# Detection of Dimethyl Ether in the Central Region of the MWC 480 Protoplanetary Disk

Yoshihide Yamato, Yuri Aikawa, Viviana V. Guzmán, Kenji Furuya, Shota Notsu, Gianni Cataldi, Karin I. Öberg, Chunhua Qi, Charles J. Law, Jane Huang, et al.

► To cite this version:

Yoshihide Yamato, Yuri Aikawa, Viviana V. Guzmán, Kenji Furuya, Shota Notsu, et al. Detection of Dimethyl Ether in the Central Region of the MWC 480 Protoplanetary Disk. The Astrophysical Journal, 2024, 974, 10.3847/1538-4357/ad6981. insu-04836801

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# Detection of Dimethyl Ether in the Central Region of the MWC 480 Protoplanetary Disk

Yoshihide Yamato¹, Yuri Aikawa¹, Viviana V. Guzmán², Kenji Furuya¹,³, Shota Notsu⁴,¹,⁵, Gianni Cataldi³, Karin I. Öberg⁶, Chunhua Qi⁶, Charles J. Law⁷,¹,², Jane Huang⁸, Richard Teague⁹, and Romane Le Gal¹⁰,¹¹

¹Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; yyamato.as@gmail.com

²Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile

³National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan

⁴Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

⁵Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

⁶Center for Astrophysics—Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

⁷Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA

⁸Department of Astronomy, Columbia University, 538 W. 120th Street, Pupin Hall, New York, NY, 10027, USA

⁹Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

¹⁰Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), Université Grenoble Alpes, CNRS, 38000 Grenoble, France

¹¹Institut de Radioastronomie Millimétrique (IRAM), 38406 Saint-Martin d'Hères, France

Received 2024 June 18; revised 2024 July 28; accepted 2024 July 29; published 2024 October 7

## Abstract

Characterizing the chemistry of complex organic molecules (COMs) at the epoch of planet formation provides insights into the chemical evolution of the interstellar medium (ISM) and the origin of organic materials in our solar system. We report a detection of dimethyl ether (CH₃OCH₃) in the disk around the Herbig Ae star MWC 480 with sensitive Atacama Large Millimeter/submillimeter Array observations. This is the first detection of CH₃OCH₃ in a nontransitional Class II disk. The spatially unresolved, compact (≈25 au in radius) nature, broad line width (~30 km s⁻¹), and high excitation temperature (~200 K) indicate the sublimation of COMs in the warm inner disk. Despite the detection of CH₃OCH₃, methanol (CH₃OH), the most abundant COM in the ISM, has not been detected, from which we constrain the column density ratio of CH₃OCH₃/CH₃OH ≳ 7. This high ratio may indicate the reprocessing of COMs during the disk phase, as well as the effect of the physical structure in the inner disk. We also find that this ratio is higher than in COM-rich transition disks recently discovered. This may indicate that in the full disk of MWC 480, COMs have experienced substantial chemical reprocessing in the innermost region, while the COM emission in the transition disks predominantly traces the inherited ice sublimating at the dust cavity edge located at larger radii (≈20 au).

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

## 1. Introduction

Complex organic molecules (COMs), defined empirically as carbon-bearing molecules with six or more atoms, serve as precursors of prebiotic molecules and thus provide insights into the origin of organic materials and life in our solar system (e.g., Ceccarelli et al. 2023). COMs are formed initially on the cold grain surfaces in dark clouds (e.g., Herbst & van Dishoeck 2009) and will be readily observable at radio wavelengths in the warm (≈100 K) environment, where they are released into the gas phase by thermal desorption. Indeed, observations with the Atacama Large Millimeter/submillimeter Array (ALMA) have detected a number of COMs in the warm inner envelopes around protostars (e.g., Jørgensen et al. 2018; Belloche et al. 2020; Yang et al. 2021) and the warm protostellar disk around the FU Ori-type outbursting star V883 Ori (van't Hoff et al. 2018; Lee et al. 2019; Yamato et al. 2024). COMs have also been detected in solar system comets, for example, 67P/Churyumov-Gerasimenko, which has been extensively studied by the Rosetta mission (Altwegg et al. 2019). While comparative studies of COM abundance ratios

suggest that COMs in comets are at least partially inherited from the interstellar medium (ISM; Drozdovskaya et al. 2019), characterizing the distributions and abundances of COMs in protoplanetary disks is crucial for understanding the chemical evolution from the ISM to planetary systems.

Recent sensitive observations with ALMA have revealed the presence of thermally desorbed oxygen-bearing COMs in a few warm transition disks around Herbig Ae stars. van der Marel et al. (2021) detected warm methanol (CH₃OH) gas localized at the asymmetric dust trap in the transition disk around IRS 48, followed by additional detections in the transition disks of HD 100546 and HD 169142 (Booth et al. 2021b, 2023). IRS 48 exhibits the highest chemical complexity among these disks, where dimethyl ether (CH₃OCH₃), methyl formate (CH₃OCHO), and ethylene oxide (c-H₂COCH₂) have been detected in addition to CH₃OH (Brunken et al. 2022; Booth et al. 2024b). Other oxygen-bearing molecules—such as SO, SO₂, and NO—have also been detected at the dust trap, indicating the presence of oxygen-rich gas and a low C/O ratio of <1 (Booth et al. 2021a; Leemker et al. 2023). HD 100546 also exhibits CH₃OCHO emission, but no CH₃OCH₃ and c-H₂COCH₂, possibly suggesting a different reservoir of COMs compared to IRS 48 (Booth et al. 2024a). In these transition disks around Herbig Ae stars, the high luminosity of the central star and the inner dust cavity provide a warmer disk condition, which makes it possible for COMs to sublimate.

<figure><img src="image_3.png" /></figure>

<footnote n="12">¹² NASA Hubble Fellowship Program Sagan Fellow.</footnote>
<table><caption>**Table 1**
Observational Details</caption><thead><tr><th rowspan="2">Date</th><th rowspan="2"># of Ant.</th><th rowspan="2">On-source Int. (min)</th><th rowspan="2">PWV (mm)</th><th rowspan="2">Baseline (m)</th><th rowspan="2">Ang. Res. (arcsec)</th><th rowspan="2">MRS (arcsec)</th><th colspan="2">Calibrators</th></tr><tr><th>Bandpass/Amplitude</th><th>Phase</th></tr></thead><tbody><tr><td colspan="9" style="text-align:center;">2021.1.00535.S (PI: Y. Yamato)</td></tr><tr><td>2022 Aug 8</td><td>42</td><td>44</td><td>0.7</td><td>15-1302</td><td>0.2</td><td>2.6</td><td>J0510+1800</td><td>J0438+3004</td></tr><tr><td>2022 Aug 9</td><td>40</td><td>44</td><td>0.2</td><td>15-1246</td><td>0.2</td><td>3.0</td><td>J0510+1800</td><td>J0438+3004</td></tr><tr><td>2022 Aug 9</td><td>40</td><td>44</td><td>0.1</td><td>15-1261</td><td>0.2</td><td>3.1</td><td>J0510+1800</td><td>J0438+3004</td></tr><tr><td>2022 Aug 10</td><td>42</td><td>44</td><td>0.4</td><td>15-1302</td><td>0.2</td><td>2.8</td><td>J0510+1800</td><td>J0438+3004</td></tr><tr><td>2022 Aug 10</td><td>41</td><td>44</td><td>0.5</td><td>15-1302</td><td>0.2</td><td>3.1</td><td>J0510+1800</td><td>J0438+3004</td></tr><tr><td colspan="9" style="text-align:center;">2021.1.00982.S (PI: V. V. Guzman)</td></tr><tr><td>2022 Aug 11</td><td>45</td><td>40</td><td>0.5</td><td>15-1302</td><td>0.2</td><td>2.9</td><td>J0510+1800</td><td>J0438+3004</td></tr><tr><td>2022 Aug 11</td><td>44</td><td>40</td><td>0.5</td><td>15-1302</td><td>0.2</td><td>2.8</td><td>J0510+1800</td><td>J0438+3004</td></tr><tr><td>2022 Aug 14</td><td>47</td><td>40</td><td>0.6</td><td>15-1302</td><td>0.2</td><td>2.8</td><td>J0510+1800</td><td>J0438+3004</td></tr></tbody></table>

