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

# Chemistry of Protostellar Clumps in the High-mass, Star-forming Filamentary Infrared Dark Cloud G034.43+00.24*

Hong-Li Liu<sup>1,11</sup>⑤, Patricio Sanhueza<sup>2,3</sup>⑤, Tie Liu<sup>4</sup>⑤, Annie Zavagno<sup>5,6</sup>, Xin-Di Tang<sup>7,8</sup>⑤, Yuefang Wu<sup>9,10</sup>⑤, and Siju Zhang<sup>5</sup>

<sup>1</sup>Departamento de Astronomía, Universidad de Concepción, Av. Esteban Iturra s/n, Distrito Universitario, 160-C, Chile; hongliliu2012@gmail.com

<sup>2</sup>National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

<sup>3</sup>Department of Astronomical Science, SOKENDAI (The Graduate University for Advanced Studies), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

<sup>4</sup>Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, People's Republic of China

<sup>5</sup>Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France

<sup>6</sup>Institut Universitaire de France (IUF), France

<sup>7</sup>Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, People's Republic of China

<sup>8</sup>Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 830011 Urumqi, People's Republic of China

<sup>9</sup>Astronomy Department, School of Physics, Peking University, 5 Yi He Yuan Road, Beijing 100871, People's Republic of China

<sup>10</sup>Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yi He Yuan Road, Beijing 100871, People's Republic of China

*Received 2020 April 15; revised 2020 August 3; accepted 2020 August 4; published 2020 September 18*

## Abstract

To search for the potential chemical dependence on physical conditions, we have carried out the study of chemistry on the nine protostellar clumps of the high-mass star-forming infrared dark cloud G034.43+00.24, with observations of several ~1 mm lines by the Atacama Pathfinder EXperiment telescope. They include CO/$^{13}$CO/$^{18}$O (2–1), HCO$^+$/H$^{13}$CO$^+$ (3–2), HCN/H$^{13}$CN (3–2), HNC (3–2), CS/C$^{34}$S (5–4), SiO (6–5), SO (6–5), p-H$_2$CO (3–2), and CH$_3$OH (5–4). All the clumps are simply grouped into two classes: high-luminosity protostellar clumps (MM1–MM4, $L_{bol} > \sim 10^3 L_\odot$) and low-luminosity protostellar clumps (MM5–MM9, $L_{bol} \ll 10^3 L_\odot$). Our observations indicate that $^{13}$CO suffers either no or low depletion in the clump environment of G034.43+00.24 as characterized by a nearly constant level of the $^{13}$CO abundance distribution. For the remaining relatively dense gas tracers, we find that their abundances tend to get enhanced in the high-luminosity protostellar clumps as opposed to the low-luminosity counterparts. We suggest that for most, if not all, of the dense gas tracers the high abundance mainly arises from both the high luminosities and associated outflows of the high-luminosity protostellar clumps while the low abundance could be due to the lack of such active star-forming activities in the low-luminosity protostellar clumps.

*Unified Astronomy Thesaurus concepts: Astrochemistry (75); Chemical abundances (224); Interstellar medium (847); Interstellar molecules (849); Star formation (1569)*

## 1. Introduction

Infrared dark clouds (IRDCs) have been the subject of great research interest since they were revealed as dark patches against the bright mid-infrared (IR) background from several large Galactic plane surveys by the Infrared Space Observatory (Perault et al. 1996), Midcourse Space Experiment (MSX; Egan et al. 1998), and Spitzer (Peretto & Fuller 2009). The first studies of IRDCs (e.g., Carey et al. 1998, 2000) suggested that they are cold (<25 K), massive (~$10^2–10^4 M_\odot$), and dense ($\ge 10^4$ cm$^{-3}$) molecular clouds, typically arranged in filamentary and/or globule structures with compact clumps/cores. Several surveys in (sub)millimeter continuum and spectral lines have followed (e.g., Teyssier et al. 2002; Pillai et al. 2006; Ragan et al. 2006; Rathborne et al. 2006, 2010; Simon et al. 2006; Dirienzo et al. 2015; Sanhueza et al. 2019) and have established that massive IRDCs harbor the precursors and early stages of high-mass star and cluster formation. The clue of high-mass star formation can be inferred from massive starless and/or prestellar cores (e.g., Yuan et al. 2018), the spectral energy distribution (SED) fit of massive young stellar objects (YSOs; e.g., Rathborne et al. 2005; Guzmán et al. 2015).

hot cores (e.g., Kurtz et al. 2000; Rathborne et al. 2011), hyper- and ultracompact H II (Churchwell 2002; Hoare et al. 2007), and some masers (e.g., class II CH$_3$OH maser) exclusively tracing high-mass star formation (e.g., Wang et al. 2014).

Moreover, IRDCs provide us with the possibility of investigating the rich chemistry of high-mass star-forming regions owing to a large variety of early evolutionary stages that the IRDCs harbor. The chemical evolution of star-forming regions starts with the starless IRDCs (e.g., van Dishoeck & Blake 1998; Miettinen 2012). Once the cloud collapse takes place, the decreasing temperature and increasing density accelerate the gas-phase depletion of molecules onto grain mantles. At this stage, the so-called dark cloud chemistry is dominated by (gas-phase) ion-molecule reactions (see van Dishoeck & Blake 1998 for a review). If the IRDCs evolve into the protostellar stage, the radiative heating and/or outflows from the central stars act to release molecules from the icy grain mantles into the gas phase. The hot core chemistry is driven by the evaporation of icy grain mantles, characterized by high abundances of fully hydrogenated molecules such as H$_2$O (e.g., Cernicharo et al. 1990; Gensheimer et al. 1996) and a rich variety of complex organic molecules (e.g., CH$_3$OH, CH$_3$CN; see Menten et al. 1988; Olmi et al. 1996). Since the chemistry of a star-forming region depends sensitively on the relevant physical conditions (e.g., temperature, density, and ionization fraction), understanding the chemical composition of IRDCs is

<footnote>* This publication is based on data acquired with the Atacama Pathfinder Experiment (APEX) under program ID 0104.F-9708. APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.</footnote>

<footnote n="11"><sup>11</sup>FONDECYT Postdoctoral Fellowship, Chile.</footnote>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Overview of the high-mass star-forming filamentary IRDC G034.43+00.24 in the Spitzer 8 µm image. The solid- and dashed-line rectangles represent the mapping areas by APEX in two frequency bands centered at 220 and 266.5 GHz, respectively. The red plus signs indicate the locations of the nine clumps identified in Rathborne et al. (2006), which are also the center positions of APEX single-pointing observations in the third frequency band centered at 255 GHz. The bottom left gives a circle with 26" in diameter, a typical beam size of the APEX observations around three frequencies. The green symbols indicate the candidate YSOs identified from the SED fit in Shepherd et al. (2007), among which the candidate high-mass ($M_* > 8 M_\odot$) YSOs are symbolized in the green circles.</figcaption></figure>

an alternative and key approach to gain insights into the physics of the early stages of high-mass star formation.

The focus of this study is on a high-mass star-forming filamentary IRDC, G034.43+00.24 (hereafter IRDC G34). It was first identified as an extinction feature in the MSX 8 µm Galactic plane survey (Simon et al. 2006). Located at 3.7 kpc (e.g., Rathborne et al. 2005; Xu et al. 2016), IRDC G34 is arranged as a straight filament (see Figure 1). This filamentary IRDC contains, based on 11"-resolution 1.2 mm observations, nine millimeter-dense clumps called MM1, MM2, ..., and MM9 in order of decreasing peak flux at 1.2 mm (Rathborne et al. 2006). Note that we do not follow the core nomenclature adopted in Rathborne et al. since the size of all sources (see Table 1) does not match the typical core size of 0.1 pc. Most recently, Soam et al. (2019) and Tang et al. (2019) reported ordered and strong (~mG) magnetic fields in this filament and argued that magnetic field plays an important role in regulating filament fragmentation into clumps. All of these clumps are coincided spatially with candidate (high-mass, $M_* > 8 M_\odot$) YSOs identified from the SEDs of mid-infrared sources by Shepherd et al. (2007), whereby they are all probably protostellar clumps (Foster et al. 2014). Particularly, some of the clumps showcase additional signs of star formation, such as an associated ultracompact H II (UC H II) region in MM2 (e.g., Bronfman et al. 1996), hot corinos (Sakai et al. 2013) and class I CH$_3$OH maser (Yanagida et al. 2014) in MM3, and

outflows in the MM1-MM4 clumps (Rathborne et al. 2005; Sanhueza et al. 2010). In the context of the bolometric luminosity, the nine clumps can be simply grouped to be two classes, high-luminosity protostellar clumps (i.e., MM1-MM4) and low-luminosity protostellar clumps (i.e., MM5-MM9). The high-luminosity protostellar clumps have luminosities of $>\sim 10^3 L_\odot$ (see Table 1), whereby they are suggested to be sites of active high-mass star formation (Rathborne et al. 2005). In contrast, the luminosities of the low-luminosity protostellar clumps are much lower, less than 200 $L_\odot$. These clumps may be either forming low-mass stars or accumulating surrounding material to form high-mass stars eventually (Foster et al. 2014). The presence of the YSOs and UC H II region, along with the high luminosities, suggests that the filamentary IRDC G34 harbors the early stages of high-mass star formation. Although several physical characteristics of the clumps of IRDC G34 have been determined (see above), their chemistry remains poorly explored owing to a lack of a systematic and comprehensive study. To our knowledge, only one previous study (Sanhueza et al. 2012) simply mentioned the chemical compositions of the nine clumps among a large sample of 159 sources through the analysis of several high-density line tracers near 90 GHz, but it was not dedicated to the clumps of IRDC G34. Particularly, chemical analysis on IRDC G34 has been carried out in detail by Sakai's team but only for the clump MM3 (Sakai et al. 2013, 2015, 2018).

