~0.29</td><td>~0.56</td><td>~69</td><td>231</td><td>1; 43; 44; 44; 15</td></tr><tr><td>J1614-1906</td><td>140</td><td>M0</td><td>0.47<sup>+0.40</sup><sub>-0.22</sub></td><td>0.62<sup>+0.32</sup><sub>-0.17</sub></td><td>~22</td><td>231</td><td>1; 43; 38; 38; 15</td></tr><tr><td>LkCa 15</td><td>157</td><td>K5</td><td>~1.00</td><td>1.14<sup>+0.03</sup><sub>-0.03</sub></td><td>214 ± 21</td><td>267</td><td>1; 39; 7; 4; 5</td></tr><tr><td>Lynds 1641 ID 1001</td><td>400</td><td>K6</td><td>~0.72</td><td>0.90<sup>+0.20</sup><sub>-0.20</sub></td><td>6.8 ± 0.1</td><td>225</td><td>45; 46; 46; 46; 47</td></tr><tr><td>Lynds 1641 ID 1006</td><td>385</td><td>M0.5</td><td>~0.45</td><td>0.45<sup>+0.08</sup><sub>-0.08</sub></td><td>5.0 ± 0.1</td><td>225</td><td>45; 48; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 1007</td><td>369</td><td>M1</td><td>~0.35</td><td>0.51<sup>+0.09</sup><sub>-0.08</sub></td><td>10 ± 0.10</td><td>225</td><td>45; 50; 51; 51; 47</td></tr><tr><td>Lynds 1641 ID 1039</td><td>380</td><td>M0</td><td>~1.50</td><td>0.48<sup>+0.06</sup><sub>-0.06</sub></td><td>21 ± 0.10</td><td>225</td><td>45; 52; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 1086</td><td>428</td><td>M0</td><td>~0.66</td><td>0.49<sup>+0.07</sup><sub>-0.07</sub></td><td>18 ± 0.10</td><td>225</td><td>53; 52; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 269</td><td>600</td><td>M3</td><td>~0.47</td><td>0.29<sup>+0.05</sup><sub>-0.05</sub></td><td>13 ± 0.10</td><td>225</td><td>45; 54; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 307</td><td>440</td><td>K7.5</td><td>~0.57</td><td>0.67<sup>+0.19</sup><sub>-0.19</sub></td><td>23 ± 0.10</td><td>225</td><td>45; 46; 46; 46; 47</td></tr><tr><td>Lynds 1641 ID 378</td><td>420</td><td>K7</td><td>~1.95</td><td>0.62<sup>+0.08</sup><sub>-0.08</sub></td><td>29 ± 0.10</td><td>225</td><td>45; 55; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 399</td><td>396</td><td>K5</td><td>~5.83</td><td>0.85<sup>+0.13</sup><sub>-0.13</sub></td><td>36 ± 0.10</td><td>225</td><td>45; 55; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 402</td><td>310</td><td>M1.5</td><td>~1.05</td><td>0.37<sup>+0.05</sup><sub>-0.05</sub></td><td>5.1 ± 0.1</td><td>225</td><td>45; 56; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 429</td><td>390</td><td>M2</td><td>~1.72</td><td>0.28<sup>+0.10</sup><sub>-0.04</sub></td><td>7.2 ± 0.1</td><td>225</td><td>45; 50; 51; 51; 47</td></tr><tr><td>Lynds 1641 ID 477</td><td>428</td><td>M2.5</td><td>~0.57</td><td>0.32<sup>+0.20</sup><sub>-0.09</sub></td><td>2.6 ± 0.1</td><td>225</td><td>53; 50; 51; 51; 47</td></tr><tr><td>Lynds 1641 ID 483</td><td>400</td><td>G1</td><td>~8.11</td><td>1.95<sup>+0.16</sup><sub>-0.16</sub></td><td>9.2 ± 0.1</td><td>225</td><td>45; 50; 51; 51; 47</td></tr><tr><td>Lynds 1641 ID 487</td><td>428</td><td>M0.5</td><td>~1.13</td><td>0.54<sup>+0.23</sup><sub>-0.23</sub></td><td>2.9 ± 0.1</td><td>225</td><td>53; 46; 46; 46; 47</td></tr><tr><td>Lynds 1641 ID 525</td><td>450</td><td>M1</td><td>~1.13</td><td>0.40<sup>+0.06</sup><sub>-0.06</sub></td><td>11 ± 0.10</td><td>225</td><td>45; 57; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 556</td><td>410</td><td>M1.5</td><td>~0.33</td><td>0.42<sup>+0.04</sup><sub>-0.04</sub></td><td>8.9 ± 0.1</td><td>225</td><td>45; 58; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 633</td><td>390</td><td>M3</td><td>~1.07</td><td>0.29<sup>+0.04</sup><sub>-0.04</sub></td><td>1.2 ± 0.1</td><td>225</td><td>45; 52; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 761</td><td>440</td><td>M0</td><td>~1.04</td><td>0.48<sup>+0.07</sup><sub>-0.07</sub></td><td>9.1 ± 0.1</td><td>225</td><td>45; 59; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 762</td><td>390</td><td>M2.5</td><td>~0.85</td><td>0.31<sup>+0.04</sup><sub>-0.04</sub></td><td>21 ± 0.10</td><td>225</td><td>45; 52; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 848</td><td>380</td><td>K7.5</td><td>~2.05</td><td>0.61<sup>+0.08</sup><sub>-0.08</sub></td><td>5.8 ± 0.1</td><td>225</td><td>45; 52; 49; 49; 47</td></tr><tr><td>Lynds 1641 ID 971</td><td>387</td><td>G9</td><td>~4.47</td><td>1.88<sup>+0.09</sup><sub>-0.09</sub></td><td>6.5 ± 0.1</td><td>225</td><td>45; 46; 46; 46; 47</td></tr><tr><td>MWC 480</td><td>156</td><td>A5</td><td>~19.60</td><td>2.11<sup>+0.06</sup><sub>-0.06</sub></td><td>427 ± 43</td><td>267</td><td>1; 21; 7; 4; 5</td></tr><tr><td>MY Lup</td><td>157</td><td>K0</td><td>0.87<sup>+0.51</sup><sub>-0.32</sub></td><td>1.23<sup>+0.09</sup><sub>-0.32</sub></td><td>~79</td><td>239</td><td>1; 36; 10; 10; 60</td></tr><tr><td>PDS 70</td><td>112</td><td>K5</td><td>~1.20</td><td>~0.83</td><td>~58</td><td>225</td><td>1; 6; 7; 7; 61</td></tr><tr><td>RU Lup</td><td>158</td><td>K7</td><td>1.45<sup>+0.85</sup><sub>-0.53</sub></td><td>0.63<sup>+0.20</sup><sub>-0.14</sub></td><td>~203</td><td>239</td><td>1; 36; 10; 10; 60</td></tr><tr><td>RW Aur A</td><td>139</td><td>K0</td><td>4.37<sup>+1.66</sup><sub>-0.65</sub></td><td>1.58<sup>+0.32</sup><sub>-0.14</sub></td><td>56 ± 7.8</td><td>337</td><td>62; 13; 20; 20; 21</td></tr><tr><td>RW Aur B</td><td>139</td><td>K6.5</td><td>1.00<sup>+0.26</sup><sub>-0.07</sub></td><td>0.66<sup>+0.12</sup><sub>-0.03</sub></td><td>13 ± 2.9</td><td>337</td><td>62; 13; 20; 20; 21</td></tr></tbody></table>
<table><caption>**Table 6**