However, the current limited detections of sublimated COMs in transition disks preclude a comparison of the COM reservoir between other Herbig and T Tauri disks. Although weak, extended emission of CH$_3$OH has been detected in the outer cold region (≥30 au) of the nearest T Tauri disk TW Hya, its low abundance relative to H$_2$ (~10$^{-11}$–10$^{-12}$) suggests a nonthermal desorption origin (Walsh et al. 2016). Similar attempts to detect CH$_3$OH in the disks around Herbig Ae stars HD 163296 and MWC 480 yielded nondetections (HD 163296; Carney et al. 2019) or tentative detections (MWC 480; Loomis et al. 2018a) in the outer cold region. In this paper, we present the first detection of CH$_3$OCH$_3$ in the MWC 480 protoplanetary disk. MWC 480 is a ~7 Myr old (Montesinos et al. 2009) Class II Herbig Ae star with a stellar mass of $M_* \approx 2.1 M_{\odot}$ (Simon et al. 2019; Teague et al. 2021) in the Taurus–Auriga star-forming region ($d \approx 162$ pc; Gaia Collaboration et al. 2018). The MWC 480 disk, which has been observed in the Molecules with ALMA at Planet-forming Scales ALMA Large Program (Öberg et al. 2021), is one of the most extensively studied disks around Herbig Ae stars. The MWC 480 disk exhibits a prominent dust ring at $\approx 98$ au (Long et al. 2018; Liu et al. 2019; Sierra et al. 2021), and the velocity perturbation observed in CO and its isotopologue emission suggests the presence of a planet at $\approx 245$ au (Teague et al. 2021; Izquierdo et al. 2023). The outer disk (≥30 au) of MWC 480 is characterized by a gas-phase C/O $> 1$ (Bosman et al. 2021a; Jiang et al. 2023) and abundant hydrocarbons and cyanides (Guzmán et al. 2021; Ilee et al. 2021), while the inner disk chemistry is unknown.

We describe the ALMA Band 7 observations in Section 2. We present the detection of CH$_3$OCH$_3$, the tentative detection of CH$_3$OCHO, and the nondetection of CH$_3$OH, from which we constrain the excitation condition and column densities based on the spectral analysis in Section 3. We discuss the results in Section 4, and finally summarize the study in Section 5.

## 2. Observations

We observed the MWC 480 disk in Band 7 during ALMA Cycle 8 (project code: 2021.1.00535.S; PI: Y. Yamato). Observations were carried out in five execution blocks. Observational details including the observation date, number of antennas, on-source integration time, mean precipitable water vapor (PWV), baseline coverage, angular resolution, maximum recoverable scale (MRS), and calibrator information are summarized in Table 1.

The pipeline calibrations were performed by ALMA staff using the standard ALMA pipeline. Subsequent self-calibration and imaging were performed using the Common Astronomy Software Applications (CASA; CASA Team et al. 2022) version 6.2.1.7. The continuum visibilities were produced by averaging the line-free channels specified by visually inspecting the data cubes delivered from the observatory. The continuum emission peak on the image from each execution block was aligned to a common direction, $\alpha(\text{ICRS}) = 04^h58^m46^s279$, $\delta(\text{ICRS}) = +29^d50^m36^s40$, which was used as the disk center in the following analysis. Then, five rounds of phase-only self-calibration and one round of amplitude self-calibration were performed, resulting in a signal-to-noise ratio (S/N) increase by a factor of $\approx 15$. The solutions were then applied to the entire data set. The continuum emission was finally subtracted from the line visibilities by fitting a first-order polynomial to line-free channels using the CASA task uvcontsub.

We focused on the spectral window originally dedicated for continuum acquisition, which covered a frequency range of 298.88–300.75 GHz and had a spectral resolution of 1.1 MHz ($\approx 1.1$ km s$^{-1}$). This spectral window contained a number of CH$_3$OCH$_3$ and CH$_3$OCHO transitions. We generated a single image cube for the entire frequency range by imaging the self-calibrated, continuum-subtracted visibilities with a channel spacing of 4 km s$^{-1}$ and a Briggs robust value of 0.5, in the modified Briggs weighting scheme of briggsbwtaper implemented in the tclean task. The large channel width of 4 km s$^{-1}$ was intended to boost the S/N of the detected CH$_3$OCH$_3$ emission while resolving its broad line width. The CLEANed image cube has a synthesized beam of $0.′′31 \times 0.′′21$ (PA = 8$^\circ.2$) and an rms noise level of 0.33 mJy beam$^{-1}$ at 4 km s$^{-1}$ channel width. The rms noise was measured on the line-free region of the image cube prior to primary beam correction. We also generated the continuum image at 1.03 mm with a Briggs robust value of −0.5 to better resolve the gap-ring structure, which is shown in the top left panel of Figure 1. Additionally, we report the detection of a new submillimeter source in the same field of view in Appendix D.

We also used the Band 7 data from another project (project code: 2021.1.00982.S; PI: V. V. Guzman). Observations were carried out in three execution blocks and their details are summarized in Table 1. The continuum spectral window of this data set, which is used in this study, covered a frequency range of 288.56–290.43 GHz, with a spectral resolution of 1.1 MHz ($\approx 1.2$ km s$^{-1}$). This spectral window covered a cluster of
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Summary of the observations toward the MWC 480 disk. Top: 1.03 mm continuum image (left) and velocity-integrated intensity maps of (tentatively) detected COMs, CH<sub>3</sub>OCH<sub>3</sub>, and CH<sub>3</sub>OCHO (others). The peak S/Ns of the right three maps are 5.4, 12, and 6.0, respectively. The white ellipse in the lower left corner indicates the synthesized beam. A scale bar of 50 au is shown in the lower right corner. Middle: spectra extracted in a circular aperture with 0.′′6 diameter toward the disk center (gray), overlaid with the best-fit spectral model for CH<sub>3</sub>OCH<sub>3</sub> (blue), CH<sub>3</sub>OCHO (orange), and the sum of both (black). The semitransparent error bars represent the associated 1σ uncertainties. The frequency ranges removed from the fit due to the atmospheric absorption or contamination from other molecular lines are shaded in gray. Bottom: zoom-in view of the middle panel in the vicinity of the (tentatively) detected features, but with the spectral axis in velocity with respect to a reference frequency of 299.11170 GHz (left), 299.89209 GHz (middle), and 300.05338 GHz (right), respectively. The vertical line segments at the bottoms of the panels indicate the center and the relative strength (at 200 K) of each transition.</figcaption></figure>

CH<sub>3</sub>OH transitions, which were used to constrain the upper limit on the CH<sub>3</sub>OH column density. We followed the same self-calibration procedure as above, but with CASA version 6.4.1. We performed three rounds of phase-only self-calibration and one round of amplitude self-calibration, which resulted in an increase in S/N by a factor of ≈5.5. We generated the image cube of the continuum spectral window in the same manner as above. The

resulting beam size and the rms noise level were 0.′′31 × 0.′′22 (PA = −35°4) and 0.67 mJy beam⁻¹ at 4 km s⁻¹ channel width.

For both data sets, the absolute flux calibration uncertainty is expected to be ≲10%, based on the Quality Assurance (QA2) criteria described in the ALMA Cycle 8 Technical Handbook. The following analyses consider only the statistical uncertainty and do not include the systematic flux calibration uncertainty.
<table><caption>**Table 2**
Results of the Spectral Analysis</caption><thead><tr><th>Molecule</th><th>T<sub>ex</sub><br>(K)</th><th>ΔV<sub>FWHM</sub><br>(km s<sup>-1</sup>)</th><th>v<sub>sys</sub><br>(km s<sup>-1</sup>)</th><th>N<br>(cm<sup>-2</sup>)</th><th>/N(CH<sub>3</sub>OH)</th></tr></thead><tbody><tr><td>CH<sub>3</sub>OCH<sub>3</sub></td><td></td><td></td><td></td><td>7.4<sup>+4.6</sup><sub>-3.2</sub> × 10<sup>15</sup></td><td>>7</td></tr><tr><td>CH<sub>3</sub>OCHO</td><td></td><td></td><td></td><td>7.4<sup>+3.6</sup><sub>-4.9</sub> × 10<sup>14</sup></td><td>>0.81<sup>a</sup></td></tr><tr><td>CH<sub>3</sub>OH</td><td>200<sup>+50</sup><sub>-50</sub></td><td>30<sup>+8</sup><sub>-6</sub></td><td>4.3<sup>+2.4</sup><sub>-2.5</sub></td><td>&lt;9.1 × 10<sup>14</sup></td><td>...</td></tr><tr><td>CH<sub>3</sub>CHO</td><td></td><td></td><td></td><td>&lt;1.9 × 10<sup>14</sup></td><td>...</td></tr></tbody></table>