Here we report our new molecular line observations toward IRDC G34. The aim of these observations is to carry out a comprehensive study to determine both the kinematics on multiple scales from the filament down to clumps and the chemical composition of the clumps. In this work, we will mainly put our focus on characterizing the chemistry of the nine clumps, with an aim to understand whether the chemical composition behaves differently in the aforementioned two classes of protostellar clumps. Our observations were made in a number of molecular tracers at $\sim 1$ mm (see below): CO/$^{13}$CO/C$^{18}$O (2–1), HCO$^+$/H$^{13}$CO$^+$ (3–2), HCN/H$^{13}$CN (3–2), HNC (3–2), CS/C$^{34}$S (5–4), SiO (6–5), SO (6–5), p(ara)-H$_2$CO (3–2), and CH$_3$OH (5–4). The paper is structured as follows: molecular line observations are described in Section 2; the results and analysis follow in Sections 3 and 4; the discussion is given in Section 5, where we explain the difference of the chemistry between the two classes of protostellar clumps; and, finally, a summary is given in Section 6.

## 2. Observations

Observations of several molecular lines at $\sim 1$ mm were conducted in 2019 October, with the Atacama Pathfinder EXperiment (APEX) 12 m telescope (Güsten et al. 2006) at Llano de Chajnantor in the Chilean Andes. The receiver PI230 was used as the front end, while the new eXtended bandwidth Fast Fourier Transform Spectrometer (XFFT S) was used as the back end with 65,536 fixed spectral channels for a 4 GHz bandwidth. Our observations consist of three observing setups, each with four 4 GHz wide spectral windows.

1. Setup A was tuned to be at 220 GHz in the lower side band (LSB) to mainly cover CO/$^{13}$CO/C$^{18}$O (2–1), SO (6–5), and p-H$_2$CO (3–2) at a resolution of $\theta \sim 28''$ and $\delta V = 0.083$ km s$^{-1}$.
<table><caption>**Table 1**

Physical Parameters of Clumps</caption><thead><tr><th>Name</th><th>R.A.</th><th>Decl.</th><th>FWHM<br>(arcsec)</th><th>Size<br>(pc)</th><th>F<sup>int</sup><sub>1.2</sub><br>(Jy)</th><th>T<sub>d</sub><br>(K)</th><th>L<sub>bol</sub><br>(L<sub>☉</sub>)</th><th>Mass<br>(M<sub>☉</sub>)</th><th>log10(N(H<sub>2</sub>))<br>log10(cm<sup>-2</sup>)</th><th>log10(N<sub>scaled</sub>(H<sub>2</sub>))<br>log10(cm<sup>-2</sup>)</th></tr></thead><tbody><tr><td>MM1</td><td>18:53:18.0</td><td>01:25:24</td><td>16</td><td>0.19</td><td>4.01</td><td>38</td><td>24360</td><td>433</td><td>23.64</td><td>23.29</td></tr><tr><td>MM2</td><td>18:53:18.6</td><td>01:24:40</td><td>26</td><td>0.42</td><td>4.33</td><td>38</td><td>14304</td><td>533</td><td>23.73</td><td>23.12</td></tr><tr><td>MM3</td><td>18:53:20.4</td><td>01:28:23</td><td>24</td><td>0.38</td><td>1.02</td><td>29</td><td>934</td><td>171</td><td>23.24</td><td>22.66</td></tr><tr><td>MM4</td><td>18:53:19.0</td><td>01:24:08</td><td>24</td><td>0.38</td><td>0.86</td><td>32</td><td>1164</td><td>115</td><td>23.06</td><td>22.54</td></tr><tr><td>MM5</td><td>18:53:19.8</td><td>01:23:30</td><td>51</td><td>0.89</td><td>2.24</td><td>23</td><td>177</td><td>388</td><td>23.60</td><td>22.80</td></tr><tr><td>MM6</td><td>18:53:18.6</td><td>01:27:48</td><td>37</td><td>0.62</td><td>0.43</td><td>7–48/[19.5]</td><td>2–93</td><td>93–764</td><td>...</td><td>22.33</td></tr><tr><td>MM7</td><td>18:53:18.3</td><td>01:27:13</td><td>28</td><td>0.46</td><td>0.29</td><td>16–43/[19.5]</td><td>15–187</td><td>37–113</td><td>...</td><td>22.27</td></tr><tr><td>MM8</td><td>18:53:16.4</td><td>01:26:20</td><td>31</td><td>0.52</td><td>0.36</td><td>20.5</td><td>79</td><td>39</td><td>22.60</td><td>22.29</td></tr><tr><td>MM9</td><td>18:53:18.4</td><td>01:28:14</td><td>39</td><td>0.67</td><td>0.53</td><td>[19]</td><td>...</td><td>...</td><td>...</td><td>22.40</td></tr></tbody></table>

**Note.** Columns (2)–(6) are the coordinates, angular and physical sizes, and 1.2 mm integrated flux of clumps, respectively, which are obtained from Rathborne et al. (2006); Columns (7)–(10) present the dust temperature, bolometric luminosity, mass, and column density, all of which, except for the values in square brackets, were derived from the SED fit (Rathborne et al. 2010), while those in square brackets were retrieved from the dust temperature map of Foster et al. (2014); Column (11) is the column density derived from the flux scaled to a common size of 28''5 (see Section 5.1).

2. *Setup B* was tuned to be at 266.5 GHz in the upper side band (USB) to mainly cover HCO$^+$, HCN, and HNC (3–2) at $\theta \sim 23''3$ and $\delta V = 0.069$ km s$^{-1}$.

3. *Setup C* was tuned to be at 255 GHz in the USB to mainly cover H$^{13}$CO$^+$ (3–2), H$^{13}$CN (3–2), CS/C$^{34}$S (5–4), SiO (6–5), and CH$_3$OH (5–4) at $\theta \sim 25''8$ and $\delta V = 0.072$ km s$^{-1}$.

*Setups A/B* were employed in the on-the-fly mode to map a large extent of the filament (3'5 × 10'0 for A and 2'5 × 6'5 for B; see also solid- and dashed-line rectangles in Figure 1), while *Setup C* adopted the position-switching mode to carry out single-pointing observations toward the nine dense clumps (i.e., MM1–MM9) as indicated by plus signs in Figure 1. The maps were centered at $\alpha_{2000} = 18^h53^m17^s989$, $\delta_{2000} = +1^\circ25'24''18$ for *Setup A* and at $\alpha_{2000} = 18^h53^m18^s585$, $\delta_{2000} = +1^\circ26'13''12$ for *Setup B*. All observation data were reduced at a velocity resolution of 0.1 km s$^{-1}$ using the CLASS90 program of the IRAM’s GILDAS software (Guilloteau & Lucas 2000) and were converted to be in units of the main-beam brightness temperature ($T_{mb}$) given a main-beam efficiency of 0.8.

In this work, we focus on molecular line observations toward the nine clumps, and we will present the analysis of the observations for the entire IRDC cloud in a forthcoming paper. Thirty transitions from 14 molecular species and isotopologues have been observed in the three setups toward the nine clumps, as listed in Table 2, where the corresponding frequencies, beam sizes, and sensitivities are presented.

## 3. Results

### 3.1. Observed Line Parameters

To extract observed parameters, Gaussian fitting was performed to the spectra of all molecular species detected in the nine clumps. We first extracted the average spectra of all the transitions for these clumps over a common size. The common size is defined as the largest beam size of all detected transitions, estimated to be 28''5 from the p-H$_2$CO transitions. The use of the common size can facilitate the comparison of some parameters (e.g., velocity, abundance) between different species by reducing the measurement errors sensitive to the measurement size.