(Continued)</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>Distance<br>(pc)<br>(2)</th><th>Spectral<br>Type<br>(3)</th><th>L<sub>*</sub><br>(L<sub>☉</sub>)<br>(4)</th><th>M<sub>*</sub><br>(M<sub>☉</sub>)<br>(5)</th><th>F<sub>ν</sub><br>(mJy)<br>(6)</th><th>ν<br>(GHz)<br>(7)</th><th>References<br>(Distance; Spectral Type; L<sub>*</sub>; M<sub>*</sub>; F<sub>ν</sub>)<br>(8)</th></tr></thead><tbody><tr><td>RY Lup</td><td>153</td><td>K2</td><td>1.91<sup>+1.11</sup><sub>-0.70</sub></td><td>1.38<sup>+0.17</sup><sub>-0.23</sub></td><td>86 ± 0.68</td><td>225</td><td>1; 36; 63; 63; 24</td></tr><tr><td>SAO 206462</td><td>135</td><td>F4</td><td>~8.80</td><td>~1.70</td><td>269 ± 27</td><td>267</td><td>1; 31; 7; 7; 11</td></tr><tr><td>SU Aur</td><td>157</td><td>G8</td><td>~10.70</td><td>~2.25</td><td>71 ± 8.6</td><td>337</td><td>1; 21; 7; 7; 21</td></tr><tr><td>Sz 111</td><td>158</td><td>M1</td><td>0.21<sup>+0.12</sup><sub>-0.08</sub></td><td>0.54<sup>+0.11</sup><sub>-0.16</sub></td><td>60 ± 0.49</td><td>225</td><td>1; 64; 63; 63; 24</td></tr><tr><td>Sz 114</td><td>157</td><td>M5</td><td>0.20<sup>+0.12</sup><sub>-0.08</sub></td><td>0.17<sup>+0.04</sup><sub>-0.03</sub></td><td>~49</td><td>239</td><td>1; 36; 10; 10; 60</td></tr><tr><td>Sz 118</td><td>161</td><td>K5</td><td>~0.72</td><td>~1.10</td><td>23 ± 0.26</td><td>225</td><td>1; 36; 42; 42; 24</td></tr><tr><td>Sz 129</td><td>160</td><td>K7</td><td>~0.43</td><td>~0.82</td><td>~86</td><td>239</td><td>1; 42; 42; 42; 60</td></tr><tr><td>Sz 130</td><td>159</td><td>M2</td><td>~0.18</td><td>~0.37</td><td>2.0 ± 0.2</td><td>225</td><td>1; 42; 42; 42; 24</td></tr><tr><td>Sz 133</td><td>116</td><td>K2</td><td>1.05<sup>+0.90</sup><sub>-0.48</sub></td><td>0.98<sup>+0.12</sup><sub>-0.25</sub></td><td>27 ± 0.24</td><td>225</td><td>1; 36; 63; 63; 24</td></tr><tr><td>Sz 46</td><td>196</td><td>M1</td><td>0.40<sup>+0.05</sup><sub>-0.04</sub></td><td>0.54<sup>+0.12</sup><sub>-0.12</sub></td><td>2.8 ± 0.2</td><td>231</td><td>34; 35; 17; 18; 18</td></tr><tr><td>Sz 49</td><td>194</td><td>M0.5</td><td>0.24<sup>+0.04</sup><sub>-0.03</sub></td><td>0.57<sup>+0.13</sup><sub>-0.11</sub></td><td>11 ± 0.30</td><td>231</td><td>34; 35; 17; 18; 18</td></tr><tr><td>Sz 53</td><td>198</td><td>M1</td><td>0.40<sup>+0.05</sup><sub>-0.04</sub></td><td>0.54<sup>+0.12</sup><sub>-0.12</sub></td><td>4.8 ± 0.2</td><td>231</td><td>34; 35; 17; 18; 18</td></tr><tr><td>Sz 58</td><td>194</td><td>K5</td><td>0.82<sup>+0.14</sup><sub>-0.12</sub></td><td>1.01<sup>+0.13</sup><sub>-0.14</sub></td><td>61 ± 0.30</td><td>231</td><td>34; 35; 17; 18; 18</td></tr><tr><td>Sz 61</td><td>199</td><td>K5</td><td>1.47<sup>+0.18</sup><sub>-0.16</sub></td><td>1.07<sup>+0.19</sup><sub>-0.21</sub></td><td>45 ± 0.20</td><td>231</td><td>34; 35; 17; 18; 18</td></tr><tr><td>Sz 63</td><td>200</td><td>M3</td><td>0.29<sup>+0.04</sup><sub>-0.04</sub></td><td>0.33<sup>+0.12</sup><sub>-0.09</sub></td><td>20 ± 0.20</td><td>231</td><td>34; 35; 17; 18; 18</td></tr><tr><td>Sz 65</td><td>153</td><td>K7</td><td>~0.89</td><td>~0.75</td><td>30 ± 0.20</td><td>225</td><td>1; 42; 42; 42; 24</td></tr><tr><td>Sz 68</td><td>153</td><td>K2</td><td>~5.42</td><td>~2.15</td><td>66 ± 0.20</td><td>225</td><td>1; 42; 42; 42; 24</td></tr><tr><td>Sz 69</td><td>153</td><td>M4.5</td><td>0.09<sup>+0.05</sup><sub>-0.03</sub></td><td>0.19<sup>+0.04</sup><sub>-0.04</sub></td><td>9.4 ± 0.0</td><td>273</td><td>1; 36; 63; 63; 26</td></tr><tr><td>Sz 71</td><td>155</td><td>M1.5</td><td>0.33<sup>+0.19</sup><sub>-0.12</sub></td><td>0.46<sup>+0.12</sup><sub>-0.15</sub></td><td>69 ± 0.31</td><td>225</td><td>1; 36; 63; 63; 24</td></tr><tr><td>Sz 75</td><td>154</td><td>K6</td><td>1.45<sup>+0.79</sup><sub>-0.51</sub></td><td>0.78<sup>+0.18</sup><sub>-0.20</sub></td><td>34 ± 0.20</td><td>225</td><td>1; 36; 63; 63; 24</td></tr><tr><td>Sz 76</td><td>156</td><td>M4</td><td>~0.18</td><td>0.23<sup>+0.04</sup><sub>-0.04</sub></td><td>4.7 ± 0.3</td><td>225</td><td>1; 22; 23; 23; 24</td></tr><tr><td>Sz 82</td><td>156</td><td>K5</td><td>2.57<sup>+1.41</sup><sub>-0.91</sub></td><td>0.89<sup>+0.21</sup><sub>-0.23</sub></td><td>205 ± 6.0</td><td>225</td><td>1; 36; 63; 63; 24</td></tr><tr><td>Sz 83</td><td>158</td><td>K7</td><td>1.45<sup>+0.85</sup><sub>-0.53</sub></td><td>0.63<sup>+0.20</sup><sub>-0.14</sub></td><td>167 ± 0.34</td><td>225</td><td>1; 64; 63; 63; 24</td></tr><tr><td>Sz 84</td><td>156</td><td>M5</td><td>0.12<sup>+0.07</sup><sub>-0.05</sub></td><td>0.15<sup>+0.03</sup><sub>-0.03</sub></td><td>13 ± 0.21</td><td>225</td><td>1; 64; 63; 63; 24</td></tr><tr><td>Sz 90</td><td>160</td><td>K7</td><td>~0.42</td><td>~0.80</td><td>8.7 ± 0.2</td><td>225</td><td>1; 42; 42; 42; 24</td></tr><tr><td>Sz 91</td><td>159</td><td>M0.5</td><td>~0.30</td><td>~0.48</td><td>9.5 ± 0.4</td><td>225</td><td>1; 39; 7; 7; 24</td></tr><tr><td>Sz 98</td><td>156</td><td>K7</td><td>1.10<sup>+0.20</sup><sub>-0.30</sub></td><td>~0.60</td><td>103 ± 0.63</td><td>225</td><td>1; 36; 3; 3; 24</td></tr><tr><td>TW 3A</td><td>37</td><td>M4</td><td>0.10<sup>+0.03</sup><sub>-0.03</sub></td><td>0.29<sup>+0.01</sup><sub>-0.01</sub></td><td>~37</td><td>226</td><td>1; 65; 66; 66; 67</td></tr><tr><td>TW Hya</td><td>60</td><td>M2</td><td>~0.40</td><td>~1.00</td><td>737 ± 3.0</td><td>266</td><td>1; 39; 7; 7; 68</td></tr><tr><td>V1094 Sco</td><td>155</td><td>K6</td><td>~1.15</td><td>~0.86<sup>+0.18</sup><sub>-0.18</sub></td><td>180 ± 7.0</td><td>225</td><td>1; 22; 23; 23; 24</td></tr><tr><td>V4046 Sgr</td><td>71</td><td>K5</td><td>~0.70</td><td>~1.10</td><td>415 ± 42</td><td>267</td><td>1; 39; 7; 7; 11</td></tr></tbody></table>