**Note.**

# 3. Detection of Dimethyl Ether

## 3.1. Spectra and Spatial Distributions

We first extracted the integrated spectra by spatially integrating the image cubes in a circular aperture with 0.6" diameter toward the disk center, which securely covers the apparent emission, as shown later. The middle panel of Figure 1 shows the resulting spectrum of the data containing detected CH<sub>3</sub>OCH<sub>3</sub> transitions. We found a clear spectral feature at ~299.89 GHz, whose line center spectrally coincides with the rest frequencies of CH<sub>3</sub>OCH<sub>3</sub> transitions, taking into account the spectral shift due to the systemic velocity (5.1 km s<sup>-1</sup>; Piétu et al. 2007; Teague et al. 2021). This feature is a blend of 12 individual CH<sub>3</sub>OCH<sub>3</sub> transitions (see Table 3 in Appendix B for their spectroscopic properties) and shows a peak S/N of 9.4. In addition, two tentative features are seen at ~299.12 and ~300.06 GHz, which also match with the frequencies of the CH<sub>3</sub>OCH<sub>3</sub> and CH<sub>3</sub>OCHO transitions. These features show peak S/Ns of 3.4 and 4.7, respectively. Because they are lower than the typical detection criteria of 5σ and the latter is blended between different species (CH<sub>3</sub>OCH<sub>3</sub> and CH<sub>3</sub>OCHO), these transitions were considered tentative detections. The zoomed-in spectra around these features are presented in the bottom panels of Figure 1, along with the expected spectral locations of the CH<sub>3</sub>OCH<sub>3</sub> and CH<sub>3</sub>OCHO transitions. We note that an exhaustive search for other molecular lines that could be responsible for those features was performed, but only CH<sub>3</sub>OCH<sub>3</sub> and CH<sub>3</sub>OCHO can consistently explain these features among the lines of detectable species/transitions in a typical disk environment.

The velocity-integrated intensity maps of these spectral features, generated using the Python package `bettermodels` (Teague & Foreman-Mackey 2018), with no flux threshold for pixel inclusion, are shown in the top panels of Figure 1, along with the 1.03 mm dust continuum image. These maps show the spatially unresolved, compact emission with a peak S/N of ~5–12.

Several strong transitions of CH<sub>3</sub>OH and CH<sub>3</sub>CHO were covered by the data from the project 2021.1.00982.S (see Table 3 in Appendix B for the covered strong transitions), but they have not been detected in the image plane.

We also verified the (tentative) detections/nondetections of these COMs by matched filter analysis (Loomis et al. 2018b). The details and resulting response spectra are shown in Appendix A. The ~5σ–17σ responses for CH<sub>3</sub>OCH<sub>3</sub> and CH<sub>3</sub>OCHO signify the detection of them, although it should be noted that the detected features are a blend of multiple transitions.

## 3.2. Spectral Analysis

To characterize the excitation temperatures and column densities of molecules, we employ a spectral fit with a simple local thermodynamic equilibrium (LTE) slab model. We basically followed the method described in Yamato et al. (2024), which is detailed in Appendix B. Briefly, the entire spectra are modeled with a total of seven free parameters: beam-averaged column densities of four molecular species (N(CH<sub>3</sub>OCH<sub>3</sub>), N(CH<sub>3</sub>OCHO), N(CH<sub>3</sub>OH), and N(CH<sub>3</sub>CHO)), the excitation temperature T<sub>ex</sub>, which is assumed to be common among different species/ transitions, the line width ΔV (including the broadening due to disk rotation), and the systemic velocity v<sub>sys</sub>. Although CH<sub>3</sub>OH and CH<sub>3</sub>CHO are not detected, this simultaneous fit allows us to constrain the upper limit on their column densities. The assumption of a common excitation temperature for all COM species reflects the expectation that different COMs sublimate simultaneously with water ice.

We explore the parameter space with the affine-invariant Markov Chain Monte Carlo (MCMC) algorithm implemented in the Python package `emcee` (Foreman-Mackey et al. 2013). The details of this fitting procedure are described in Appendix B. Figure 1 compares the best-fit model with the observed spectra. The three observed spectral features are well reproduced by the best-fit model. While the robust feature at ~299.89 GHz is explained by the cluster of CH<sub>3</sub>OCH<sub>3</sub> transitions with relatively low upper-state energies of 36–45 K, two other tentative features correspond to the transitions of CH<sub>3</sub>OCH<sub>3</sub> and CH<sub>3</sub>OCHO, with higher upper-state energies of 187–447 K (see Table 3). The result of the fit is summarized in Table 2. The excitation temperature and line width are constrained to be T<sub>ex</sub> = 200<sup>+50</sup><sub>-50</sub> K and ΔV = 30<sup>+8</sup><sub>-6</sub> km s<sup>-1</sup>, where the uncertainties are 16th and 84th percentiles (corresponding to 1σ) of the posterior distributions. The resulting beam-averaged column densities of CH<sub>3</sub>OCH<sub>3</sub> and CH<sub>3</sub>OCHO are 7.4<sup>+4.6</sup><sub>-3.2</sub> × 10<sup>15</sup> and 7.4<sup>+3.6</sup><sub>-4.9</sub> × 10<sup>14</sup> cm<sup>-2</sup>, respectively. The upper limits on the column densities of CH<sub>3</sub>OH and CH<sub>3</sub>CHO are also constrained to be <9.1 × 10<sup>14</sup> and <1.9 × 10<sup>14</sup> cm<sup>-2</sup>, respectively. These upper limits are the 99.85th percentile (corresponding to 3σ) of the posterior distributions. From these constraints on column densities, we derive a CH<sub>3</sub>OCH<sub>3</sub>/CH<sub>3</sub>OH column density ratio (lower limit) of >7.

# 4. Discussion

## 4.1. Chemical Origin of the COM Emission

The spatially unresolved, compact (~25 au radius) CH<sub>3</sub>OCH<sub>3</sub> emission with a broad line width (~30 km s<sup>-1</sup>) and a high excitation temperature (~200 K; see Section 3.2) indicates warm CH<sub>3</sub>OCH<sub>3</sub> gas in the inner disk. This is the first observational

<footnote>a The median value of N(CH<sub>3</sub>OCHO) divided by the N(CH<sub>3</sub>OH) upper limit, which should be viewed as tentative. See Appendix B for details.</footnote>
evidence of COMs sublimated in the inner warm region of the MWC 480 disk. The observed broad line width is also reproduced with the emission originating from ≲20 to 30 au radii, where the disk temperature is ≳80–100 K, by a more detailed radiative transfer calculation (see Appendix C), further supporting this interpretation. Indeed, the peak brightness temperature of the CO J = 2–1 emission already reaches ≳70 K at ≈50 au (Law et al. 2021), implying that the inner disk of MWC 480 is warm enough (≈100 K) for COMs to sublimate. Furthermore, a super-resolution image of the Band 6 continuum emission shows a steep brightness increase inside a 20 au radius (Yamaguchi et al. 2024), potentially suggesting the sublimation of water and COMs there that modify the dust optical properties. As the (sub)millimeter dust continuum emission from the midplane is optically thick in the inner ≲40 au (Sierra et al. 2021), the observed CH$_{3}$OCH$_{3}$ emission likely traces the disk surface layer.

The CH$_{3}$OCH$_{3}$ detection and its icy origin lead us to expect the detection of other COMs, particularly the most abundant COM in the ISM, CH$_{3}$OH. However, the CH$_{3}$OH transitions ($E_u \gtrsim 40$ K) have not been detected in the present observations, resulting in a high CH$_{3}$OCH$_{3}$/CH$_{3}$OH column density ratio of >7 (Section 3.2). This value is considerably higher than the typical values in the warm inner envelopes around protostars (e.g., Jørgensen et al. 2018; Yang et al. 2021), by more than 1 order of magnitude. This may indicate the efficient formation of CH$_{3}$OCH$_{3}$ during the disk phase. While it is anticipated that CH$_{3}$OH predominantly forms via hydrogenation of CO on the cold grain surfaces in the dark cloud (e.g., Watanabe & Kouchi 2002), more complex species, such as CH$_{3}$OCH$_{3}$ and CH$_{3}$OCHO, may also form on the grain surface in situ in disks. Disk chemical models suggest that radical–radical reactions on the warm (~30–50 K) grain surfaces may enhance the abundance of CH$_{3}$OCH$_{3}$ and CH$_{3}$OCHO (e.g., Furuya & Aikawa 2014):

$$CH_3 + CH_3O \rightarrow CH_3OCH_3, \quad (1)$$

$$HCO + CH_3O \rightarrow CH_3OCHO, \quad (2)$$

where the ingredient radicals (CH$_{3}$, CH$_{3}$O, and HCO) are the photodissociation products of CH$_{3}$OH ice (Öberg et al. 2009). These reactions, triggered by energetic processes such as UV, X-rays, and cosmic rays, will play a pivotal role in determining the abundance of COMs during the disk phase. Additionally, gas-phase formation in the inner warm region may also be a contributing factor, as suggested by astrochemical models (Taquet et al. 2016; Garrod et al. 2022).