<table><caption>**Table 2**

Summary of Observed Molecular Lines</caption><thead><tr><th>Line</th><th>F<sub>rest</sub><br>(GHz)</th><th>Beam<br>(arcsec)</th><th>rms<br>(K)</th><th>Mapping Size<br>(arcsec × arcsec)</th></tr></thead><tbody><tr><td>H<sub>2</sub>CO (3<sub>03</sub> – 2<sub>02</sub>)</td><td>218.222</td><td>28.6</td><td>0.09</td><td>210 × 600</td></tr><tr><td>H<sub>2</sub>CO (3<sub>22</sub> – 2<sub>21</sub>)</td><td>218.476</td><td>28.6</td><td>0.09</td><td>210 × 600</td></tr><tr><td>CH<sub>3</sub>OH (4<sub>2</sub> – 3<sub>1</sub>) E</td><td>218.440</td><td>28.6</td><td>0.09</td><td>210 × 600</td></tr><tr><td>H<sub>2</sub>CO (3<sub>21</sub> – 2<sub>20</sub>)</td><td>218.760</td><td>28.5</td><td>0.09</td><td>210 × 600</td></tr><tr><td>C<sup>18</sup>O (2–1)</td><td>219.560</td><td>28.4</td><td>0.09</td><td>210 × 600</td></tr><tr><td>SO (6–5)</td><td>219.949</td><td>28.4</td><td>0.09</td><td>210 × 600</td></tr><tr><td><sup>13</sup>CO (2–1)</td><td>220.399</td><td>28.3</td><td>0.09</td><td>210 × 600</td></tr><tr><td>CO (2–1)</td><td>230.538</td><td>27.1</td><td>0.11</td><td>210 × 600</td></tr><tr><td>C<sup>34</sup>S (5–4)</td><td>241.016</td><td>25.9</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) E</td><td>241.700</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>-1</sub> – 4<sub>-1</sub>) E</td><td>241.767</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) A<sup>+</sup></td><td>241.791</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) A<sup>+</sup></td><td>241.807</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>241.813</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>241.830</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) A<sup>-</sup></td><td>241.833</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>-</sup></td><td>241.842</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>241.844</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>241.852</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>1</sub> – 4<sub>1</sub>) E</td><td>241.879</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>+</sup></td><td>241.888</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>241.904</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>241.905</td><td>25.8</td><td>0.02</td><td>...</td></tr><tr><td>CS (5–4)</td><td>244.936</td><td>25.5</td><td>0.03</td><td>...</td></tr><tr><td>H<sup>13</sup>CN (3–2)</td><td>259.012</td><td>24.1</td><td>0.03</td><td>...</td></tr><tr><td>H<sup>13</sup>CO<sup>+</sup> (3–2)</td><td>260.255</td><td>24.0</td><td>0.03</td><td>...</td></tr><tr><td>SiO (6–5)</td><td>260.518</td><td>24.0</td><td>0.03</td><td>...</td></tr><tr><td>HCN (3–2)</td><td>265.886</td><td>23.5</td><td>0.10</td><td>150 × 390</td></tr><tr><td>HCO<sup>+</sup> (3–2)</td><td>267.558</td><td>23.3</td><td>0.10</td><td>150 × 390</td></tr><tr><td>HNC (3–2)</td><td>271.981</td><td>22.9</td><td>0.11</td><td>150 × 390</td></tr></tbody></table>

**Note.** The rms is estimated at a velocity resolution of 0.1 km s$^{-1}$.

Regardless of p-H$_2$CO and CH$_3$OH, we made use of the single-Gaussian model from the *spectral-cube* python package (Ginsburg 2016) in the fitting to the other species. Although HCN and H$^{13}$CN theoretically have the hyperfine components in their spectra, the actual spectra measured here appear approximately to be a single component, whereby both species were accounted for in the single-Gaussian fitting. We show in
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Example plot of the single-Gaussian fitting. The fitting (red curve) is conducted for 12 out of 14 molecular species detected in MM1. For a good demonstration, the signals above 1 rms are highlighted in the gray shade, corresponding to the velocity range displayed in the upper-left corner of each panel. The dashed line is the systemic velocity inferred from the multi-Gaussian fitting of CH<sub>3</sub>OH (see Figure 3).</figcaption></figure>

Figure 2 an example of the fitting to the average spectra over the common size for 12 transitions toward MM1. As shown in the figure, the single-Gaussian model does not fit the spectrum of CO (2–1) well at all, which happens to all clumps. This result could be due to either the possibility of more than two velocity components or a severe optical depth effect. We therefore will not analyze the parameters of CO (2–1) in this work. For other transitions the fitting is rather good, even though some of them, such as <sup>13</sup>CO (2–1) and HCN (3–2), have two peaks in the spectra, which, however, will not significantly affect the results of the following analysis.

Regarding p-H₂CO and CH₃OH, we performed the multi-Gaussian fitting (Ginsburg 2016), as they have more than one rotational transitions observed in each clump. That is, multiple Gaussian components are implemented to simultaneously fit all detected transitions of a species, assuming the same line width (FWHM) for all the components and fixed separations between different transitions. Figure 3 shows an example of the multi-Gaussian fitting, the fitting to several transitions of CH₃OH

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Example plot of the multiple-Gaussian fitting. The red curve represents the fitting to the multiple transitions of both CH<sub>3</sub>OH at 241 GHz (top panel) and p-H<sub>2</sub>CO at 218 GHz (bottom panel) detected in MM1. Note that in the bottom panel CH<sub>3</sub>OH (4<sub>2</sub>–3<sub>1</sub>) E is fitted simultaneously along with p-H<sub>2</sub>CO transitions.</figcaption></figure>

detected in MM1. Note that in the fitting the spectra of p-H₂CO were smoothed to have a velocity resolution of 0.4 km s⁻¹ in order to improve the signal-to-noise ratio especially for the two weak transitions (i.e., 3<sub>22</sub>–2<sub>21</sub> and 3<sub>21</sub>–2<sub>20</sub>). In addition, CH₃OH (4<sub>2</sub> – 3<sub>1</sub>) E at 218.44 GHz, which is in between the 3<sub>03</sub>–2<sub>02</sub> and 3<sub>22</sub>–2<sub>21</sub> transitions of p-H₂CO, was simultaneously fitted along with the p-H₂CO transitions. In all, the fitting performs very well for both species toward the nine clumps.