**Note.** Column 1: Star+disk system from the literature sample (Section 2.4). Columns 2 and 3: Estimated distance and spectral type, respectively. Columns 4 and 5: Stellar luminosity and central stellar mass, respectively. For known multi-star systems, the total central stellar luminosity and mass are reported, while the spectral type is reported for the primary star. Where available, the stellar luminosities and stellar masses are taken from references that have calculated or updated values using parallaxes from recent studies (e.g., Gaia; Column 2). Dynamical stellar masses are used when known in the literature; otherwise, stellar masses were estimated in the literature using stellar evolutionary models. Values for which no error is given in the literature are marked with a tilde. Columns 5 and 6: Millimeter-wavelength continuum flux and corresponding continuum frequency, respectively. Column 7: Reference numbers for the distance, spectral type, stellar luminosity, stellar mass, and mm continuum flux, in that order.

**References.** (1) Gaia Collaboration et al. (2016), Gaia Collaboration et al. (2018); (2) Herbig (1977); (3) van der Marel et al. (2019); (4) Simon et al. (2019); (5) Öberg et al. (2010); (6) Wenger et al. (2000); (7) Dong et al. (2018); (8) Czekala et al. (2015); (9) Luhman & Mamajek (2012); (10) Andrews et al. (2018b); (11) Öberg et al. (2011); (12) Herbig & Bell (1988); (13) Herczeg & Hillenbrand (2014); (14) Long et al. (2018); (15) Pegues et al. (2020); (16) Spezzi et al. (2008); (17) value from Spezzi et al. (2008), scaled to distance in column 2; (18) Villenave et al. (2021); (19) Pecaut & Mamajek (2013); (20) Akeson et al. (2019); (21) Andrews et al. (2013); (22) Alcalá et al. (2014, 2017); (23) Sanchis et al. (2020); (24) Ansdell et al. (2018); (25) Pegues et al. (2021b); (26) Pegues et al. (2021a); (27) Vieira et al. (2003); (28) van der Plas et al. (2019); (29) The & Molster (1994); (30) Miley et al. (2019); (31) Fairlamb et al. (2015); (32) Murphy & Paunzen (2017); (33) Fedele et al. (2017); (34) Galli et al. (2021); (35) Spezzi et al. (2013); (36) Alcalá et al. (2017); (37) Manara et al. (2017); (38) Hendler et al. (2020); (39) van der Marel et al. (2016); (40) assumed based on stellar mass; (41) Ansdell et al. (2017); (42) Alcalá et al. (2019); (43) Barenfeld et al. (2016); (44) Pegues et al. (2020) via methodology of Andrews et al. (2018b); (45) Bailer-Jones et al. (2021); (46) Fang et al. (2013) at a distance of 450 pc; (47) Grant et al. (2021); (48) Hernández et al. (2008, 2009); (49) Kim et al. (2016) at a distance of 414 pc; (50) Fang et al. (2009); (51) Fang et al. (2009) at a distance of 450 pc; (52) Hsu et al. (2012); (53) Kounkel et al. (2017); (54) Hernández et al. (2008); (55) Allen (1995); Fang et al. (2009); Hsu et al. (2012); (56) Fang et al. (2009),citehsuetal2012; (57) Fang et al. (2013); (58) based on Allen & Davis (2008); Hsu et al. (2012); (59) Allen & Davis (2008); (60) Andrews et al. (2018b); (61) Facchini et al. (2021); (62) used in Akeson et al. (2019), based on Gaia (e.g., Gaia Collaboration et al. (2016, 2018)); (63) Andrews et al. (2018a); (64) Alcalá et al. (2014); (65) Kellogg et al. (2017); (66) scaled from value in Kellogg et al. (2017) by Czekala et al. (2021); (67) Czekala et al. (2021); (68) Kastner et al. (2015).

We refer to the original references for retrieval and discussion of all star+disk systems and their characteristics.

## A.2. Molecular Line Fluxes for the Literature Sample

Tables 7 through 16 list molecular line fluxes and upper limits for the 10 target molecular lines (Table 3) observed from

the literature disk sample (Section 2.4), in alphabetical order by molecular line. The fluxes here are written exactly as they were reported in the literature. They are scaled separately by distance and/or frequency in our figures for consistent comparison.