The derived column density ratio (CH$_{3}$OCH$_{3}$/CH$_{3}$OH > 7) is still higher than the prediction of those models, which is lower than unity (e.g., Furuya & Aikawa 2014; Taquet et al. 2016). This implies that some additional effect is necessary to explain the observations. One possibility is the difference in the distributions between CH$_{3}$OCH$_{3}$ and CH$_{3}$OH. The slightly higher binding energy of CH$_{3}$OH (≈5000–6600 K; Ferrero et al. 2020; Minissale et al. 2022) than of CH$_{3}$OCH$_{3}$ (≈4000–6500 K; Ligterink & Minissale 2023) may cause the CH$_{3}$OH ice to sublimate in warmer regions (i.e., smaller radii) and the CH$_{3}$OH emission to be more heavily beam-diluted (see Appendix C for a test of this possibility). Additionally, dust continuum emission may selectively hide the CH$_{3}$OH emission that could originate from a more inner region (e.g., Bosman et al. 2021b; Yamato et al. 2024).

Taken together, we suggest that the substantial reprocessing of COMs during the disk phase, in conjunction with an additional effect of the difference in spatial distributions, could explain the detection of CH$_{3}$OCH$_{3}$ but not CH$_{3}$OH. Further

<figure><img src="image_5.png" /><figcaption>Figure 2. Comparison of the column density ratios of CH$_{3}$OCH$_{3}$ and CH$_{3}$OCHO relative to CH$_{3}$OH in three disks: HD 100546 (Booth et al. 2024a), Oph IRS 48 (Booth et al. 2024b), and MWC 480 (this work). The upward and downward triangles indicate lower and upper limits, respectively. Given the tentative detection (see Section 3.2), the value of CH$_{3}$OCHO in MWC 480 is plotted with transparency. The shaded region indicates the typical range of protostellar values measured by extensive line surveys (PILS—Jørgensen et al. 2016; CALYPSO—Belloche et al. 2020; PEACHES—Yang et al. 2021).</figcaption></figure>

observations at higher resolution and sensitivity are required to more accurately constrain the distributions and abundance ratio of COMs, thus enabling a better inference of the reprocessing of COMs during the disk phase.

## 4.2. Comparison to other Disks

### 4.2.1. COM-rich Transition Disks

Recent deep-line surveys have identified the emission from the warm gas of sublimated COMs in three transition disks: HD 100546, Oph IRS 48, and HD 169142 (Booth et al. 2023, 2024a, 2024b). Multiple species have been detected in Oph IRS 48 (CH$_{3}$OH, CH$_{3}$OCH$_{3}$, CH$_{3}$OCHO, and c-H$_{2}$COCH$_{2}$) and HD 100546 (CH$_{3}$OH and CH$_{3}$OCHO), while only CH$_{3}$OH has been detected in HD 169142. On the other hand, in full disks, no previous detections of sublimated COMs have been reported, and the present study provides the first evidence of icy organic sublimates. Figure 2 compares the abundance ratios of CH$_{3}$OCH$_{3}$ and CH$_{3}$OCHO relative to CH$_{3}$OH among HD 100546 (Booth et al. 2024a), Oph IRS 48 (Booth et al. 2024b), and MWC 480. The abundance ratios exhibit a considerable degree of variability among the sources, with particularly elevated values in the full disk of MWC 480. This suggests the potentially different chemistry and/or the effect of varying physical structures between transition and full disks. The abundance ratios in these transition disks may be determined predominantly by the sublimation of inherited ices at the dust cavity edge (or dust trap) located at larger radii (≈20 au; e.g., Booth et al. 2021b). In contrast, in the full disk of MWC 480, COM emission is likely to originate from more inner regions (≈20–30 au), where COMs may have experienced more reprocessing by, e.g., X-ray-driven chemistry and/or gas-phase chemistry in the disk surface layer. Indeed, the higher fractional abundance of HCO$^{+}$ relative to CO in MWC 480 compared to HD 100546 and IRS 48 may indicate a
higher X-ray flux (Aikawa et al. 2021; Booth et al. 2024a, 2024b), although the lower HCO$^+$ abundance in HD 100546 and IRS 48 could be attributed to the presence of gas-phase H$_2$O that destroys HCO$^+$ (e.g., Leemker et al. 2021).

It should also be noted that we obtain a tight upper limit on the CH$_3$CHO column density, which is less than ~3% of the CH$_3$OCH$_3$ column. This is in a similar trend to the nondetection of CH$_3$CHO in the IRS 48 and HD 100546 disks (Booth et al. 2024a, 2024b), while in contrast to the protostellar envelopes/disks where CH$_3$CHO exists in a comparable amount of CH$_3$OCH$_3$ (e.g., Jørgensen et al. 2018; Yang et al. 2021; Yamato et al. 2024). This might indicate that the specific reactions and/or branching ratios that are relevant to CH$_3$CHO formation/destruction may be at work in the disk phase, as suggested by the detection of c-H$_2$COCH$_2$, the isomer of CH$_3$CHO, in IRS 48 (Booth et al. 2024b).

### 4.2.2. Nondetection in HD 163296

It is also noteworthy that the same transitions of CH$_3$OCH$_3$ toward another well-studied full disk around the Herbig Ae star HD 163296 have not been detected with a similar sensitivity and resolution in the same project.<fnref n="13" /> This suggests that the chemistry and/or physical structure in the inner disk may be different between MWC 480 and HD 163296. The CO snow-line in MWC 480 is located at a larger radius (~100 au) than in HD 163296 (~65 au), as demonstrated in a comprehensive thermochemical modeling study (Zhang et al. 2021), implying a warmer inner disk of MWC 480. The higher rotation temperature of the HC$_3$N emission in the inner ≲10 au (~90 K versus ~60 K) may also indicate that the inner region is warmer in MWC 480 (Ilee et al. 2021). Furthermore, the slightly higher mass accretion rate and bolometric luminosity of MWC 480 (Montesinos et al. 2009; Mendigutía et al. 2013; Fairlamb et al. 2015) may result in a greater quantity of organics being delivered to the inner disk (e.g., Banzatti et al. 2023).

## 5. Summary and Conclusion

We present ALMA Band 7 observations toward the disk around the Herbig Ae star MWC 480. We detected the line emission of CH$_3$OCH$_3$ and tentatively detected CH$_3$OCHO for the first time in the MWC 480 disk. This is the first detection of CH$_3$OCH$_3$ in a nontransitional Class II disk. The compact, spatially unresolved nature at a resolution of ~0''3 (or ~25 au radius), the broad line width (~30 km s$^{-1}$ in FWHM), and the high excitation temperature (~200 K) indicate ice sublimation in the warm inner disk.

Furthermore, we present a nondetection of CH$_3$OH, which results in a high CH$_3$OCH$_3$/CH$_3$OH column density ratio of >7. This high ratio may be explained by the efficient formation of CH$_3$OCH$_3$ during the disk phase, triggered by UV, X-ray, and cosmic rays, with a possible difference in the spatial distributions between CH$_3$OCH$_3$ and CH$_3$OH. We compare the column density ratios of COMs with those in COM-rich transition disks that have recently been discovered. The higher ratios in the full disk of MWC 480 may point to substantial chemical reprocessing of COMs in the innermost region, while

those in transition disks may predominantly trace the sublimation of inherited ice at the dust trap located at ≳20 au.

The detection of CH$_3$OCH$_3$ in MWC 480 provides the first evidence of icy organic sublimates in a full disk. Further observations, including the confirmation of the detection with other transitions, and detailed disk chemical modeling are necessary to better understand the inner disk chemistry. In particular, future JWST observations will provide complementary information about the COM abundances in gas and solid phases, which can be compared with the gas-phase abundances estimated from ALMA observations.

## Acknowledgments

We thank the anonymous referee for constructive comments that improved the manuscript. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2021.1.00535.S and ADS/JAO.ALMA#2021.1.00982.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSTC 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. Y.Y. acknowledges support by a Grant-in-Aid for the Japan Society for the Promotion of Science (JSPS) Fellows (KAKENHI grant No. JP23KJ0636). Y.A. acknowledges support by MEXT/JSPS Grants-in-Aid for Scientific Research (KAKENHI) grant Nos. JP20H05847, JP21H04495, and JP24K00674 and NAOJ ALMA Scientific Research grant code 2019-13B. V.V.G. gratefully acknowledges support from FONDECYT Regular 1221352 and ANID CATA-BASAL project FB210003. K.F. acknowledges support by MEXT/JSPS Grants-in-Aid for Scientific Research (KAKENHI) grant No. JP21K13967. S.N. is grateful for support from Grants-in-Aid for JSPS (Japan Society for the Promotion of Science) Fellows grant No. JP23KJ0329 and MEXT/JSPS Grants-in-Aid for Scientific Research (KAKENHI) grant Nos. JP20K22376, JP23K13155, JP20H05845, and JP23H05441. Support for C.J.L. was provided by NASA through the NASA Hubble Fellowship grant No. HST-HF2-51535.001-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.