Both single and multi-Gaussian fittings provide the following line parameters: the main-beam temperature T<sub>mb</sub>, velocity centroid V<sub>lsr</sub>, and line width FHWM, as listed in Table 3, where the derived integrated line intensities ∫T<sub>mb</sub>dV can also be seen in the last column. The uncertainties of these parameters mainly arise either from the sensitivity of the spectra or from the velocity resolution. As a result, we find that the species CO/<sup>13</sup>CO/C<sup>18</sup>O, HCO<sup>+</sup>/H<sup>13</sup>CO<sup>+</sup>, HCN, HNC, CS, and CH<sub>3</sub>OH have a very high detection rate between 89% and
<table><caption>**Table 3**
Observed Parameters of Line Transitions</caption><thead><tr><th>Name</th><th>Line</th><th>V<sub>lsr</sub><br>(km s<sup>-1</sup>)</th><th>T<sub>mb</sub><br>(K)</th><th>FHWM<br>(km s<sup>-1</sup>)</th><th>∫T<sub>mb</sub>dV<br>(K km s<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="20">MM1</td><td>CO (2–1)</td><td>59.8 ± 0.1</td><td>9.44 ± 0.21</td><td>8.5 ± 0.2</td><td>97.7 ± 0.6</td></tr><tr><td><sup>13</sup>CO (2–1)</td><td>58.0 ± 0.1</td><td>7.26 ± 0.05</td><td>4.5 ± 0.2</td><td>35.9 ± 0.1</td></tr><tr><td>C<sup>18</sup>O (2–1)</td><td>57.8 ± 0.1</td><td>2.23 ± 0.05</td><td>3.4 ± 0.2</td><td>8.1 ± 0.1</td></tr><tr><td>HCO<sup>+</sup> (3–2)</td><td>57.9 ± 0.1</td><td>3.09 ± 0.03</td><td>6.1 ± 0.2</td><td>20.4 ± 0.1</td></tr><tr><td>H<sup>13</sup>CO<sup>+</sup> (3–2)</td><td>57.9 ± 0.1</td><td>0.83 ± 0.02</td><td>3.1 ± 0.2</td><td>2.8 ± 0.1</td></tr><tr><td>HCN (3–2)</td><td>57.9 ± 0.1</td><td>2.16 ± 0.08</td><td>10.7 ± 0.2</td><td>24.1 ± 0.2</td></tr><tr><td>H<sup>13</sup>CN (3–2)</td><td>57.7 ± 0.1</td><td>0.66 ± 0.02</td><td>6.4 ± 0.2</td><td>4.7 ± 0.1</td></tr><tr><td>CS (5–4)</td><td>57.8 ± 0.1</td><td>2.55 ± 0.04</td><td>7.0 ± 0.2</td><td>20.9 ± 0.1</td></tr><tr><td>C<sup>34</sup>S (5–4)</td><td>58.0 ± 0.1</td><td>0.50 ± 0.01</td><td>4.8 ± 0.2</td><td>2.5 ± 0.1</td></tr><tr><td>HNC (3–2)</td><td>57.8 ± 0.1</td><td>1.96 ± 0.03</td><td>4.6 ± 0.2</td><td>9.8 ± 0.1</td></tr><tr><td>SiO (6–5)</td><td>57.5 ± 0.1</td><td>0.31 ± 0.02</td><td>11.9 ± 0.2</td><td>4.0 ± 0.1</td></tr><tr><td>SO (6–5)</td><td>57.7 ± 0.1</td><td>0.97 ± 0.05</td><td>4.9 ± 0.2</td><td>5.0 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>03</sub> – 2<sub>02</sub>)</td><td>57.9 ± 0.4</td><td>1.83 ± 0.04</td><td>5.2 ± 0.4</td><td>10.1 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>22</sub> – 2<sub>21</sub>)</td><td>57.9 ± 0.4</td><td>0.60 ± 0.04</td><td>5.2 ± 0.4</td><td>3.3 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>21</sub> – 2<sub>20</sub>)</td><td>57.9 ± 0.4</td><td>0.54 ± 0.04</td><td>5.2 ± 0.4</td><td>3.0 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (4<sub>2</sub> – 3<sub>1</sub>) E</td><td>57.9 ± 0.4</td><td>1.15 ± 0.04</td><td>5.0 ± 0.4</td><td>6.1 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) E</td><td>57.8 ± 0.1</td><td>1.47 ± 0.03</td><td>4.9 ± 0.1</td><td>7.7 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>−1</sub> – 4<sub>−1</sub>) E</td><td>57.8 ± 0.1</td><td>3.06 ± 0.03</td><td>4.9 ± 0.1</td><td>16.0 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) A<sup>+</sup></td><td>57.8 ± 0.1</td><td>3.50 ± 0.03</td><td>4.9 ± 0.1</td><td>18.4 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) A<sup>+</sup></td><td>57.8 ± 0.1</td><td>0.21 ± 0.03</td><td>4.9 ± 0.1</td><td>1.1 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>−4</sub>) E</td><td>57.8 ± 0.1</td><td>0.16 ± 0.03</td><td>4.9 ± 0.1</td><td>0.8 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>57.8 ± 0.1</td><td>0.15 ± 0.03</td><td>4.9 ± 0.1</td><td>0.8 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) A<sup>−</sup></td><td>57.8 ± 0.1</td><td>0.33 ± 0.03</td><td>4.9 ± 0.1</td><td>1.7 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>−</sup></td><td>57.8 ± 0.1</td><td>0.33 ± 0.03</td><td>4.9 ± 0.1</td><td>1.7 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>57.8 ± 0.1</td><td>0.13 ± 0.03</td><td>4.9 ± 0.1</td><td>0.7 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>−3</sub> – 4<sub>−3</sub>) E</td><td>57.8 ± 0.1</td><td>0.22 ± 0.03</td><td>4.9 ± 0.1</td><td>1.1 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>1</sub> – 4<sub>1</sub>) E</td><td>57.8 ± 0.1</td><td>0.89 ± 0.03</td><td>4.9 ± 0.1</td><td>4.7 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>+</sup></td><td>57.8 ± 0.1</td><td>0.37 ± 0.03</td><td>4.9 ± 0.1</td><td>1.9 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>−2</sub> – 4<sub>−2</sub>) E</td><td>57.8 ± 0.1</td><td>0.37 ± 0.03</td><td>4.9 ± 0.1</td><td>1.9 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>57.8 ± 0.1</td><td>0.94 ± 0.03</td><td>4.9 ± 0.1</td><td>4.9 ± 0.1</td></tr><tr><td rowspan="20">MM2</td><td>CO (2–1)</td><td>59.8 ± 0.1</td><td>10.83 ± 0.45</td><td>10.6 ± 0.2</td><td>111.2 ± 1.3</td></tr><tr><td><sup>13</sup>CO (2–1)</td><td>56.5 ± 0.1</td><td>10.13 ± 0.06</td><td>4.8 ± 0.2</td><td>55.8 ± 0.1</td></tr><tr><td>C<sup>18</sup>O (2–1)</td><td>57.1 ± 0.1</td><td>4.18 ± 0.04</td><td>4.5 ± 0.2</td><td>20.3 ± 0.1</td></tr><tr><td>HCO<sup>+</sup> (3–2)</td><td>57.3 ± 0.1</td><td>3.63 ± 0.04</td><td>6.4 ± 0.2</td><td>25.2 ± 0.1</td></tr><tr><td>H<sup>13</sup>CO<sup>+</sup> (3–2)</td><td>57.6 ± 0.1</td><td>0.79 ± 0.01</td><td>3.9 ± 0.2</td><td>3.2 ± 0.1</td></tr><tr><td>HCN (3–2)</td><td>57.2 ± 0.1</td><td>2.03 ± 0.11</td><td>11.6 ± 0.2</td><td>23.7 ± 0.3</td></tr><tr><td>H<sup>13</sup>CN (3–2)</td><td>57.8 ± 0.1</td><td>0.49 ± 0.01</td><td>5.3 ± 0.2</td><td>3.0 ± 0.1</td></tr><tr><td>CS (5–4)</td><td>57.9 ± 0.1</td><td>3.15 ± 0.04</td><td>6.1 ± 0.2</td><td>22.0 ± 0.1</td></tr><tr><td>C<sup>34</sup>S (5–4)</td><td>57.9 ± 0.1</td><td>0.26 ± 0.01</td><td>4.5 ± 0.2</td><td>1.2 ± 0.1</td></tr><tr><td>HNC (3–2)</td><td>57.6 ± 0.1</td><td>2.15 ± 0.03</td><td>4.8 ± 0.2</td><td>11.2 ± 0.1</td></tr><tr><td>SiO (6–5)</td><td>57.4 ± 0.1</td><td>0.25 ± 0.02</td><td>7.7 ± 0.2</td><td>2.0 ± 0.1</td></tr><tr><td>SO (6–5)</td><td>57.6 ± 0.1</td><td>0.87 ± 0.05</td><td>5.0 ± 0.2</td><td>4.4 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>03</sub> – 2<sub>02</sub>)</td><td>57.9 ± 0.4</td><td>2.02 ± 0.04</td><td>5.7 ± 0.4</td><td>12.2 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>22</sub> – 2<sub>21</sub>)</td><td>57.9 ± 0.4</td><td>0.54 ± 0.04</td><td>5.7 ± 0.4</td><td>3.3 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>21</sub> – 2<sub>20</sub>)</td><td>57.9 ± 0.4</td><td>0.51 ± 0.04</td><td>5.7 ± 0.4</td><td>3.1 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (4<sub>2</sub> – 3<sub>1</sub>) E</td><td>58.0 ± 0.4</td><td>0.73 ± 0.04</td><td>5.6 ± 0.4</td><td>4.3 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) E</td><td>57.8 ± 0.1</td><td>0.85 ± 0.02</td><td>5.7 ± 0.1</td><td>5.1 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>−1</sub> – 4<sub>−1</sub>) E</td><td>57.8 ± 0.1</td><td>2.33 ± 0.02</td><td>5.7 ± 0.1</td><td>14.1 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) A<sup>+</sup></td><td>57.8 ± 0.1</td><td>2.78 ± 0.02</td><td>5.7 ± 0.1</td><td>16.9 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>−4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) A<sup>−</sup></td><td>57.8 ± 0.1</td><td>0.09 ± 0.02</td><td>5.7 ± 0.1</td><td>0.5 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>−</sup></td><td>57.8 ± 0.1</td><td>0.10 ± 0.02</td><td>5.7 ± 0.1</td><td>0.6 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>−3</sub> – 4<sub>−3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>1</sub> – 4<sub>1</sub>) E</td><td>57.8 ± 0.1</td><td>0.41 ± 0.02</td><td>5.7 ± 0.1</td><td>2.5 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>−2</sub> – 4<sub>−2</sub>) E</td><td>57.8 ± 0.1</td><td>0.45 ± 0.02</td><td>5.7 ± 0.1</td><td>2.7 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>57.8 ± 0.1</td><td>0.20 ± 0.02</td><td>5.7 ± 0.1</td><td>1.2 ± 0.1</td></tr><tr><td>MM3</td><td>CO (2–1)</td><td>61.0 ± 0.1</td><td>10.06 ± 0.58</td><td>7.5 ± 0.2</td><td>95.3 ± 1.4</td></tr></tbody></table>
<table><caption>**Table 3**