We refer to the original references for retrieval of all line fluxes, how the fluxes were measured, and a discussion of the star+disk systems.
<table><caption>**Table 7**
$^{12}$CO 2–1 Fluxes for the Literature Sample</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th><sup>12</sup>CO 2–1 Flux<br>(mJy km s<sup>-1</sup>)<br>(2)</th><th>Reference<br>(3)</th></tr></thead><tbody><tr><td>AA Tau</td><td>8200 ± 160 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>AK Sco</td><td>2210 ± 10 (A)</td><td>Czekala et al. (2015)</td></tr><tr><td>AS 205</td><td>19,230 ± 270 (A)</td><td>Salyk et al. (2014)</td></tr><tr><td>AS 209</td><td>7790 ± 45 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>CM Cha</td><td>670 ± 40 (A)</td><td>Villenave et al. (2021)</td></tr><tr><td>CQ Tau</td><td>3100 ± 180 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>DM Tau</td><td>14,870 ± 120 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>FP Tau</td><td>781 ± 11 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>GM Aur</td><td>19,844 ± 74 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>HD 100453</td><td>2340 ± 60 (A)</td><td>van der Plas et al. (2019)</td></tr><tr><td>HD 142527</td><td>~17,400 (A)</td><td>Perez et al. (2015)</td></tr><tr><td>HD 163296</td><td>45,246 ± 102 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>HD 169142</td><td>14,000 ± 2000 (A)</td><td>Fedele et al. (2017)</td></tr><tr><td>Hn 24 A</td><td>350 ± 40 (A)</td><td>Villenave et al. (2021)</td></tr><tr><td>IM Lup</td><td>22,342 ± 94 (S)</td><td>Öberg et al. (2021a)</td></tr><tr><td>J0432+1827</td><td>1847 ± 13 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1100-7619</td><td>1306 ± 6 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1545-3417</td><td>738 ± 10 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1609-1908</td><td>860 ± 21 (A)</td><td>Anderson et al. (2019)</td></tr><tr><td>LkCa 15</td><td>13,940 ± 150 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>Lynds 1641 ID 1001</td><td>830 ± 50 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 1006</td><td>180 ± 30 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 1007</td><td>280 ± 30 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 1039</td><td>930 ± 40 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 1086</td><td>260 ± 30 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 269</td><td>610 ± 30 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 307</td><td>370 ± 40 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 378</td><td>2930 ± 40 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 399</td><td>1320 ± 60 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 402</td><td>520 ± 40 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 429</td><td>600 ± 40 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 477</td><td>260 ± 30 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 483</td><td>180 ± 20 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 487</td><td>200 ± 200 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 525</td><td>670 ± 40 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 556</td><td>290 ± 30 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 633</td><td>230 ± 20 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 761</td><td>1010 ± 40 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 762</td><td>1570 ± 30 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 848</td><td>450 ± 40 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 971</td><td>410 ± 40 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>MWC 480</td><td>23,226 ± 63 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>PDS 70</td><td>6109 ± 26 (A)</td><td>Facchini et al. (2021)</td></tr><tr><td>RU Lup</td><td>~20,000 (A)</td><td>Huang et al. (2020a)</td></tr><tr><td>RW Aur A</td><td>3420 ± 30 (A)</td><td>Rodriguez et al. (2018)</td></tr><tr><td>RW Aur B</td><td>860 ± 30 (A)</td><td>Rodriguez et al. (2018)</td></tr><tr><td>SAO 206462</td><td>10,390 ± 210 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>Sz 46</td><td>120 ± 10 (A)</td><td>Villenave et al. (2021)</td></tr><tr><td>Sz 49</td><td>610 ± 40 (A)</td><td>Villenave et al. (2021)</td></tr><tr><td>Sz 53</td><td>840 ± 60 (A)</td><td>Villenave et al. (2021)</td></tr><tr><td>Sz 58</td><td>4010 ± 240 (A)</td><td>Villenave et al. (2021)</td></tr><tr><td>Sz 61</td><td>1310 ± 80 (A)</td><td>Villenave et al. (2021)</td></tr><tr><td>Sz 63</td><td>1040 ± 70 (A)</td><td>Villenave et al. (2021)</td></tr><tr><td>Sz 69</td><td>2001 ± 15 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>Sz 91</td><td>4932 ± 23 (A)</td><td>Canovas et al. (2016)</td></tr><tr><td>TW 3A</td><td>~526 (A)</td><td>Czekala et al. (2021)</td></tr><tr><td>TW Hya</td><td>17800 ± 267 (A)</td><td>Calahan et al. (2021)</td></tr><tr><td>V4046 Sgr</td><td>29300 ± 540 (A)</td><td>Kastner et al. (2018)</td></tr></tbody></table>

Note. Columns 1 and 2: $^{12}$CO 2–1 line fluxes for sources from the literature sample (Section 2.4). 3σ upper limits are marked with <. Fluxes from ALMA, the SMA, and the IRAM PdBI are marked with (A), (S), and (I), respectively. When fluxes from multiple instruments are available, we select the flux from the instrument with the highest spatial resolution to decrease the chance of contamination from external sources of emission. Column 3: Reference.

<table><caption>**Table 8**
$^{13}$CO 2–1 Fluxes from the Literature Sample</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th><sup>13</sup>CO 2–1 Flux<br>(mJy km s<sup>-1</sup>)<br>(2)</th><th>Reference<br>(3)</th></tr></thead><tbody><tr><td>AK Sco</td><td>540 ± 10 (A)</td><td>Czekala et al. (2015)</td></tr><tr><td>AS 205</td><td>1860 ± 40 (A)</td><td>Salyk et al. (2014)</td></tr><tr><td>AS 209</td><td>2269 ± 38 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>CX Tau</td><td>~400 (I)</td><td>Piétu et al. (2014)</td></tr><tr><td>DE Tau</td><td>~400 (I)</td><td>Piétu et al. (2014)</td></tr><tr><td>DS Tau</td><td>~500 (I)</td><td>Piétu et al. (2014)</td></tr><tr><td>EX Lup</td><td>760 ± 200 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>FP Tau</td><td>232 ± 8 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>GM Aur</td><td>5028 ± 48 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>HD 100453</td><td>900 ± 30 (A)</td><td>van der Plas et al. (2019)</td></tr><tr><td>HD 100546</td><td>~12,872 (A)</td><td>Miley et al. (2019)</td></tr><tr><td>HD 142527</td><td>~11,400 (A)</td><td>Perez et al. (2015)</td></tr><tr><td>HD 163296</td><td>15,885 ± 80 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>HD 169142</td><td>7600 ± 600 (A)</td><td>Fedele et al. (2017)</td></tr><tr><td>IM Lup</td><td>8370 ± 83 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>J0432+1827</td><td>537 ± 9 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1100-7619</td><td>417 ± 6 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1545-3417</td><td>218 ± 8 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J15560210-3655282</td><td>450 ± 100 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>J16000236-4222145</td><td>976 ± 325 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>J16070384-3911113</td><td>805 ± 117 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>J1609-1908</td><td>118 ± 26 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>J16090141-3925119</td><td>642 ± 79 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>J16102955-3922144</td><td>137 ± 38 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>J1612-1859</td><td>103 ± 20 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>J1614-1906</td><td>75 ± 22 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>Lynds 1641 ID 1039</td><td>190 ± 20 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 269</td><td>90 ± 20 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 307</td><td>210 ± 40 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 378</td><td>620 ± 40 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 399</td><td>160 ± 20 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 525</td><td>260 ± 30 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 556</td><td>240 ± 30 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 762</td><td>370 ± 30 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 971</td><td>200 ± 30 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>MWC 480</td><td>8361 ± 57 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>PDS 70</td><td>1846 ± 14 (A)</td><td>Facchini et al. (2021)</td></tr><tr><td>RU Lup</td><td>1200 ± 200 (A)</td><td>Huang et al. (2020a)</td></tr><tr><td>RW Aur A</td><td>260 ± 30 (A)</td><td>Rodriguez et al. (2018)</td></tr><tr><td>RW Aur B</td><td>100 ± 30 (A)</td><td>Rodriguez et al. (2018)</td></tr><tr><td>RY Lup</td><td>2502 ± 416 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>SU Aur</td><td>1100 ± 400 (I)</td><td>Piétu et al. (2014)</td></tr><tr><td>Sz 111</td><td>2187 ± 512 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>Sz 118</td><td>80 ± 26 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>Sz 133</td><td>282 ± 73 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>Sz 63</td><td>210 ± 30 (A)</td><td>Villenave et al. (2021)</td></tr><tr><td>Sz 69</td><td>345 ± 13 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>Sz 75</td><td>1000 ± 290 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>Sz 76</td><td>620 ± 100 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>Sz 82</td><td>5893 ± 166 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>Sz 83</td><td>1076 ± 152 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>Sz 84</td><td>489 ± 89 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>Sz 91</td><td>1097 ± 190 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>TW 3A</td><td>~86 (A)</td><td>Czekala et al. (2021)</td></tr><tr><td>TW Hya</td><td>2720 ± 180 (A)</td><td>Qi et al. (2013c)</td></tr><tr><td>V1094 Sco</td><td>13,800 ± 4300 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>V4046 Sgr</td><td>8440 ± 450 (A)</td><td>Kastner et al. (2018)</td></tr></tbody></table>