Facility: ALMA.

Software: CASA (CASA Team et al. 2022), bettermoments (Teague & Foreman-Mackey 2018), VISIBLE (Loomis et al. 2018b), keplerian_mask.py (Teague 2020), emcee (Foreman-Mackey et al. 2013), corner.py (Foreman-Mackey 2016), RADMC-3D (Dullemond et al. 2012), Numpy (Harris et al. 2020), Scipy (Virtanen et al. 2020), Matplotlib (Hunter 2007), Astropy (Astropy Collaboration et al. 2013, 2018, 2022), cmocean (Thyng et al. 2016).

## Appendix A
Matched Filter Analysis

We performed the matched filter analysis (Loomis et al. 2018b), which is commonly used to extract weak molecular line emission in protoplanetary disks, to verify the detections/ nondetections of COMs. As a filter kernel, we used a simple Keplerian rotating disk model generated with the Python script keplerian_mask (Teague 2020), assuming a disk inclination

<footnote n="13">¹³ It should be noted that the physical scales traced by these observations are slightly different between MWC 480 and HD 163296, since the angular resolutions (~0''3) are the same but the distances are different (162 pc versus 101 pc).</footnote>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Matched filter responses. The horizontal dotted line marks 5σ. The relative intensities of the covered lines at 200 K are shown by the vertical segments for CH₃OCH₃ (blue), CH₃OCHO (orange), CH₃OH (red), and CH₃CHO (green). A cluster of CH₃OCH₃ lines at ~299.89 GHz exhibits a ~17σ response, while the tentative features at ~299.12 GHz and ~300.06 GHz show ~5σ–6σ responses.</figcaption></figure>

angle of 37°0, position angle of 148°0, and central stellar mass of 2.1 M⊙ (Öberg et al. 2021; Teague et al. 2021). The outer radius of the disk was set as 0.115, based on the extent of the emission in the velocity-integrated intensity maps. The Python package VISIBLE (Loomis et al. 2018b) was used to compute the cross-correlation between the observed visibilities and the filter kernel. The resultant response spectra are shown in Figure 3. The brightest cluster of CH₃OCH₃ lines at ~299.89 GHz shows a ~17σ filter response, while the two tentative features at ~299.12 GHz and ~300.06 GHz exhibit ~5σ–6σ filter responses. The response spectra do not exhibit any ≥5σ responses of CH₃OH and CH₃CHO transitions.

## Appendix B
### Details of Spectral Analysis

Here, we describe the details of the spectral analysis (Section 3.2) for deriving the excitation temperature and column density of COMs. We fit the entire spectra with an LTE slab model based on the basic radiative transfer equations (e.g., Yamato et al. 2024). The emergent intensity of the line emission $I_ν$ at each frequency is modeled as

$$I_{\nu} = (B_{\nu}(T_{\text{ex}}) - B_{\nu}(T_{\text{CMB}}))(1 - e^{-\tau_{\nu}}), \quad (\text{B1})$$

where $B_ν$ is the Planck function of the blackbody radiation, $T_{ex}$ is the excitation temperature, $T_{CMB} = 2.73$ K is the temperature of the cosmic microwave background, and $\tau_ν$ is the optical depth profile. We assume that $T_{ex}$ is shared by different species of COMs, i.e., the emitting regions are common among different COMs, given that COMs are expected to sublimate at a similar temperature. The optical depth profile $\tau_ν$ is computed as

$$\tau_{\nu} = \sum_{i} \frac{c^2 A_{\text{ul},i} N_{\text{u},i}}{8 \pi \nu^2} \left[ \exp\left(\frac{h\nu}{k_B T_{\text{ex}}}\right) - 1 \right] \phi_{\nu,i}, \quad (\text{B2})$$

where c is the speed of light, $A_{ul,i}$ is the Einstein A coefficient of the i'th transition, $N_{u,i}$ is the upper-state column density of the i'th transition, h is the Planck constant, $k_B$ is the Boltzmann constant, and $\phi_{\nu,i}$ is the line profile function of the i'th transition. The summation i goes over different transitions of all molecular species. The upper-state column density $N_{u,i}$ is related to the

total column density N as

$$N_{u,i} = \frac{N}{Q(T_{\text{ex}})} g_{u,i} \exp \left( - \frac{E_{u,i}}{k_B T_{\text{ex}}} \right), \quad (\text{B3})$$

where Q is the partition function, $g_u$ is the upper-state degeneracy, and $E_u$ is the upper-state energy. The line profile function $\phi_{\nu,i}$ in Equation (B2) is defined as

$$\phi_{\nu,i} = \frac{1}{\sqrt{2\pi}\sigma_{\nu,i}} \exp\left[-\frac{(\nu - \nu_{0,i} - \delta\nu_i)^2}{2\sigma_{\nu,i}^2}\right], \quad (\text{B4})$$

where $\sigma_{\nu,i}$ is the frequency line width, $\nu_{0,i}$ is the rest frequency of the i'th transition, and $\delta\nu_i$ is the frequency shift due to the systemic velocity $v_{sys}$. The frequency line width $\sigma_{\nu,i}$ is related to the velocity line width in FWHM $\Delta V_{FWHM}$ as $\sigma_{\nu,i} = \frac{\nu_{0,i} \Delta V_{FWHM}}{c \sqrt{8 \ln 2}}$<fnref n="14" />

Within this model, we consider >600 transitions of four molecular species—CH₃OCH₃, CH₃OCHO, CH₃OH, and CH₃CHO—whose Einstein A coefficient is larger than $10^{-8}$ s⁻¹ and upper-state energy is lower than 1000 K. Table 3 lists the strong transitions and their spectroscopic properties that effectively constrain the column density (and its upper limit). The spectroscopic data of these transitions, including partition functions, are taken from the Cologne Database for Molecular Spectroscopy (Müller et al. 2001, 2005; Endres et al. 2016) for CH₃OCH₃ and CH₃OH and the Jet Propulsion Laboratory database (Pickett et al. 1998) for CH₃OCHO and CH₃CHO. In total, we include seven free parameters in the fit: the excitation temperature $T_{ex}$, the velocity line width $\Delta V_{FWHM}$, the systemic velocity $v_{sys}$, and the logarithm of the column densities of these four species. We note that here we assume a Gaussian line shape (Equation (B4)) and the line width is the one after including the broadening due to Keplerian rotation rather than the intrinsic one. This predominantly reflects the underlying assumption that the emission is optically thin, where only the velocity-integrated intensity matters for column density derivation. The actual line shape may be different from Gaussian, as usually seen in the