(Continued)</caption><thead><tr><th>Name</th><th>Line</th><th>V<sub>lsr</sub><br>(km s<sup>-1</sup>)</th><th>T<sub>mb</sub><br>(K)</th><th>FHWM<br>(km s<sup>-1</sup>)</th><th>∫T<sub>mb</sub>dV<br>(K km s<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="24">MM4</td><td><sup>13</sup>CO (2–1)</td><td>58.8 ± 0.1</td><td>5.82 ± 0.08</td><td>3.7 ± 0.2</td><td>25.2 ± 0.2</td></tr><tr><td>C<sup>18</sup>O (2–1)</td><td>58.9 ± 0.1</td><td>1.52 ± 0.04</td><td>2.7 ± 0.2</td><td>4.3 ± 0.1</td></tr><tr><td>HCO<sup>+</sup> (3–2)</td><td>59.8 ± 0.1</td><td>1.77 ± 0.03</td><td>5.3 ± 0.2</td><td>10.6 ± 0.1</td></tr><tr><td>H<sup>13</sup>CO<sup>+</sup> (3–2)</td><td>59.4 ± 0.1</td><td>0.33 ± 0.02</td><td>2.0 ± 0.2</td><td>0.7 ± 0.1</td></tr><tr><td>HCN (3–2)</td><td>60.1 ± 0.1</td><td>1.30 ± 0.07</td><td>9.4 ± 0.2</td><td>14.1 ± 0.2</td></tr><tr><td>H<sup>13</sup>CN (3–2)</td><td>59.4 ± 0.1</td><td>0.22 ± 0.02</td><td>4.8 ± 0.2</td><td>1.1 ± 0.1</td></tr><tr><td>CS (5–4)</td><td>59.5 ± 0.1</td><td>1.17 ± 0.04</td><td>7.0 ± 0.2</td><td>9.7 ± 0.1</td></tr><tr><td>C<sup>34</sup>S (5–4)</td><td>59.3 ± 0.1</td><td>0.08 ± 0.02</td><td>3.2 ± 0.2</td><td>0.2 ± 0.1</td></tr><tr><td>HNC (3–2)</td><td>59.5 ± 0.1</td><td>1.04 ± 0.03</td><td>3.7 ± 0.2</td><td>4.2 ± 0.1</td></tr><tr><td>SiO (6–5)</td><td>57.9 ± 0.2</td><td>0.14 ± 0.02</td><td>14.7 ± 0.3</td><td>1.6 ± 0.1</td></tr><tr><td>SO (6–5)</td><td>60.2 ± 0.1</td><td>0.31 ± 0.04</td><td>5.5 ± 0.2</td><td>1.4 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>03</sub> – 2<sub>02</sub>)</td><td>60.0 ± 0.4</td><td>0.95 ± 0.03</td><td>6.1 ± 0.4</td><td>6.1 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>22</sub> – 2<sub>21</sub>)</td><td>60.0 ± 0.4</td><td>0.23 ± 0.03</td><td>6.1 ± 0.4</td><td>1.5 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>21</sub> – 2<sub>20</sub>)</td><td>60.0 ± 0.4</td><td>0.25 ± 0.03</td><td>6.1 ± 0.4</td><td>1.6 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (4<sub>2</sub> – 3<sub>1</sub>) E</td><td>60.0 ± 0.4</td><td>0.42 ± 0.03</td><td>6.2 ± 0.4</td><td>2.7 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) E</td><td>59.7 ± 0.1</td><td>0.59 ± 0.03</td><td>5.8 ± 0.1</td><td>3.7 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>-1</sub> – 4<sub>-1</sub>) E</td><td>59.7 ± 0.1</td><td>1.67 ± 0.03</td><td>5.8 ± 0.1</td><td>10.3 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) A<sup>+</sup></td><td>59.7 ± 0.1</td><td>2.09 ± 0.03</td><td>5.8 ± 0.1</td><td>12.9 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4.4</sub> – 4<sub>4.4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) A<sup>-</sup></td><td>59.7 ± 0.1</td><td>0.07 ± 0.03</td><td>5.8 ± 0.1</td><td>0.4 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>-</sup></td><td>59.7 ± 0.1</td><td>0.07 ± 0.03</td><td>5.8 ± 0.1</td><td>0.4 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3.3</sub> – 4<sub>3.3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>1</sub> – 4<sub>1</sub>) E</td><td>59.7 ± 0.1</td><td>0.26 ± 0.03</td><td>5.8 ± 0.1</td><td>1.6 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2.2</sub> – 4<sub>2.2</sub>) E</td><td>59.7 ± 0.1</td><td>0.12 ± 0.03</td><td>5.8 ± 0.1</td><td>0.7 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>59.7 ± 0.1</td><td>0.30 ± 0.03</td><td>5.8 ± 0.1</td><td>1.8 ± 0.1</td></tr><tr><td>CO (2–1)</td><td>59.3 ± 0.1</td><td>9.36 ± 0.40</td><td>10.2 ± 0.2</td><td>88.2 ± 1.1</td></tr><tr><td><sup>13</sup>CO (2–1)</td><td>56.7 ± 0.1</td><td>9.16 ± 0.05</td><td>3.8 ± 0.2</td><td>38.7 ± 0.1</td></tr><tr><td>C<sup>18</sup>O (2–1)</td><td>57.1 ± 0.1</td><td>3.70 ± 0.04</td><td>3.1 ± 0.2</td><td>12.3 ± 0.1</td></tr><tr><td>HCO<sup>+</sup> (3–2)</td><td>56.8 ± 0.1</td><td>1.77 ± 0.03</td><td>5.3 ± 0.2</td><td>10.4 ± 0.1</td></tr><tr><td>H<sup>13</sup>CO<sup>+</sup> (3–2)</td><td>57.5 ± 0.1</td><td>0.44 ± 0.02</td><td>2.4 ± 0.2</td><td>1.1 ± 0.1</td></tr><tr><td>HCN (3–2)</td><td>55.8 ± 0.1</td><td>0.91 ± 0.06</td><td>8.8 ± 0.2</td><td>8.4 ± 0.1</td></tr><tr><td>H<sup>13</sup>CN (3–2)</td><td>57.3 ± 0.1</td><td>0.18 ± 0.02</td><td>3.4 ± 0.2</td><td>0.6 ± 0.1</td></tr><tr><td>CS (5–4)</td><td>57.3 ± 0.1</td><td>1.09 ± 0.02</td><td>4.2 ± 0.2</td><td>5.3 ± 0.1</td></tr><tr><td>C<sup>34</sup>S (5–4)</td><td>57.2 ± 0.1</td><td>0.10 ± 0.01</td><td>2.1 ± 0.2</td><td>0.2 ± 0.1</td></tr><tr><td>HNC (3–2)</td><td>57.4 ± 0.1</td><td>1.06 ± 0.03</td><td>4.0 ± 0.2</td><td>4.4 ± 0.1</td></tr><tr><td>SiO (6–5)</td><td>56.5 ± 0.1</td><td>0.14 ± 0.02</td><td>4.4 ± 0.2</td><td>0.6 ± 0.1</td></tr><tr><td>SO (6–5)</td><td>57.3 ± 0.1</td><td>0.46 ± 0.03</td><td>3.2 ± 0.2</td><td>1.5 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>03</sub> – 2<sub>02</sub>)</td><td>57.6 ± 0.4</td><td>1.03 ± 0.03</td><td>4.4 ± 0.4</td><td>4.8 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>22</sub> – 2<sub>21</sub>)</td><td>57.6 ± 0.4</td><td>0.25 ± 0.03</td><td>4.4 ± 0.4</td><td>1.2 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>21</sub> – 2<sub>20</sub>)</td><td>57.6 ± 0.4</td><td>0.24 ± 0.03</td><td>4.4 ± 0.4</td><td>1.1 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (4<sub>2</sub> – 3<sub>1</sub>) E</td><td>57.6 ± 0.4</td><td>0.33 ± 0.03</td><td>4.4 ± 0.4</td><td>1.5 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) E</td><td>57.3 ± 0.1</td><td>0.34 ± 0.02</td><td>4.3 ± 0.1</td><td>1.6 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>-1</sub> – 4<sub>-1</sub>) E</td><td>57.3 ± 0.1</td><td>1.26 ± 0.02</td><td>4.3 ± 0.1</td><td>5.8 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) A<sup>+</sup></td><td>57.3 ± 0.1</td><td>1.56 ± 0.02</td><td>4.3 ± 0.1</td><td>7.2 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4.4</sub> – 4<sub>4.4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) A<sup>-</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>-</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3.3</sub> – 4<sub>3.3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>1</sub> – 4<sub>1</sub>) E</td><td>57.3 ± 0.1</td><td>0.13 ± 0.02</td><td>4.3 ± 0.1</td><td>0.6 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2.2</sub> – 4<sub>2.2</sub>) E</td><td>57.3 ± 0.1</td><td>0.11 ± 0.02</td><td>4.3 ± 0.1</td><td>0.5 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>57.3 ± 0.1</td><td>0.13 ± 0.02</td><td>4.3 ± 0.1</td><td>0.6 ± 0.1</td></tr><tr><td>CO (2–1)</td><td>57.3 ± 0.1</td><td>5.21 ± 0.27</td><td>8.6 ± 0.2</td><td>56.7 ± 0.7</td></tr><tr><td><sup>13</sup>CO (2–1)</td><td>57.6 ± 0.1</td><td>5.62 ± 0.03</td><td>3.6 ± 0.2</td><td>21.0 ± 0.1</td></tr></tbody></table>
<table><caption>**Table 3**
(Continued)</caption><thead><tr><th>Name</th><th>Line</th><th>V<sub>lsr</sub><br>(km s<sup>-1</sup>)</th><th>T<sub>mb</sub><br>(K)</th><th>FHWM<br>(km s<sup>-1</sup>)</th><th>∫T<sub>mb</sub>dV<br>(K km s<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="24">MM6</td><td>C<sup>18</sup>O (2–1)</td><td>57.8 ± 0.1</td><td>2.59 ± 0.03</td><td>2.2 ± 0.2</td><td>6.0 ± 0.1</td></tr><tr><td>HCO<sup>+</sup> (3–2)</td><td>57.6 ± 0.1</td><td>0.95 ± 0.02</td><td>3.5 ± 0.2</td><td>3.5 ± 0.1</td></tr><tr><td>H<sup>13</sup>CO<sup>+</sup> (3–2)</td><td>57.9 ± 0.1</td><td>0.13 ± 0.02</td><td>2.2 ± 0.2</td><td>0.3 ± 0.1</td></tr><tr><td>HCN (3–2)</td><td>56.7 ± 0.1</td><td>0.36 ± 0.03</td><td>6.5 ± 0.2</td><td>2.3 ± 0.1</td></tr><tr><td>H<sup>13</sup>CN (3–2)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CS (5–4)</td><td>57.8 ± 0.1</td><td>0.25 ± 0.02</td><td>2.1 ± 0.2</td><td>0.5 ± 0.1</td></tr><tr><td>C<sup>34</sup>S (5–4)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>HNC (3–2)</td><td>57.8 ± 0.1</td><td>0.59 ± 0.02</td><td>2.4 ± 0.2</td><td>1.4 ± 0.1</td></tr><tr><td>SiO (6–5)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>SO (6–5)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO (3<sub>03</sub> – 2<sub>02</sub>)</td><td>58.0 ± 0.4</td><td>0.30 ± 0.02</td><td>2.8 ± 0.4</td><td>0.9 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>22</sub> – 2<sub>21</sub>)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO (3<sub>21</sub> – 2<sub>20</sub>)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (4<sub>2</sub> – 3<sub>1</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>1</sub> – 4<sub>1</sub>) E</td><td>57.6 ± 0.1</td><td>0.14 ± 0.03</td><td>3.4 ± 0.1</td><td>0.5 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) A<sup>+</sup></td><td>57.6 ± 0.1</td><td>0.17 ± 0.03</td><td>3.4 ± 0.1</td><td>0.6 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) A<sup>−</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>−</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>1</sub> – 4<sub>1</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CO (2–1)</td><td>59.8 ± 0.1</td><td>6.99 ± 0.23</td><td>4.8 ± 0.2</td><td>70.2 ± 0.7</td></tr><tr><td><sup>13</sup>CO (2–1)</td><td>58.4 ± 0.1</td><td>5.14 ± 0.05</td><td>3.3 ± 0.2</td><td>18.7 ± 0.1</td></tr><tr><td>C<sup>18</sup>O (2–1)</td><td>58.3 ± 0.1</td><td>1.90 ± 0.05</td><td>2.1 ± 0.2</td><td>4.2 ± 0.1</td></tr><tr><td>HCO<sup>+</sup> (3–2)</td><td>58.4 ± 0.1</td><td>0.73 ± 0.03</td><td>2.8 ± 0.2</td><td>2.2 ± 0.1</td></tr><tr><td>H<sup>13</sup>CO<sup>+</sup> (3–2)</td><td>58.3 ± 0.1</td><td>0.09 ± 0.01</td><td>1.3 ± 0.2</td><td>0.1 ± 0.1</td></tr><tr><td>HCN (3–2)</td><td>57.9 ± 0.1</td><td>0.25 ± 0.03</td><td>4.8 ± 0.2</td><td>1.3 ± 0.1</td></tr><tr><td>H<sup>13</sup>CN (3–2)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CS (5–4)</td><td>58.4 ± 0.1</td><td>0.19 ± 0.01</td><td>2.4 ± 0.2</td><td>0.5 ± 0.1</td></tr><tr><td>C<sup>34</sup>S (5–4)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>HNC (3–2)</td><td>58.3 ± 0.1</td><td>0.46 ± 0.03</td><td>2.1 ± 0.2</td><td>1.0 ± 0.1</td></tr><tr><td>SiO (6–5)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>SO (6–5)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO (3<sub>03</sub> – 2<sub>02</sub>)</td><td>58.6 ± 0.4</td><td>0.25 ± 0.04</td><td>2.7 ± 0.4</td><td>0.7 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>22</sub> – 2<sub>21</sub>)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO (3<sub>21</sub> – 2<sub>20</sub>)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (4<sub>2</sub> – 3<sub>1</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>1</sub> – 4<sub>1</sub>) E</td><td>58.2 ± 0.1</td><td>0.13 ± 0.02</td><td>3.4 ± 0.1</td><td>0.5 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) A<sup>+</sup></td><td>58.2 ± 0.1</td><td>0.16 ± 0.02</td><td>3.4 ± 0.1</td><td>0.6 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) A<sup>−</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>−</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>1</sub> – 4<sub>1</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>MM7</td><td>CO (2–1)</td><td>59.7 ± 0.1</td><td>6.07 ± 0.13</td><td>3.5 ± 0.2</td><td>63.1 ± 0.4</td></tr><tr><td><sup>13</sup>CO (2–1)</td><td>58.0 ± 0.1</td><td>4.63 ± 0.06</td><td>3.4 ± 0.2</td><td>16.9 ± 0.1</td></tr><tr><td>C<sup>18</sup>O (2–1)</td><td>58.0 ± 0.1</td><td>1.92 ± 0.04</td><td>2.4 ± 0.2</td><td>4.8 ± 0.1</td></tr></tbody></table>
<table><caption>**Table 3**
(Continued)</caption><thead><tr><th>Name</th><th>Line</th><th>V<sub>lsr</sub><br>(km s<sup>-1</sup>)</th><th>T<sub>mb</sub><br>(K)</th><th>FHWM<br>(km s<sup>-1</sup>)</th><th>∫T<sub>mb</sub>dV<br>(K km s<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="24">MM8</td><td>HCO<sup>+</sup> (3–2)</td><td>57.7 ± 0.1</td><td>0.38 ± 0.03</td><td>3.3 ± 0.2</td><td>1.2 ± 0.1</td></tr><tr><td>H<sup>13</sup>CO<sup>+</sup> (3–2)</td><td>57.5 ± 0.1</td><td>0.07 ± 0.01</td><td>2.0 ± 0.2</td><td>0.1 ± 0.1</td></tr><tr><td>HCN (3–2)</td><td>57.7 ± 0.1</td><td>0.13 ± 0.03</td><td>4.6 ± 0.3</td><td>0.4 ± 0.1</td></tr><tr><td>H<sup>13</sup>CN (3–2)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CS (5–4)</td><td>58.1 ± 0.1</td><td>0.09 ± 0.01</td><td>2.4 ± 0.2</td><td>0.2 ± 0.1</td></tr><tr><td>C<sup>34</sup>S (5–4)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>HNC (3–2)</td><td>57.9 ± 0.1</td><td>0.24 ± 0.03</td><td>2.6 ± 0.2</td><td>0.6 ± 0.1</td></tr><tr><td>SiO (6–5)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>SO (6–5)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO (3<sub>03</sub> – 2<sub>02</sub>)</td><td>58.3 ± 0.4</td><td>0.15 ± 0.04</td><td>2.4 ± 0.4</td><td>0.4 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>22</sub> – 2<sub>21</sub>)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO (3<sub>21</sub> – 2<sub>20</sub>)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (4<sub>2</sub> – 3<sub>1</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>−1</sub> – 4<sub>−1</sub>) E</td><td>57.7 ± 0.1</td><td>0.06 ± 0.02</td><td>3.1 ± 0.1</td><td>0.2 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) A<sup>+</sup></td><td>57.7 ± 0.1</td><td>0.06 ± 0.02</td><td>3.1 ± 0.1</td><td>0.3 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>−4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) A<sup>−</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>−</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>−3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>1</sub> – 4<sub>1</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>−2</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CO (2–1)</td><td>59.4 ± 0.1</td><td>7.23 ± 0.13</td><td>2.3 ± 0.2</td><td>63.3 ± 0.4</td></tr><tr><td><sup>13</sup>CO (2–1)</td><td>58.4 ± 0.1</td><td>6.28 ± 0.06</td><td>1.9 ± 0.2</td><td>17.6 ± 0.1</td></tr><tr><td>C<sup>18</sup>O (2–1)</td><td>57.7 ± 0.1</td><td>2.20 ± 0.07</td><td>2.7 ± 0.2</td><td>4.6 ± 0.1</td></tr><tr><td>HCO<sup>+</sup> (3–2)</td><td>57.5 ± 0.1</td><td>0.43 ± 0.02</td><td>3.7 ± 0.2</td><td>1.6 ± 0.1</td></tr><tr><td>H<sup>13</sup>CO<sup>+</sup> (3–2)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>HCN (3–2)</td><td>57.2 ± 0.1</td><td>0.14 ± 0.03</td><td>4.6 ± 0.2</td><td>0.5 ± 0.1</td></tr><tr><td>H<sup>13</sup>CN (3–2)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CS (5–4)</td><td>57.4 ± 0.1</td><td>0.07 ± 0.01</td><td>3.3 ± 0.2</td><td>0.2 ± 0.1</td></tr><tr><td>C<sup>34</sup>S (5–4)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>HNC (3–2)</td><td>57.5 ± 0.1</td><td>0.28 ± 0.02</td><td>3.1 ± 0.2</td><td>0.8 ± 0.1</td></tr><tr><td>SiO (6–5)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>SO (6–5)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO (3<sub>03</sub> – 2<sub>02</sub>)</td><td>57.9 ± 0.4</td><td>0.11 ± 0.05</td><td>3.3 ± 0.9</td><td>0.4 ± 0.1</td></tr><tr><td>H<sub>2</sub>CO (3<sub>22</sub> – 2<sub>21</sub>)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO (3<sub>21</sub> – 2<sub>20</sub>)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (4<sub>2</sub> – 3<sub>1</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>−1</sub> – 4<sub>−1</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) A<sup>+</sup></td><td>57.5 ± 0.1</td><td>0.06 ± 0.02</td><td>5.8 ± 0.1</td><td>0.3 ± 0.1</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>−4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) A<sup>−</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>−</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>−3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>1</sub> – 4<sub>1</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>−2</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>MM9</td><td>CO (2–1)</td><td>59.9 ± 0.1</td><td>7.91 ± 0.58</td><td>6.3 ± 0.2</td><td>63.9 ± 1.3</td></tr><tr><td><sup>13</sup>CO (2–1)</td><td>58.4 ± 0.1</td><td>5.36 ± 0.08</td><td>3.6 ± 0.2</td><td>22.0 ± 0.1</td></tr><tr><td>C<sup>18</sup>O (2–1)</td><td>58.3 ± 0.1</td><td>1.79 ± 0.05</td><td>2.2 ± 0.2</td><td>4.3 ± 0.1</td></tr><tr><td>HCO<sup>+</sup> (3–2)</td><td>58.6 ± 0.1</td><td>0.88 ± 0.02</td><td>3.2 ± 0.2</td><td>3.0 ± 0.1</td></tr></tbody></table>
<table><caption>**Table 3**
(Continued)</caption><thead><tr><th>Name</th><th>Line</th><th>V<sub>lsr</sub><br>(km s<sup>-1</sup>)</th><th>T<sub>mb</sub><br>(K)</th><th>FHWM<br>(km s<sup>-1</sup>)</th><th>∫T<sub>mb</sub>dV<br>(K km s<sup>-1</sup>)</th></tr></thead><tbody><tr><td></td><td>H<sup>13</sup>CO<sup>+</sup> (3–2)</td><td>58.7 ± 0.1</td><td>0.10 ± 0.01</td><td>1.6 ± 0.2</td><td>0.1 ± 0.1</td></tr><tr><td></td><td>HCN (3–2)</td><td>58.5 ± 0.1</td><td>0.30 ± 0.03</td><td>5.7 ± 0.2</td><td>1.7 ± 0.1</td></tr><tr><td></td><td>H<sup>13</sup>CN (3–2)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CS (5–4)</td><td>58.8 ± 0.1</td><td>0.29 ± 0.01</td><td>2.2 ± 0.2</td><td>0.8 ± 0.1</td></tr><tr><td></td><td>C<sup>34</sup>S (5–4)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>HNC (3–2)</td><td>58.6 ± 0.1</td><td>0.47 ± 0.03</td><td>2.6 ± 0.2</td><td>1.2 ± 0.1</td></tr><tr><td></td><td>SiO (6–5)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>SO (6–5)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>H<sub>2</sub>CO (3<sub>03</sub> – 2<sub>02</sub>)</td><td>59.0 ± 0.4</td><td>0.44 ± 0.04</td><td>4.2 ± 0.4</td><td>1.9 ± 0.1</td></tr><tr><td></td><td>H<sub>2</sub>CO (3<sub>22</sub> – 2<sub>21</sub>)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>H<sub>2</sub>CO (3<sub>21</sub> – 2<sub>20</sub>)</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (4<sub>2</sub> – 3<sub>1</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>−1</sub> – 4<sub>−1</sub>) E</td><td>58.6 ± 0.1</td><td>0.19 ± 0.02</td><td>3.4 ± 0.1</td><td>0.7 ± 0.1</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>0</sub> – 4<sub>0</sub>) A<sup>+</sup></td><td>58.6 ± 0.1</td><td>0.22 ± 0.02</td><td>3.4 ± 0.1</td><td>0.8 ± 0.1</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>−4</sub> – 4<sub>−4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>4</sub> – 4<sub>4</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) A<sup>−</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>−</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>3</sub> – 4<sub>3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>−3</sub> – 4<sub>−3</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>1</sub> – 4<sub>1</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) A<sup>+</sup></td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>−2</sub> – 4<sub>−2</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>CH<sub>3</sub>OH (5<sub>2</sub> – 4<sub>2</sub>) E</td><td>...</td><td>...</td><td>...</td><td>...</td></tr></tbody></table>