Note. Column 1: Star+disk system from the literature sample (Section 2.4). Column 2: $^{13}$CO 2–1 line fluxes for protoplanetary disks from the literature sample (Section 2.4). 3σ upper limits are marked with <. Fluxes from ALMA, the SMA, and the IRAM PdBI are marked with (A), (S), and (I), respectively. When fluxes from multiple instruments are available, we select the flux from the instrument with the highest spatial resolution to decrease the chance of contamination from external sources of emission. Only detected $^{13}$CO 2–1 line fluxes are shown from Ansdell et al. (2018). Column 3: Reference.
<table><caption>**Table 9**
C<sup>18</sup>O 2–1 Fluxes from the Literature Sample</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>C<sup>18</sup>O 2–1 Flux<br>(mJy km s<sup>-1</sup>)<br>(2)</th><th>Reference<br>(3)</th></tr></thead><tbody><tr><td>AK Sco</td><td>200 ± 10 (A)</td><td>Czekala et al. (2015)</td></tr><tr><td>AS 205</td><td>340 ± 30 (A)</td><td>Salyk et al. (2014)</td></tr><tr><td>AS 209</td><td>538 ± 27 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>CI Tau</td><td>549 ± 17 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>CW Tau</td><td>~200 (I)</td><td>Piétu et al. (2014)</td></tr><tr><td>DM Tau</td><td>998 ± 17 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>DO Tau</td><td>210 ± 12 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>FP Tau</td><td>91 ± 7 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>GM Aur</td><td>1092 ± 39 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>HD 100453</td><td>390 ± 20 (A)</td><td>van der Plas et al. (2019)</td></tr><tr><td>HD 100546</td><td>~2948 (A)</td><td>Miley et al. (2019)</td></tr><tr><td>HD 142527</td><td>~3400 (A)</td><td>Perez et al. (2015)</td></tr><tr><td>HD 143006</td><td>135 ± 10 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>HD 163296</td><td>5783 ± 51 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>HD 169142</td><td>3900 ± 500 (A)</td><td>Fedele et al. (2017)</td></tr><tr><td>IM Lup</td><td>1592 ± 60 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>J0432+1827</td><td>148 ± 9 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1100-7619</td><td>71 ± 4 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1545-3417</td><td>89 ± 10 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J16000236-4222145</td><td>52 ± 15 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>J1604-2130</td><td>1267 ± 7 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>J1609-1908</td><td>43 ± 17 (A)</td><td>Anderson et al. (2019)</td></tr><tr><td>LkCa 15</td><td>619 ± 18 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>Lynds 1641 ID 378</td><td>150 ± 30 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>Lynds 1641 ID 556</td><td>80 ± 20 (A)</td><td>Grant et al. (2021)</td></tr><tr><td>MWC 480</td><td>3017 ± 41 (A)</td><td>Öberg et al. (2021a)</td></tr><tr><td>PDS 70</td><td>465 ± 9 (A)</td><td>Facchini et al. (2021)</td></tr><tr><td>RU Lup</td><td>340 ± 30 (A)</td><td>Huang et al. (2020a)</td></tr><tr><td>RW Aur A</td><td>90 ± 30 (A)</td><td>Rodriguez et al. (2018)</td></tr><tr><td>RW Aur B</td><td>&lt;90 (A)</td><td>Rodriguez et al. (2018)</td></tr><tr><td>RY Lup</td><td>765 ± 67 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>Sz 111</td><td>586 ± 83 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>Sz 69</td><td>38 ± 7 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>Sz 75</td><td>120 ± 30 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>Sz 82</td><td>1325 ± 63 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>Sz 83</td><td>543 ± 111 (A)</td><td>Ansdell et al. (2018)</td></tr><tr><td>TW Hya</td><td>570 ± 85 (A)</td><td>Calahan et al. (2021)</td></tr><tr><td>V4046 Sgr</td><td>1184 ± 18 (A)</td><td>Pegues et al. (2020)</td></tr></tbody></table>

**Note.** Column 1: Star+disk system from the literature sample (Section 2.4). Column 2: C<sup>18</sup>O 2–1 line fluxes for protoplanetary disks from the literature sample (Section 2.4). 3σ upper limits are marked with <. Fluxes from ALMA, the SMA, and the IRAM PdBI are marked with (A), (S), and (I), respectively. When fluxes from multiple instruments are available, we select the flux from the instrument with the highest spatial resolution to decrease the chance of contamination from external sources of emission. Only detected C<sup>18</sup>O 2–1 fluxes are shown from Ansdell et al. (2018). Column 3: Reference.
<table><caption>**Table 10**
C₂H 3–2 Fluxes from the Literature Sample</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>C<sub>2</sub>H 3–2 Flux<br>(mJy km s<sup>-1</sup>)<br>(2)</th><th>Reference<br>(3)</th></tr></thead><tbody><tr><td>AS 209</td><td>2709 ± 290 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>CI Tau</td><td>1041 ± 110 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>DM Tau</td><td>2013 ± 200 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>DO Tau</td><td>&lt;62 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>FP Tau</td><td>140 ± 18 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>GM Aur</td><td>801 ± 141 (A)</td><td>Guzmán et al. (2021)</td></tr><tr><td>HD 143006</td><td>435 ± 56 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>HD 163296</td><td>4396 ± 440 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>IM Lup</td><td>1450 ± 170 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>J0432+1827</td><td>288 ± 19 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1100-7619</td><td>1180 ± 17 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1545-3417</td><td>61 ± 12 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1604-2130</td><td>2632 ± 270 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>J16070384-3911113</td><td>&lt;16 (A)</td><td>Miotello et al. (2019)</td></tr><tr><td>J16070854-3914075</td><td>647 ± 55 (A)</td><td>Miotello et al. (2019)</td></tr><tr><td>J1609-1908</td><td>563 ± 70 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>J16090141-3925119</td><td>200 ± 46 (A)</td><td>Miotello et al. (2019)</td></tr><tr><td>J1612-1859</td><td>&lt;97 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>J1614-1906</td><td>&lt;100 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>LkCa 15</td><td>2435 ± 250 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>MWC 480</td><td>1846 ± 190 (A)</td><td>Bergner et al. (2019)</td></tr><tr><td>MY Lup</td><td>284 ± 21 (A)</td><td>Miotello et al. (2019)</td></tr><tr><td>PDS 70</td><td>1930 ± 25 (A)</td><td>Facchini et al. (2021)</td></tr><tr><td>Sz 114</td><td>383 ± 32 (A)</td><td>Miotello et al. (2019)</td></tr><tr><td>Sz 129</td><td>107 ± 16 (A)</td><td>Miotello et al. (2019)</td></tr><tr><td>Sz 68</td><td>&lt;5 (A)</td><td>Miotello et al. (2019)</td></tr><tr><td>Sz 69</td><td>28 ± 8 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>Sz 71</td><td>1101 ± 46 (A)</td><td>Miotello et al. (2019)</td></tr><tr><td>Sz 98</td><td>1101 ± 57 (A)</td><td>Miotello et al. (2019)</td></tr><tr><td>TW Hya</td><td>6990 ± 60 (S)</td><td>Kastner et al. (2015)</td></tr><tr><td>V4046 Sgr</td><td>2962 ± 300 (A)</td><td>Bergner et al. (2019)</td></tr></tbody></table>