<footnote n="14">¹⁴ This sort of column density retrieval, including the functions of querying molecular spectroscopic data, is packaged into a public Python code specfit (https://github.com/yyamato-as/specfit).</footnote>
<table><caption>**Table 3**
Spectroscopic Properties of Major Molecular Lines</caption><thead><tr><th>Transition</th><th>ν<sub>0</sub><br>(GHz)</th><th>log<sub>10</sub>A<sub>ul</sub><br>(s<sup>-1</sup>)</th><th>g<sub>u</sub></th><th>E<sub>u</sub><br>(K)</th></tr></thead><tbody><tr><td colspan="5" style="text-align:center; font-weight:bold;">Dimethyl Ether (CH<sub>3</sub>OCH<sub>3</sub>)</td></tr><tr><td>5<sub>4,2</sub>-4<sub>3,1</sub> AE</td><td>299.884130</td><td>-3.763</td><td>22</td><td>36.1</td></tr><tr><td>5<sub>4,2</sub>-4<sub>3,2</sub> EA</td><td>299.886128</td><td>-3.769</td><td>44</td><td>36.2</td></tr><tr><td>5<sub>4,1</sub>-4<sub>3,1</sub> AE</td><td>299.886804</td><td>-3.769</td><td>44</td><td>36.1</td></tr><tr><td>5<sub>4,1</sub>-4<sub>3,2</sub> AE</td><td>299.888802</td><td>-3.763</td><td>66</td><td>36.1</td></tr><tr><td>5<sub>4,1</sub>-4<sub>3,1</sub> EE</td><td>299.889910</td><td>-3.784</td><td>176</td><td>36.1</td></tr><tr><td>5<sub>4,2</sub>-4<sub>3,2</sub> EE</td><td>299.890981</td><td>-3.784</td><td>176</td><td>36.1</td></tr><tr><td>5<sub>4,2</sub>-4<sub>3,1</sub> AA</td><td>299.892089</td><td>-3.763</td><td>66</td><td>36.1</td></tr><tr><td>5<sub>4,1</sub>-4<sub>3,2</sub> AA</td><td>299.896762</td><td>-3.763</td><td>110</td><td>36.1</td></tr><tr><td>8<sub>3,5</sub>-7<sub>2,6</sub> AE</td><td>299.899470</td><td>-4.007</td><td>34</td><td>45.4</td></tr><tr><td>8<sub>3,5</sub>-7<sub>2,6</sub> EA</td><td>299.899781</td><td>-4.008</td><td>68</td><td>45.4</td></tr><tr><td>8<sub>3,5</sub>-7<sub>2,6</sub> EE</td><td>299.903025</td><td>-4.008</td><td>272</td><td>45.4</td></tr><tr><td>8<sub>3,5</sub>-7<sub>2,6</sub> AA</td><td>299.906424</td><td>-4.007</td><td>102</td><td>45.4</td></tr><tr><td>29<sub>6,23</sub>-29<sub>5,24</sub> AA</td><td>300.062299</td><td>-3.825</td><td>590</td><td>447.5</td></tr><tr><td>29<sub>6,23</sub>-29<sub>5,24</sub> EE</td><td>300.062707</td><td>-3.825</td><td>944</td><td>447.5</td></tr><tr><td>29<sub>6,23</sub>-29<sub>5,24</sub> AE</td><td>300.063115</td><td>-3.825</td><td>354</td><td>447.5</td></tr><tr><td>29<sub>6,23</sub>-29<sub>5,24</sub> EA</td><td>300.063116</td><td>-3.825</td><td>236</td><td>447.5</td></tr><tr><td colspan="5" style="text-align:center; font-weight:bold;">Methyl Formate (CH<sub>3</sub>OCHO)</td></tr><tr><td>23<sub>5,18</sub>-22<sub>5,17</sub> E, v<sub>t</sub> = 0</td><td>299.097870</td><td>-3.410</td><td>94</td><td>183.9</td></tr><tr><td>25<sub>3,22</sub>-24<sub>3,21</sub> E, v<sub>t</sub> = 1</td><td>299.111695</td><td>-3.406</td><td>102</td><td>387.6</td></tr><tr><td>23<sub>5,18</sub>-22<sub>5,17</sub> A, v<sub>t</sub> = 0</td><td>299.123858</td><td>-3.410</td><td>94</td><td>183.9</td></tr><tr><td>27<sub>1,26</sub>-26<sub>2,25</sub> A, v<sub>t</sub> = 1</td><td>299.128806</td><td>-4.243</td><td>110</td><td>397.2</td></tr><tr><td>27<sub>2,26</sub>-26<sub>2,25</sub> A, v<sub>t</sub> = 1</td><td>299.131294</td><td>-3.397</td><td>110</td><td>397.2</td></tr><tr><td>24<sub>8,16</sub>-23<sub>8,16</sub> E, v<sub>t</sub> = 0</td><td>299.131361</td><td>-4.932</td><td>98</td><td>220.4</td></tr><tr><td>27<sub>1,26</sub>-26<sub>1,25</sub> A, v<sub>t</sub> = 1</td><td>299.133165</td><td>-3.397</td><td>110</td><td>397.2</td></tr><tr><td>27<sub>2,26</sub>-26<sub>1,25</sub> A, v<sub>t</sub> = 1</td><td>299.135510</td><td>-4.243</td><td>110</td><td>397.2</td></tr><tr><td>12<sub>6,6</sub>-11<sub>5,7</sub> A, v<sub>t</sub> = 0</td><td>299.162434</td><td>-4.481</td><td>50</td><td>70.0</td></tr><tr><td>24<sub>7,18</sub>-23<sub>7,17</sub> E, v<sub>t</sub> = 0</td><td>300.020579</td><td>-3.426</td><td>98</td><td>210.9</td></tr><tr><td>24<sub>7,18</sub>-23<sub>7,17</sub> A, v<sub>t</sub> = 0</td><td>300.035621</td><td>-3.425</td><td>98</td><td>210.9</td></tr><tr><td>24<sub>6,19</sub>-23<sub>6,18</sub> E, v<sub>t</sub> = 0</td><td>300.046229</td><td>-3.415</td><td>98</td><td>202.7</td></tr><tr><td>24<sub>6,19</sub>-23<sub>6,18</sub> A, v<sub>t</sub> = 0</td><td>300.063893</td><td>-3.415</td><td>98</td><td>202.7</td></tr><tr><td>26<sub>3,24</sub>-25<sub>3,23</sub> E, v<sub>t</sub> = 0</td><td>300.070744</td><td>-3.398</td><td>106</td><td>206.8</td></tr><tr><td>34<sub>2,32</sub>-34<sub>1,33</sub> A, v<sub>t</sub> = 0</td><td>300.071349</td><td>-4.903</td><td>138</td><td>340.3</td></tr><tr><td>34<sub>3,32</sub>-34<sub>2,33</sub> A, v<sub>t</sub> = 0</td><td>300.072325</td><td>-4.903</td><td>138</td><td>340.3</td></tr><tr><td>26<sub>3,24</sub>-25<sub>3,23</sub> A, v<sub>t</sub> = 0</td><td>300.079079</td><td>-3.398</td><td>106</td><td>206.8</td></tr><tr><td colspan="5" style="text-align:center; font-weight:bold;">Methanol (CH<sub>3</sub>OH)</td></tr><tr><td>4<sub>-3,2</sub>-5<sub>-2,4</sub> E, v<sub>t</sub> = 0</td><td>288.705624</td><td>-4.734</td><td>36</td><td>70.9</td></tr><tr><td>6<sub>0,6</sub>-5<sub>0,5</sub> E, v<sub>t</sub> = 0</td><td>289.939377</td><td>-3.976</td><td>52</td><td>61.8</td></tr><tr><td>6<sub>1,6</sub>-5<sub>1,5</sub> E, v<sub>t</sub> = 0</td><td>290.069747</td><td>-3.987</td><td>52</td><td>54.3</td></tr><tr><td>6<sub>0,6</sub>-5<sub>0,5</sub> A, v<sub>t</sub> = 0</td><td>290.110637</td><td>-3.975</td><td>52</td><td>48.7</td></tr><tr><td>6<sub>5,2</sub>-5<sub>5,1</sub> E, v<sub>t</sub> = 0</td><td>290.117786</td><td>-4.489</td><td>52</td><td>172.8</td></tr><tr><td>6<sub>-5,1</sub>-5<sub>-5,0</sub> E, v<sub>t</sub> = 0</td><td>290.138889</td><td>-4.489</td><td>52</td><td>184.8</td></tr><tr><td>6<sub>5,2</sub>-5<sub>5,0</sub> A, v<sub>t</sub> = 0</td><td>290.145085</td><td>-4.487</td><td>52</td><td>186.6</td></tr><tr><td>6<sub>5,1</sub>-5<sub>5,1</sub> A, v<sub>t</sub> = 0</td><td>290.145085</td><td>-4.487</td><td>52</td><td>186.6</td></tr><tr><td>6<sub>4,3</sub>-5<sub>4,2</sub> A, v<sub>t</sub> = 0</td><td>290.161348</td><td>-4.229</td><td>52</td><td>129.1</td></tr><tr><td>6<sub>4,2</sub>-5<sub>4,1</sub> A, v<sub>t</sub> = 0</td><td>290.161352</td><td>-4.229</td><td>52</td><td>129.1</td></tr><tr><td>6<sub>4,3</sub>-5<sub>4,2</sub> E, v<sub>t</sub> = 0</td><td>290.162356</td><td>-4.230</td><td>52</td><td>136.6</td></tr><tr><td>6<sub>-4,2</sub>-5<sub>-4,1</sub> E, v<sub>t</sub> = 0</td><td>290.183289</td><td>-4.227</td><td>52</td><td>144.7</td></tr><tr><td>6<sub>2,5</sub>-5<sub>2,4</sub> A, v<sub>t</sub> = 0</td><td>290.184674</td><td>-4.023</td><td>52</td><td>86.5</td></tr><tr><td>6<sub>3,4</sub>-5<sub>3,3</sub> A, v<sub>t</sub> = 0</td><td>290.189515</td><td>-4.100</td><td>52</td><td>98.5</td></tr><tr><td>6<sub>3,3</sub>-5<sub>3,2</sub> A, v<sub>t</sub> = 0</td><td>290.190549</td><td>-4.100</td><td>52</td><td>98.5</td></tr><tr><td>6<sub>3,3</sub>-5<sub>3,2</sub> E, v<sub>t</sub> = 0</td><td>290.209695</td><td>-4.097</td><td>52</td><td>111.5</td></tr><tr><td>6<sub>-3,4</sub>-5<sub>-3,3</sub> E, v<sub>t</sub> = 0</td><td>290.213180</td><td>-4.098</td><td>52</td><td>96.5</td></tr><tr><td>6<sub>-1,5</sub>-5<sub>-1,4</sub> E, v<sub>t</sub> = 0</td><td>290.248685</td><td>-3.975</td><td>52</td><td>69.8</td></tr><tr><td>6<sub>2,4</sub>-5<sub>2,3</sub> A, v<sub>t</sub> = 0</td><td>290.264068</td><td>-4.022</td><td>52</td><td>86.5</td></tr><tr><td>6<sub>2,4</sub>-5<sub>2,3</sub> E, v<sub>t</sub> = 0</td><td>290.307281</td><td>-4.024</td><td>52</td><td>74.7</td></tr><tr><td>6<sub>-2,5</sub>-5<sub>-2,4</sub> E, v<sub>t</sub> = 0</td><td>290.307738</td><td>-4.029</td><td>52</td><td>71.0</td></tr><tr><td colspan="5" style="text-align:center; font-weight:bold;">Acetaldehyde (CH<sub>3</sub>CHO)</td></tr><tr><td>15<sub>6,10</sub>-14<sub>6,9</sub> A, v<sub>t</sub> = 0</td><td>289.149836</td><td>-3.157</td><td>62</td><td>192.2</td></tr></tbody></table>