100% for at least 3 rms signals, while the remaining H<sup>13</sup>CN, C<sup>34</sup>S, SiO, SO, and p-H<sub>2</sub>CO have a detection rate of less than 50%.

## 3.2. Line Optical Depth and Column Density

Regardless of p-H<sub>2</sub>CO and CH<sub>3</sub>OH, we calculated in this section the excitation temperature and column density for other species. With the assumption of local thermodynamic equilibrium (LTE), the excitation temperature, T<sub>ex</sub>, can be calculated as follows (e.g., Garden et al. 1991; Liu et al. 2015):

$$T_{\text{mb}} = [J_{\nu}(T_{\text{ex}}) - J_{\nu}(T_{\text{bg}})](1 - e^{-\tau})f, \quad (1)$$

where the cosmic background radiation T<sub>bg</sub> is 2.73 K, f is the beam filling factor, and J is defined as a function of the temperature T in the form

$$J(T) = \frac{h\nu}{k} \frac{1}{e^{h\nu/kT} - 1}, \quad (2)$$

where h and k are the Planck and Boltzmann constants, respectively, and ν is the transition frequency. Based on Equation (1), the optical depth can be arranged to relate to the intensities of two different isotopologues of a given species as follows:

$$\frac{T_{\text{mb}}^1}{T_{\text{mb}}^2} \approx \frac{1 - e^{-\eta}}{1 - e^{-\tau_2}} \approx \frac{1 - e^{-\eta}}{1 - e^{-\tau_1/R}}, \quad (3)$$

where τ<sub>1</sub> and τ<sub>2</sub> are the optical depths of the two isotopologues and R is the isotope ratio between the two isotopologues. The isotope pairs analyzed here include <sup>13</sup>CO/C<sup>18</sup>O, HCO<sup>+</sup>/H<sup>13</sup>CO<sup>+</sup>, HCN/H<sup>13</sup>CN, and CS/C<sup>34</sup>S, in which R is

assumed to be 7.4 for [<sup>13</sup>CO]/[C<sup>18</sup>O] (Areal et al. 2018), 55 for [<sup>12</sup>C]/[<sup>13</sup>C] (e.g., Wannier et al. 1982; Garay et al. 2010; Sanhueza et al. 2010), and 22.5 for [<sup>32</sup>S]/[<sup>34</sup>S] (e.g., Blake et al. 1994; Garay et al. 2010).

In practice, we did not calculate T<sub>ex</sub> directly via Equation (1) since the beam filling factor for most of the species is not always unity, and instead varies with the source, making the calculation uncertain. For example, inspecting the velocity-integrated intensity map of the high-density gas tracers (e.g., HCO<sup>+</sup>, HCN, and HNC), we find that their emission in MM5-MM9 is not as extended as in MM1-MM4, which could result in different beam filling factors for the same tracer in different sources. Actually, under the LTE assumption, all levels of the species are supposed to be populated with the same T<sub>ex</sub> in a single source. We therefore assume the source-averaged dust temperature, T<sub>d</sub> (see Table 1), as T<sub>ex</sub> of all species investigated here for each source. Note that since Rathborne et al. (2010) did not provide a good constraint on T<sub>d</sub> for the MM6, MM7, and MM9 clumps (see Table 1), we retrieved them from the dust temperature map of Foster et al. (2014). As a result, these T<sub>d</sub> values are comparable to that of MM8. In general, an isotope pair combines an optically thick species and a relatively optically thin counterpart. If both the optically thick and thin species of an isotope pair have significant detection (i.e., the peak of the spectrum ≥3 rms), their intensities will be directly used to derive τ from Equation (3). Second, if only the optically thick species of an isotope pair has significant detection, τ will be derived from Equation (3) assuming that the optically thin counterpart has 1 rms detection as an upper limit. Third, for those species whose isotopologues were not
detected or observed, we treat them as optically thin tracers and thus obtain upper limit values to their $\tau$ (see Table 4).

Given $T_{ex}$ and $\tau$ (see above), the column density of a molecular species $N$ can be estimated under the LTE assumption as follows (e.g., Mangum & Shirley 2015; Yuan et al. 2016):

$$N = \frac{3h}{8\pi^3\mu^2} \frac{k(T_{ex} + hB/3k)}{hBJ_u} \frac{e^{(E_u/kT_{ex})}}{e^{(h\nu/kT_{ex})} - 1} \times \frac{1}{J(T_{ex}) - J(T_{bg})} \frac{\tau}{1 - e^{-\tau}} \int T_{mb} dV, \quad (4)$$

where $\mu$ is the permanent dipole moment of the molecule, $B$ is the rotational constant of the molecule, and $J_u$ and $E_u$ are the rotational quantum number and energy of the upper state, respectively.

The computed $\tau$ and $N$ can be found in Table 4. For a further check, the column densities of $^{13}$CO and CS we derived were compared with the results by Sanhueza et al. (2010), who had carried out observations of CO/$^{13}$CO/C$^{18}$O (3–2) and CS/C$^{34}$S (2–1) toward MM1 and MM2 of the IRDC G34 cloud. It turns out that our results are in good agreement with theirs (see Table 7 of Sanhueza et al) within a typical factor of <2.

### 3.3. Rotational Temperatures and Column Densities of p-H₂CO and CH₃OH

Regarding p-H₂CO and CH₃OH, their rotational temperature and column density are computed in this section through a rotational diagram (RD) analysis (e.g., Goldsmith & Langer 1999; Garay et al. 2010; Yuan et al. 2018) since both species have more than one rotational transition observed. Assuming the optically thin and LTE conditions, the RD analysis relates to the integrated line intensity $\int T_{mb} dV$, rotational temperature $T_{rot}$, and column density $N$ via

$$\frac{3k \int T_{mb} dV}{8\pi^3 \mu^2 \nu S} = \frac{N_u}{g_u} = \frac{N}{Q(T_{rot})} e^{-\frac{E_u}{kT_{rot}}}, \quad (5)$$

where $S$ is the line strength of the transition, $N_u$ and $g_u$ are the column density and the degeneracy of the upper state, respectively, and $Q(T_{rot})$ is the rotational partition function.

Figure 4 presents an example of the RD analysis for CH₃OH and p-H₂CO. In practice, the RD analysis for a species was only conducted on the clumps with significant measurements (i.e., the spectrum peak ≥3 rms) in at least two transitions. In total, the RD analysis for CH₃OH was performed toward all of the nine clumps except for MM8, while the same analysis for p-H₂CO was done toward the MM1-MM4 clumps.

As a result, the $T_{rot}$ and $N$ parameters can be derived from the RD analysis, as shown in the upper-left corner of each panel in the figure.