**Note.** Column 1: Star+disk system from the literature sample (Section 2.4). Column 2: C₂H 3–2 line fluxes for protoplanetary disks from the literature sample (Section 2.4). 3σ upper limits are marked with <. Fluxes from ALMA, the SMA, and the IRAM PdBI are marked with (A), (S), and (I), respectively. When fluxes from multiple instruments are available, we select the flux from the instrument with the highest spatial resolution to decrease the chance of contamination from external sources of emission. Column 3: Reference.

<table><caption>**Table 11**
CS 5–4 Fluxes from the Literature Sample</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>CS 5–4 Flux<br>(mJy km s<sup>-1</sup>)<br>(2)</th><th>Reference<br>(3)</th></tr></thead><tbody><tr><td>CI Tau</td><td>756 ± 17 (A)</td><td>Le Gal et al. (2019)</td></tr><tr><td>DM Tau</td><td>388 ± 9 (A)</td><td>Le Gal et al. (2019)</td></tr><tr><td>DO Tau</td><td>159 ± 6 (A)</td><td>Le Gal et al. (2019)</td></tr><tr><td>LkCa 15</td><td>1050 ± 14 (A)</td><td>Le Gal et al. (2019)</td></tr><tr><td>MWC 480</td><td>536 ± 19 (A)</td><td>Le Gal et al. (2019)</td></tr><tr><td>PDS 70</td><td>450 ± 6 (A)</td><td>Facchini et al. (2021)</td></tr></tbody></table>

**Note.** Column 1: Star+disk system from the literature sample (Section 2.4). Column 2: C₂H 3–2 line fluxes for protoplanetary disks from the literature sample (Section 2.4). 3σ upper limits are marked with <. Fluxes from ALMA, the SMA, and the IRAM PdBI are marked with (A), (S), and (I), respectively. When fluxes from multiple instruments are available, we select the flux from the instrument with the highest spatial resolution to decrease the chance of contamination from external sources of emission. Column 3: Reference.
<table><caption>**Table 12**

DCN 3–2 Fluxes from the Literature Sample</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>DCN 3–2 Flux<br>(mJy km s<sup>-1</sup>)<br>(2)</th><th>Reference<br>(3)</th></tr></thead><tbody><tr><td>AA Tau</td><td>&lt;270 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>AS 205</td><td>&lt;270 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>AS 209</td><td>340 ± 30 (A)</td><td>Huang et al. (2017)</td></tr><tr><td>CI Tau</td><td>27 ± 10 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>CQ Tau</td><td>&lt;255 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>DM Tau</td><td>&lt;46 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>DO Tau</td><td>&lt;29 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>FP Tau</td><td>&lt;6 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>GM Aur</td><td>37 ± 6 (A)</td><td>Cataldi et al. (2021)</td></tr><tr><td>HD 142527</td><td>&lt;390 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>HD 143006</td><td>82 ± 15 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>HD 163296</td><td>120 ± 20 (A)</td><td>Huang et al. (2017)</td></tr><tr><td>IM Lup</td><td>90 ± 13 (A)</td><td>Huang et al. (2017)</td></tr><tr><td>J0432+1827</td><td>&lt;18 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1100-7619</td><td>15 ± 3 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1545-3417</td><td>&lt;14 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1604-2130</td><td>295 ± 31 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>J1609-1908</td><td>67 ± 18 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>J1612-1859</td><td>&lt;36 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>J1614-1906</td><td>&lt;51 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>LkCa 15</td><td>280 ± 30 (A)</td><td>Huang et al. (2017)</td></tr><tr><td>MWC 480</td><td>70 ± 20 (A)</td><td>Huang et al. (2017)</td></tr><tr><td>PDS 70</td><td>209 ± 11 (A)</td><td>Facchini et al. (2021)</td></tr><tr><td>SAO 206462</td><td>&lt;435 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>Sz 69</td><td>&lt;9 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>TW Hya</td><td>641 ± 20 (A)</td><td>Öberg et al. (2021b)</td></tr><tr><td>V4046 Sgr</td><td>680 ± 210 (A)</td><td>Kastner et al. (2018)</td></tr></tbody></table>

**Note.** Column 1: Star+disk system from the literature sample (Section 2.4). Column 2: DCN 3–2 line fluxes for protoplanetary disks from the literature sample (Section 2.4). 3σ upper limits are marked with <. Fluxes from ALMA, the SMA, and the IRAM PdBI are marked with (A), (S), and (I), respectively. When fluxes from multiple instruments are available, we select the flux from the instrument with the highest spatial resolution to decrease the chance of contamination from external sources of emission. Column 3: Reference.

<table><caption>**Table 13**

DCO$^{+}$ 3–2 Fluxes from the Literature Sample</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>DCO<sup>+</sup> 3–2 Flux<br>(mJy km s<sup>-1</sup>)<br>(2)</th><th>Reference<br>(3)</th></tr></thead><tbody><tr><td>AA Tau</td><td>&lt;270 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>AS 205</td><td>&lt;300 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>AS 209</td><td>480 ± 30 (A)</td><td>Huang et al. (2017)</td></tr><tr><td>CQ Tau</td><td>&lt;345 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>DM Tau</td><td>710 ± 40 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>GM Aur</td><td>&lt;225 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>HD 142527</td><td>&lt;405 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>HD 163296</td><td>1290 ± 40 (A)</td><td>Huang et al. (2017)</td></tr><tr><td>HD 169142</td><td>730 ± 73 (A)</td><td>Carney et al. (2018)</td></tr><tr><td>IM Lup</td><td>490 ± 20 (A)</td><td>Huang et al. (2017)</td></tr><tr><td>LkCa 15</td><td>400 ± 30 (A)</td><td>Huang et al. (2017)</td></tr><tr><td>MWC 480</td><td>420 ± 30 (A)</td><td>Huang et al. (2017)</td></tr><tr><td>RU Lup</td><td>54 ± 16 (A)</td><td>Huang et al. (2020a)</td></tr><tr><td>SAO 206462</td><td>&lt;510 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>TW Hya</td><td>1904 ± 19 (A)</td><td>Öberg et al. (2021b)</td></tr><tr><td>V4046 Sgr</td><td>510 ± 20 (A)</td><td>Kastner et al. (2018)</td></tr></tbody></table>