<table><caption>**Table 3**
(Continued)</caption><thead><tr><th>Transition</th><th>ν<sub>0</sub><br>(GHz)</th><th>log<sub>10</sub>A<sub>ul</sub><br>(s<sup>-1</sup>)</th><th>g<sub>u</sub></th><th>E<sub>u</sub><br>(K)</th></tr></thead><tbody><tr><td>15<sub>6,9</sub>-14<sub>6,8</sub> A, v<sub>t</sub> = 0</td><td>289.149853</td><td>-3.157</td><td>62</td><td>192.2</td></tr><tr><td>15<sub>6,9</sub>-14<sub>6,8</sub> E, v<sub>t</sub> = 0</td><td>289.163254</td><td>-3.157</td><td>62</td><td>192.2</td></tr><tr><td>15<sub>6,10</sub>-14<sub>6,9</sub> E, v<sub>t</sub> = 0</td><td>289.200782</td><td>-3.157</td><td>62</td><td>192.1</td></tr><tr><td>7<sub>2,5</sub>-6<sub>1,6</sub> E, v<sub>t</sub> = 0</td><td>289.238518</td><td>-4.459</td><td>30</td><td>35.1</td></tr><tr><td>15<sub>5,11</sub>-14<sub>5,10</sub> A, v<sub>t</sub> = 0</td><td>289.272555</td><td>-3.132</td><td>62</td><td>167.5</td></tr><tr><td>15<sub>5,10</sub>-14<sub>5,9</sub> A, v<sub>t</sub> = 0</td><td>289.273794</td><td>-3.132</td><td>62</td><td>167.5</td></tr><tr><td>15<sub>5,10</sub>-14<sub>5,9</sub> E, v<sub>t</sub> = 0</td><td>289.316721</td><td>-3.132</td><td>62</td><td>167.4</td></tr><tr><td>15<sub>5,11</sub>-14<sub>5,10</sub> E, v<sub>t</sub> = 0</td><td>289.322537</td><td>-3.132</td><td>62</td><td>167.4</td></tr><tr><td>15<sub>4,12</sub>-14<sub>4,11</sub> A, v<sub>t</sub> = 0</td><td>289.506111</td><td>-3.112</td><td>62</td><td>147.2</td></tr><tr><td>15<sub>4,11</sub>-14<sub>4,10</sub> A, v<sub>t</sub> = 0</td><td>289.558746</td><td>-3.112</td><td>62</td><td>147.3</td></tr><tr><td>15<sub>4,11</sub>-14<sub>4,10</sub> E, v<sub>t</sub> = 0</td><td>289.562297</td><td>-3.112</td><td>62</td><td>147.2</td></tr><tr><td>15<sub>4,12</sub>-14<sub>4,11</sub> E, v<sub>t</sub> = 0</td><td>289.585326</td><td>-3.112</td><td>62</td><td>147.2</td></tr><tr><td>15<sub>3,13</sub>-14<sub>3,12</sub> A, v<sub>t</sub> = 0</td><td>289.602928</td><td>-3.097</td><td>62</td><td>131.5</td></tr><tr><td>15<sub>3,13</sub>-14<sub>3,12</sub> E, v<sub>t</sub> = 0</td><td>289.716267</td><td>-3.101</td><td>62</td><td>131.4</td></tr><tr><td>7<sub>2,5</sub>-6<sub>1,6</sub> A, v<sub>t</sub> = 0</td><td>289.942040</td><td>-4.371</td><td>30</td><td>35.0</td></tr><tr><td>16<sub>0,16</sub>-15<sub>0,15</sub> E, v<sub>t</sub> = 0</td><td>299.174780</td><td>-3.038</td><td>66</td><td>123.6</td></tr><tr><td>16<sub>0,16</sub>-15<sub>0,15</sub> A, v<sub>t</sub> = 0</td><td>299.218175</td><td>-3.038</td><td>66</td><td>123.5</td></tr></tbody></table>

**Note.** For (tentatively) detected species (CH₃OCH₃ and CH₃OCHO), all transitions that could be responsible for the observed signals are listed. For nondetected species (CH₃OH and CH₃CHO), only strong transitions (log₁₀Aₐ₉₁(s⁻¹) ≥ -5 and Eₐ₉₁ ≤ 200 K) that effectively constrain the column upper limits are listed.

spectra of rotating disks, but given the unresolved emission, the low S/N, and the low spectral resolution, we do not perform a detailed line shape modeling here and it is partly addressed in the radiative transfer modeling (Appendix C).

We explore the parameter space with the affine-invariant MCMC algorithm implemented in the Python package `emcee` (Foreman-Mackey et al. 2013) using uniform prior distributions:

$$ 
\begin{align*}
T_{\text{ex}} (\text{K}) &= U(5, 1000), \\
\Delta V_{\text{FWHM}} (\text{km s}^{-1}) &= U(0.0, 100.0), \\
v_{\text{sys}} (\text{km s}^{-1}) &= U(-5.0, 15.0), \\
\log_{10} N (\text{X}) (\text{cm}^{-2}) &= U(10.0, 20.0),
\end{align*}
$$

where $U(a, b)$ denotes the uniform distributions between $a$ and $b$, and $X$ represents the four molecular species. We exclude the following frequency ranges from the fit, due to the contamination from other molecular lines and telluric absorption lines: [288.85, 289.00], [289.62, 289.66], [300.13, 300.18], and [300.64, 300.70] in gigahertz. We run 50 walkers for 10,000 steps to sample the posterior distributions, and the initial 2000 steps are discarded as burn-in. Figure 4 shows the parameter covariances and marginalized posterior distributions. Only the upper limits on the CH₃OH and CH₃CHO column density are constrained due to the nondetection. The median value of the systemic velocity (≈4.3 km s⁻¹) is slightly shifted from the literature value (≈5.1 km s⁻¹; Piétu et al. 2007; Teague et al. 2021) due to the coarse channel width (≈4 km s⁻¹), and these are consistent within the uncertainty. From these posterior samples, we compute the column density ratios relative to CH₃OH, where N(CH₃OCH₃)/N(CH₃OH) is constrained to be >7 (0.15th percentile, corresponding to 3σ for a Gaussian distribution). For CH₃OCHO, while the 0.15th percentile of N(CH₃OCH₃)/N(CH₃OH) is very small, due to the long tail in
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Parameter covariances and marginalized posterior distributions for the MCMC spectral fit. The values and errors at the top of each panel indicate the 16th, 50th (median), and 84th percentiles of the posterior distributions.</figcaption></figure>

the posterior distributions of N(CH₃OCHO) (see Figure 4), the median values of N(CH₃OCHO) divided by the upper limit of N(CH₃OH) are listed in Table 2 and added in Figure 2 as a reference. For CH₃CHO, we do not obtain meaningful constraints due to the nondetection.

## Appendix C
### Parametric Disk Model

Here, we demonstrate a detailed radiative transfer calculation of a disk model that aims to constrain what spatial distributions of COMs in the inner disk can explain the observed CH₃OCH₃

emission with a more realistic disk physical structure. Additionally, we examine if the difference in the spatial distributions of CH₃OCH₃ and CH₃OH can explain the nondetection of CH₃OH, as discussed in Section 4.1.