**Note.** Column 1: Star+disk system from the literature sample (Section 2.4). Column 2: DCO$^{+}$ 3–2 line fluxes for protoplanetary disks from the literature sample (Section 2.4). 3σ upper limits are marked with <. Fluxes from ALMA, the SMA, and the IRAM PdBI are marked with (A), (S), and (I), respectively. When fluxes from multiple instruments are available, we select the flux from the instrument with the highest spatial resolution to decrease the chance of contamination from external sources of emission. Column 3: Reference.
<table><caption>**Table 14**
H₂CO 3–2 Fluxes from the Literature Sample</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>H<sub>2</sub>CO 3–2 Flux<br>(mJy km s<sup>-1</sup>)<br>(2)</th><th>Reference<br>(3)</th></tr></thead><tbody><tr><td>AA Tau</td><td>160 ± 80 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>AS 205</td><td>&lt;240 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>AS 209</td><td>292 ± 28 (A)</td><td>Guzmán et al. (2021)</td></tr><tr><td>CI Tau</td><td>420 ± 12 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>CQ Tau</td><td>&lt;315 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>DM Tau</td><td>462 ± 17 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>DO Tau</td><td>85 ± 17 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>FP Tau</td><td>&lt;15 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>GM Aur</td><td>900 ± 57 (A)</td><td>Guzmán et al. (2021)</td></tr><tr><td>HD 142527</td><td>270 ± 110 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>HD 143006</td><td>161 ± 9 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>HD 163296</td><td>813 ± 78 (A)</td><td>Guzmán et al. (2021)</td></tr><tr><td>IM Lup</td><td>752 ± 72 (A)</td><td>Guzmán et al. (2021)</td></tr><tr><td>J0432+1827</td><td>80 ± 8 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1100-7619</td><td>100 ± 4 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1545-3417</td><td>&lt;14 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1604-2130</td><td>821 ± 6 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>J1609-1908</td><td>43 ± 13 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>J1612-1859</td><td>&lt;26 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>J1614-1906</td><td>&lt;30 (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>LkCa 15</td><td>662 ± 24* (A)</td><td>Pegues et al. (2020)</td></tr><tr><td>MWC 480</td><td>144 ± 39 (A)</td><td>Guzmán et al. (2021)</td></tr><tr><td>PDS 70</td><td>710 ± 7 (A)</td><td>Facchini et al. (2021)</td></tr><tr><td>SAO 206462</td><td>&lt;450 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>Sz 69</td><td>&lt;11 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>TW Hya</td><td>283 ± 4 (A)</td><td>Terwisscha van Scheltinga et al. (2021)</td></tr><tr><td>V4046 Sgr</td><td>1218 ± 40* (A)</td><td>Pegues et al. (2020)</td></tr></tbody></table>

**Note.** Column 1: Star+disk system from the literature sample (Section 2.4). Column 2: H₂CO 3–2 line fluxes for protoplanetary disks from the literature sample (Section 2.4). 3σ upper limits are marked with <. Fluxes from ALMA, the SMA, and the IRAM PdBI are marked with (A), (S), and (I), respectively. When fluxes from multiple instruments are available, we select the flux from the instrument with the highest spatial resolution to decrease the chance of contamination from external sources of emission. H₂CO 3–2 fluxes that are actually H₂CO 4–3 fluxes used as proxies are marked with a ‘*’. Column 3: Reference.

<table><caption>**Table 15**
HCN 3–2 Fluxes from the Literature Sample</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>HCN 3–2 Flux<br>(mJy km s<sup>-1</sup>)<br>(2)</th><th>Reference<br>(3)</th></tr></thead><tbody><tr><td>AA Tau</td><td>590 ± 130 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>AS 205</td><td>&lt;510 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>AS 209</td><td>2965 ± 297 (A)</td><td>Guzmán et al. (2021)</td></tr><tr><td>CI Tau</td><td>1271 ± 390 (S)</td><td>Bergner et al. (2019)</td></tr><tr><td>CQ Tau</td><td>&lt;750 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>DM Tau</td><td>2966 ± 400 (S)</td><td>Bergner et al. (2019)</td></tr><tr><td>DO Tau</td><td>&lt;1252 (S)</td><td>Bergner et al. (2019)</td></tr><tr><td>FP Tau</td><td>272 ± 17 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>GM Aur</td><td>1792 ± 180 (A)</td><td>Guzmán et al. (2021)</td></tr><tr><td>HD 142527</td><td>&lt;1155 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>HD 143006</td><td>2149 ± 220 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>HD 163296</td><td>7346 ± 736 (A)</td><td>Guzmán et al. (2021)</td></tr><tr><td>IM Lup</td><td>2409 ± 242 (A)</td><td>Guzmán et al. (2021)</td></tr><tr><td>J0432+1827</td><td>600 ± 18 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1100-7619</td><td>1655 ± 24 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1545-3417</td><td>120 ± 14 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>J1604-2130</td><td>8507 ± 850 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>J16085324-3914401</td><td>162 ± 19 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>J1609-1908</td><td>1048 ± 110 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>J1612-1859</td><td>&lt;79 (A)</td><td>Bergner et al. (2020)</td></tr><tr><td>J1614-1906</td><td>130 ± 40 (A)</td><td>Bergner et al. (2020)</td></tr></tbody></table>

<table><caption>**Table 15**
(Continued)</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>HCN 3–2 Flux<br>(mJy km s<sup>-1</sup>)<br>(2)</th><th>Reference<br>(3)</th></tr></thead><tbody><tr><td>LkCa 15</td><td>3809 ± 450 (S)</td><td>Bergner et al. (2019)</td></tr><tr><td>MWC 480</td><td>2469 ± 248 (A)</td><td>Guzmán et al. (2021)</td></tr><tr><td>SAO 206462</td><td>580 ± 150 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>Sz 118</td><td>175 ± 21 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>Sz 129</td><td>443 ± 46 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>Sz 130</td><td>&lt;37 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>Sz 65</td><td>&lt;32 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>Sz 68</td><td>&lt;24 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>Sz 69</td><td>42 ± 9 (A)</td><td>Pegues et al. (2021a)</td></tr><tr><td>Sz 90</td><td>169 ± 26 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>TW Hya</td><td>8500 ± 1700 (S)</td><td><sup>a</sup></td></tr><tr><td>V4046 Sgr</td><td>10000 ± 1000 (A)</td><td>Bergner et al. (2019)</td></tr></tbody></table>

**Note.** Column 1: Star+disk system from the literature sample (Section 2.4). Column 2: HCN 3–2 line fluxes for protoplanetary disks from the literature sample (Section 2.4). 3σ upper limits are marked with <. Fluxes from ALMA, the SMA, and the IRAM PdBI are marked with (A), (S), and (I), respectively. When fluxes from multiple instruments are available, we select the flux from the instrument with the highest spatial resolution to decrease the chance of contamination from external sources of emission. Column 3: Reference.