As the disk density and temperature structures, we used the disk model tailored to MWC 480 in Zhang et al. (2021). The details of the model are presented in the original paper. Briefly, the model assumed two populations (small and large) of dust grains, and the distributions of large dust grains are constrained from the high-resolution ALMA observations. The small dust grains are assumed to be completely coupled with the gas.
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Left and middle: gas density (left) and temperature (middle) structure of the disk model. The dashed lines in the middle panel enclose the region where molecules distribute for each boundary temperature in the models (80, 100, 120, and 140 K from outside). The upper boundary of the regions is $z/r = 0.3$, which is also depicted by the dashed lines. Right: comparison of the observed and modeled spectra of the CH$_3$OCH$_3$ transitions at ~299.89 GHz for the models, with different boundary temperatures of CH$_3$OCH$_3$ (blue for the 80 K model and orange for the 100 K model). The spectra of these models are nearly identical and overlap.</figcaption></figure>

<table><caption>**Table 4**
Disk Modeling Results</caption><thead><tr><th>r<sub>in</sub><br>(au)</th><th>r<sub>out</sub><br>(au)</th><th>(z/r)<sub>min</sub></th><th>(z/r)<sub>max</sub></th><th>T<sub>sub(CH3OCH3)</sub><br>(K)</th><th>T<sub>sub(CH3OH)</sub><br>(K)</th><th>x(CH3OCH3)</th><th>x(CH3OH)</th><th>x(CH3OCH3)/x(CH3OH)</th></tr></thead><tbody><tr><td>0.5</td><td>30</td><td>0.0</td><td>0.3</td><td>80</td><td>100</td><td>3.0 × 10<sup>-8</sup></td><td>&lt;3.1 × 10<sup>-8</sup></td><td>&gt;0.97</td></tr><tr><td>0.5</td><td>30</td><td>0.0</td><td>0.3</td><td>80</td><td>120</td><td>3.0 × 10<sup>-8</sup></td><td>&lt;1.2 × 10<sup>-7</sup></td><td>&gt;0.25</td></tr><tr><td>0.5</td><td>30</td><td>0.0</td><td>0.3</td><td>100</td><td>120</td><td>1.8 × 10<sup>-7</sup></td><td>&lt;1.0 × 10<sup>-7</sup></td><td>&gt;1.8</td></tr><tr><td>0.5</td><td>30</td><td>0.0</td><td>0.3</td><td>100</td><td>140</td><td>1.8 × 10<sup>-7</sup></td><td>&lt;5.8 × 10<sup>-7</sup></td><td>&gt;0.31</td></tr></tbody></table>

While in the original model, the gas temperature structure is computed by thermochemical modeling, given the uncertainty of the modeling in the inner region, here we simply assume that the gas temperature is the same as the dust temperature (surface-area-weighted over both populations) computed with thermal Monte Carlo simulations using the RADMC-3D package (Dullemond et al. 2012). The gas density and temperature distributions used for the present calculations are shown in Figure 5.

The distributions of the warm CH$_3$OCH$_3$ and CH$_3$OH gas are assumed based on the dust temperature, which controls the ice sublimation. Since in general the sublimation temperature of a specific molecule is proportional to the binding energy of the molecule (e.g., Furuya & Aikawa 2014), the sublimation temperature ratio between CH$_3$OCH$_3$ and CH$_3$OH should be the same as the binding energy ratio. Although the absolute values of binding energy for CH$_3$OCH$_3$ and CH$_3$OH show some scatter, depending on the experimental methods and the composition of the surface, there is a trend that CH$_3$OCH$_3$ has a lower binding energy than CH$_3$OH. The experimental estimates of the binding energies of CH$_3$OCH$_3$ (≈4000 K; Lattelais et al. 2011) and CH$_3$OH (≈5000–5500 K; Ferrero et al. 2020; Minissale et al. 2022) thus lead to the sublimation temperature ratio of $T_{sub(CH3OH)}/T_{sub(CH3OCH3)} \approx 1.2-1.4$. Based on this ratio and the gas density of the inner disk, we constructed a series of models in which CH$_3$OCH$_3$ and CH$_3$OH distribute with a constant abundance $x$ (relative to the H$_2$ molecule) in the regions where the dust temperature is higher than 80 and 100 K for CH$_3$OCH$_3$ and higher than 100, 120, and 140 K for CH$_3$OH. This emulates the drastic gas-phase abundance increases due to thermal desorption. We additionally set an upper boundary of $z/r \lesssim 0.3$, reflecting the expectation that molecules can be destroyed in the irradiated surface of the disk (e.g., Walsh et al. 2014). Outside these boundaries, the abundances of CH$_3$OH and CH$_3$OCH$_3$ are set to zero, which is a suitable first-order approximation, given the steep (exponential) dependence of gaseous molecular abundances

on visual extinction and dust temperature around its sublimation temperature. The dependence of COM abundances on other parameters such as gas density is weaker (e.g., Hasegawa et al. 1992; Furuya & Aikawa 2014). This resulted in the distributions of warm CH$_3$OCH$_3$ and CH$_3$OH gas all within ~20–30 au (the middle panel of Figure 5). The velocity structure of the warm gas is assumed to be in Keplerian rotation with a known stellar mass of $2.1M_\odot$ (e.g., Teague et al. 2021).

For the models with different combinations of boundary temperatures, as listed in Table 4, which have at maximum a sublimation temperature ratio of 1.5 as the most extreme case, we compute image cubes covering the frequency ranges of the robustly detected CH$_3$OCH$_3$ transitions at ~299.89 GHz and the undetected CH$_3$OH transitions at 290.0–290.4 GHz using the RADMC-3D package (Dullemond et al. 2012). The resulting image cubes are convolved with the observing beam, continuum-subtracted, and spatially integrated to create the disk-integrated spectra, which are then compared to the observed spectra. The abundances $x(CH_3OCH_3)$ and $x(CH_3OH)$ are adjusted to fit the observed spectra.

The right panel of Figure 5 compares the best-fit model with the observed spectrum of CH$_3$OCH$_3$ at ~299.89 GHz. Both models with boundary temperatures of 80 and 100 K reproduce the spectrum well, particularly the broad line width. Furthermore, the nondetection of CH$_3$OH places constraints on the abundance ratio $x(CH_3OCH_3)/x(CH_3OH)$ of the warm gas, as listed in Table 4. The constrained lower limits of the abundance ratios span ~0.3–2, depending on the combination of the boundary temperatures. These values are lower than the column density ratio derived in the spectral analysis in Section 3.2 (where the same distributions are implicitly assumed) by a factor of ~3–20. This indicates that the spatial distribution is a critical factor in measuring the correct abundance ratio. Further spatially resolved observations are necessary. It should be noted that the disk physical structure used in this modeling is an extrapolation of the outer disk structure. The actual structure can vary—for example, due to
the accretion heating that could modify the temperature structure of the inner disk. Constraining the inner disk structure is also essential to inferring the inner disk chemistry.

## Appendix D
### Detection of a New Submillimeter Source

We report the detection of a new submillimeter source at a separation of ≈5.2 from the central star with a position angle of 212° (Figure 6). The emission is compact and reaches a peak S/N of ≈7 on the image made with a Briggs robust value of 0.5. The coordinate of the continuum peak is α(J2000) = 4h58m46s066; δ(J2000) = 29d50m32s04. To characterize the emission, we performed a Gaussian fit to this component with the CASA task `imfit`, which obtained a deconvolved size of 0.33 × 0.13 (PA = 179°) and a flux density of 0.32 mJy. Although the origin of this emission is unclear, this component is located outside the $^{12}$CO gas disk analyzed in Teague et al. (2021), and thus it is unlikely that the emission is attributed to the circumplanetary disk around a forming planet in the MWC 480 disk. It is possible that this originates from the disk or envelope around a field star or from a background galaxy.

<figure><img src="image_9.png" /><figcaption>**Figure 6.** 1.03 mm continuum image made with a Briggs robust value of 0.5. The inset panel presents the zoomed-in view of the vicinity of the new submillimeter source. The beam size of 0.31 × 0.21 (PA = 82°) is shown in the lower left corner of each panel. A scale bar is shown in the lower right corner of each panel.</figcaption></figure>

## ORCID iDs

Yoshihide Yamato https://orcid.org/0000-0003-4099-6941

Yuri Aikawa https://orcid.org/0000-0003-3283-6884

Viviana V. Guzmán https://orcid.org/0000-0003-4784-3040

Kenji Furuya https://orcid.org/0000-0002-2026-8157

Shota Notsu https://orcid.org/0000-0003-2493-912X

Gianni Cataldi https://orcid.org/0000-0002-2700-9676

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

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

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

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

Richard Teague https://orcid.org/0000-0003-1534-5186

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

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