<footnote><sup>a</sup> From Qi et al. (2008) as reported in Cleeves et al. (2015).</footnote>
<table><caption>**Table 16**
HCO$^{+}$ 3–2 Fluxes from the Literature Sample</caption><thead><tr><th>Star+Disk<br>System<br>(1)</th><th>HCO<sup>+</sup> 3–2 Flux<br>(mJy km s<sup>-1</sup>)<br>(2)</th><th>Reference<br>(3)</th></tr></thead><tbody><tr><td>AA Tau</td><td>2320 ± 190 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>AS 205</td><td>2500 ± 120 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>AS 209</td><td>4020 ± 160 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>CQ Tau</td><td>750 ± 240 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>DM Tau</td><td>5340 ± 120 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>GM Aur</td><td>6960 ± 60 (A)</td><td>Huang et al. (2020b)</td></tr><tr><td>HD 142527</td><td>7790 ± 530 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>IM Lup</td><td>9670 ± 290 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>J16085324-3914401</td><td>254 ± 26 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>LkCa 15</td><td>5190 ± 170 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>MWC 480</td><td>4650 ± 310 (S)</td><td>Öberg et al. (2010)</td></tr><tr><td>SAO 206462</td><td>1180 ± 220 (S)</td><td>Öberg et al. (2011)</td></tr><tr><td>Sz 118</td><td>376 ± 38 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>Sz 129</td><td>595 ± 61 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>Sz 130</td><td>184 ± 20 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>Sz 65</td><td>219 ± 23 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>Sz 68</td><td>183 ± 20 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>Sz 90</td><td>298 ± 31 (A)</td><td>Anderson et al. (2022)</td></tr><tr><td>TW Hya</td><td>12900 ± 2120 (S)</td><td>Cleeves et al. (2015)</td></tr><tr><td>V4046 Sgr</td><td>11430 ± 240 (S)</td><td>Öberg et al. (2011)</td></tr></tbody></table>

**Note.** Column 1: Star+disk system from the literature sample (Section 2.4). Column 2: HCO$^{+}$ 3–2 line fluxes for protoplanetary disks from the literature sample (Section 2.4). 3σ upper limits are marked with <. Fluxes from ALMA, the SMA, and the IRAM PdBI are marked with (A), (S), and (I), respectively. When fluxes from multiple instruments are available, we select the flux from the instrument with the highest spatial resolution to decrease the chance of contamination from external sources of emission. Column 3: Reference.
## Appendix B
Unbiased Spectra for all SMA Bands

Figure 10 displays the spectral data across all SMA spectral bands for the 230 GHz and 240 GHz SMA receivers.

<figure><img src="image_12.png" /><figcaption>**Figure 10.** Unbiased spectra (i.e., spectra extracted with uniform elliptical masks; Section 3.1) for all disks (split by row) and for all bands observed with the 230 GHz (top panels) and 240 GHz (bottom panels) SMA receiver. The spectra per band are allowed to overlap in the panels at the band edges; in our analysis, however, we excluded the ~0.1 GHz of each band edge to avoid artifacts. The 10 target molecular lines (Table 3) are marked with dashed pink lines.</figcaption></figure>
# Appendix C
## Channel Maps

### C.1. Channel Maps for HD 34282

Figures 11 and 12 display channel maps for molecular line emission detected or tentatively detected from HD 34282.

<figure><img src="image_13.png" /><figcaption>**Figure 11.** $^{12}$CO 2–1 (first panel), $^{13}$CO 2–1 (second panel), C$^{18}$O 2–1 (third panel), HCO$^{+}$ 3–2 (fourth panel), and HCN 3–2 (fifth panel) emission from HD 34282. Emission is shown above $1\sigma$, where $\sigma$ is the channel rms (Table 5). The estimated disk center is marked with a gray plus. Keplerian masks are overlaid with dashed lines, and synthesized beams are drawn in the lower left corner of the entire figure.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure 12.** CS 5–4 (first panel), C₂H 3–2 (second panel), and DCO⁺ 3–2 (third panel) emission from HD 34282. Emission is shown above 1σ, where σ is the channel rms (Table 5). The estimated disk center is marked with a gray plus. Keplerian masks are overlaid with dashed lines, and synthesized beams are drawn as gray ellipses in the lower left corner of the entire figure.</figcaption></figure>
## C.2. Channel Maps for HD 36112

Figure 13 displays channel maps for molecular line emission detected or tentatively detected from HD 36112.

<figure><img src="image_15.png" /><figcaption>**Figure 13.** $^{12}$CO 2–1 (first panel), $^{13}$CO 2–1 (second panel), C$^{18}$O 2–1 (third panel), HCO$^{+}$ 3–2 (fourth panel), CS 5–4 (fifth panel), and DCO$^{+}$ 3–2 (sixth panel) emission from HD 36112. Emission is shown above $1\sigma$, where $\sigma$ is the channel rms (Table 5). The estimated disk center is marked with a gray plus. Keplerian masks are overlaid with dashed lines, and synthesized beams are drawn as gray ellipses in the lower left corner of the entire figure.</figcaption></figure>
### C.3. Channel Maps for HD 142666

Figure 14 displays channel maps for molecular line emission detected or tentatively detected from HD 142666.

<figure><img src="image_16.png" /></figure>

**Figure 14.** $^{12}$CO 2–1 (first panel), $^{13}$CO 2–1 (second panel), C$^{18}$O 2–1 (third panel), HCO$^{+}$ 3–2 (fourth panel), and HCN 3–2 (fifth panel) emission from HD 142666. Emission is shown above 1σ, where σ is the channel rms (Table 5). The estimated disk center is marked with a gray plus. Keplerian masks are overlaid with dashed lines, and synthesized beams are drawn as gray ellipses in the lower left corner of the entire figure.
### C.4. Channel Maps for HD 144432

Figure 15 displays channel maps for molecular line emission detected or tentatively detected from HD 144432.

<figure><img src="image_17.png" /><figcaption>Figure 15. $^{12}$CO 2–1 (first panel), $^{13}$CO 2–1 (second panel), and DCN 3–2 (third panel) emission from HD 144432. Emission is shown above $1\sigma$, where $\sigma$ is the channel rms (Table 5). The estimated disk center is marked with a gray plus. Keplerian masks are overlaid with dashed lines, and synthesized beams are drawn as gray ellipses in the lower left corner of the entire figure.</figcaption></figure>

## Appendix D
ALMA Archival Observations of HD 34282

Figure 16 displays images of HD 34282 emission extracted directly from the ALMA archive for the observing project 2013.1.00658.S.

<figure><img src="image_18.png" /><figcaption>Figure 16. Images of HD 34282 emission extracted directly from the ALMA archive for the ALMA observing project 2013.1.00658.S. The emission shown is for the 350 GHz continuum (leftmost two panels), $^{12}$CO 3–2 emission (middle two panels), and HCO$^{+}$ 4–3 emission (rightmost two panels). The emission was observed in either the compact (Comp.) or extended (Ext.) configurations. Synthesized beams are drawn in white in the bottom right corner of each panel. The continuum images share the same color bar, as do the line emission images. Both color bars are in log scale to emphasize faint emission.</figcaption></figure>
ORCID iDs

Jamila Pegues https://orcid.org/0000-0002-6042-1486

Karin I. Öberg https://orcid.org/0000-0001-8798-1347

Chunhua Qi https://orcid.org/0000-0001-8642-1786

Sean M. Andrews https://orcid.org/0000-0003-2253-2270

Jane Huang https://orcid.org/0000-0001-6947-6072

Charles J. Law https://orcid.org/0000-0003-1413-1776

Romane Le Gal https://orcid.org/0000-0003-1837-3772

Luca Matrà https://orcid.org/0000-0003-4705-3188

David J. Wilner https://orcid.org/0000-0003-1526-7587

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