### 3.4. Abundance of Molecular Species

The chemical composition of a molecular cloud is usually reflected by the abundance of molecules with respect to the molecular hydrogen. We calculated the abundance of species Y as the ratio of its column density, $N(Y)$, to the H₂ column density, $N(H_2)$. The species Y here stands for $^{13}$CO, HCO⁺, HCN, CS, HNC, SiO, SO, p-H₂CO, and CH₃OH. The isotopologues of the species Y (if any) were not taken into

<table><caption>**Table 4**

Derived Line Parameters</caption><thead><tr><th>Name</th><th>Line</th><th>τ</th><th>log10(N)<br>(cm<sup>-2</sup>)</th><th>log10(X)</th></tr></thead><tbody><tr><td rowspan="10">MM1</td><td><sup>13</sup>CO</td><td>2.72 ± 0.02</td><td>16.7 ± 14.6</td><td>-6.6 ± -8.7</td></tr><tr><td>HCO<sup>+</sup></td><td>17.18 ± 0.23</td><td>14.0 ± 11.9</td><td>-9.3 ± -11.4</td></tr><tr><td>HCN</td><td>20.08 ± 0.93</td><td>14.4 ± 12.5</td><td>-8.9 ± -10.8</td></tr><tr><td>CS</td><td>4.86 ± 0.09</td><td>14.2 ± 12.1</td><td>-9.1 ± -11.1</td></tr><tr><td>HNC</td><td>...</td><td>12.9 ± 10.6</td><td>-10.4 ± -12.7</td></tr><tr><td>SiO</td><td>...</td><td>12.7 ± 10.8</td><td>-10.6 ± -12.4</td></tr><tr><td>SO</td><td>...</td><td>13.4 ± 11.6</td><td>-9.9 ± -11.7</td></tr><tr><td>H<sub>2</sub>CO</td><td>...</td><td>14.4 ± 13.2</td><td>-8.9 ± -10.1</td></tr><tr><td>CH<sub>3</sub>OH</td><td>...</td><td>15.1 ± 14.7</td><td>-8.2 ± -8.6</td></tr><tr><td rowspan="10">MM2</td><td><sup>13</sup>CO</td><td>3.94 ± 0.03</td><td>17.1 ± 14.7</td><td>-6.0 ± -8.4</td></tr><tr><td>HCO<sup>+</sup></td><td>13.49 ± 0.18</td><td>14.0 ± 11.8</td><td>-9.1 ± -11.3</td></tr><tr><td>HCN</td><td>15.37 ± 1.05</td><td>14.2 ± 12.2</td><td>-8.9 ± -10.9</td></tr><tr><td>CS</td><td>1.97 ± 0.02</td><td>13.9 ± 12.0</td><td>-9.3 ± -11.1</td></tr><tr><td>HNC</td><td>...</td><td>13.0 ± 10.7</td><td>-10.2 ± -12.5</td></tr><tr><td>SiO</td><td>...</td><td>12.4 ± 10.7</td><td>-10.7 ± -12.4</td></tr><tr><td>SO</td><td>...</td><td>13.3 ± 11.6</td><td>-9.8 ± -11.5</td></tr><tr><td>H<sub>2</sub>CO</td><td>...</td><td>14.3 ± 13.0</td><td>-8.8 ± -10.1</td></tr><tr><td>CH<sub>3</sub>OH</td><td>...</td><td>15.1 ± 14.9</td><td>-8.0 ± -8.2</td></tr><tr><td rowspan="10">MM3</td><td><sup>13</sup>CO</td><td>2.23 ± 0.04</td><td>16.4 ± 14.4</td><td>-6.3 ± -8.3</td></tr><tr><td>HCO<sup>+</sup></td><td>11.15 ± 0.21</td><td>13.3 ± 11.7</td><td>-9.3 ± -11.0</td></tr><tr><td>HCN</td><td>10.10 ± 0.67</td><td>13.7 ± 12.2</td><td>-8.9 ± -10.5</td></tr><tr><td>CS</td><td>1.53 ± 0.06</td><td>13.0 ± 12.0</td><td>-9.7 ± -10.7</td></tr><tr><td>HNC</td><td>...</td><td>12.5 ± 10.5</td><td>-10.2 ± -12.1</td></tr><tr><td>SiO</td><td>...</td><td>12.4 ± 10.8</td><td>-10.3 ± -11.8</td></tr><tr><td>SO</td><td>...</td><td>12.9 ± 11.5</td><td>-9.8 ± -11.2</td></tr><tr><td>H<sub>2</sub>CO</td><td>...</td><td>14.0 ± 12.8</td><td>-8.7 ± -9.9</td></tr><tr><td>CH<sub>3</sub>OH</td><td>...</td><td>15.0 ± 14.7</td><td>-7.7 ± -7.9</td></tr><tr><td rowspan="10">MM4</td><td><sup>13</sup>CO</td><td>3.83 ± 0.03</td><td>16.9 ± 14.6</td><td>-5.7 ± -7.9</td></tr><tr><td>HCO<sup>+</sup></td><td>15.68 ± 0.29</td><td>13.5 ± 11.7</td><td>-9.0 ± -10.8</td></tr><tr><td>HCN</td><td>11.94 ± 0.92</td><td>13.5 ± 12.0</td><td>-9.0 ± -10.5</td></tr><tr><td>CS</td><td>2.22 ± 0.04</td><td>13.1 ± 11.8</td><td>-9.5 ± -10.7</td></tr><tr><td>HNC</td><td>...</td><td>12.5 ± 10.5</td><td>-10.0 ± -12.1</td></tr><tr><td>SiO</td><td>...</td><td>11.9 ± 10.6</td><td>-10.6 ± -12.0</td></tr><tr><td>SO</td><td>...</td><td>12.9 ± 11.3</td><td>-9.7 ± -11.2</td></tr><tr><td>H<sub>2</sub>CO</td><td>...</td><td>13.8 ± 12.2</td><td>-8.7 ± -10.3</td></tr><tr><td>CH<sub>3</sub>OH</td><td>...</td><td>15.2 ± 15.0</td><td>-7.3 ± -7.6</td></tr><tr><td rowspan="10">MM5</td><td><sup>13</sup>CO</td><td>4.57 ± 0.04</td><td>16.5 ± 14.2</td><td>-6.3 ± -8.6</td></tr><tr><td>HCO<sup>+</sup></td><td>8.04 ± 0.18</td><td>12.9 ± 11.7</td><td>-9.9 ± -11.1</td></tr><tr><td>HCN</td><td>≤4.75 ± 0.48</td><td>≤13.0 ± 11.3</td><td>≤-9.8 ± -11.5</td></tr><tr><td>CS</td><td>≤1.87 ± 0.17</td><td>≤12.6 ± 11.1</td><td>≤-10.2 ± -11.7</td></tr><tr><td>HNC</td><td>...</td><td>12.0 ± 10.3</td><td>-10.8 ± -12.5</td></tr><tr><td>SiO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>SO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH</td><td>...</td><td>13.8 ± 12.9</td><td>-9.0 ± -10.1</td></tr><tr><td rowspan="10">MM6</td><td><sup>13</sup>CO</td><td>3.41 ± 0.04</td><td>16.3 ± 14.4</td><td>-6.0 ± -7.9</td></tr><tr><td>HCO<sup>+</sup></td><td>6.88 ± 0.27</td><td>12.5 ± 11.5</td><td>-9.8 ± -10.9</td></tr><tr><td>HCN</td><td>≤7.17 ± 1.07</td><td>≤12.9 ± 11.4</td><td>≤-9.4 ± -10.9</td></tr><tr><td>CS</td><td>≤2.47 ± 0.15</td><td>≤12.7 ± 11.1</td><td>≤-9.6 ± -11.2</td></tr><tr><td>HNC</td><td>...</td><td>11.9 ± 10.4</td><td>-10.4 ± -12.0</td></tr><tr><td>SiO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>SO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH</td><td>...</td><td>13.7 ± -15.5</td><td>-8.6 ± -37.8</td></tr><tr><td rowspan="10">MM7</td><td><sup>13</sup>CO</td><td>3.96 ± 0.06</td><td>16.4 ± 14.3</td><td>-5.9 ± -7.9</td></tr><tr><td>HCO<sup>+</sup></td><td>10.69 ± 1.01</td><td>12.5 ± 11.6</td><td>-9.8 ± -10.7</td></tr><tr><td>HCN</td><td>≤14.86 ± 5.21</td><td>≤12.7 ± 11.7</td><td>≤-9.5 ± -10.6</td></tr><tr><td>CS</td><td>≤5.81 ± 1.10</td><td>≤12.6 ± 11.4</td><td>≤-9.6 ± -10.9</td></tr><tr><td>HNC</td><td>...</td><td>11.7 ± 10.4</td><td>-10.6 ± -11.9</td></tr><tr><td>SiO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>SO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH</td><td>...</td><td>13.2 ± 14.6</td><td>-9.1 ± -10.3</td></tr></tbody></table>
<table><caption>**Table 4**
(Continued)</caption><thead><tr><th>Name</th><th>Line</th><th>τ</th><th>log10(N)<br>(cm<sup>-2</sup>)</th><th>log10(X)</th></tr></thead><tbody><tr><td rowspan="10">MM8</td><td><sup>13</sup>CO</td><td>3.19 ± 0.04</td><td>16.4 ± 14.5</td><td>-5.9 ± -7.8</td></tr><tr><td>HCO<sup>+</sup></td><td>3.93 ± 0.24</td><td>12.5 ± 10.8</td><td>-9.8 ± -11.5</td></tr><tr><td>HCN</td><td>≤3.71 ± 3.86</td><td>≤12.8 ± 11.6</td><td>≤-9.5 ± -10.7</td></tr><tr><td>CS</td><td>≤7.88 ± 2.29</td><td>≤12.7 ± 11.6</td><td>≤-9.6 ± -10.7</td></tr><tr><td>HNC</td><td>...</td><td>11.8 ± 10.3</td><td>-10.5 ± -12.0</td></tr><tr><td>SiO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>SO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>CH<sub>3</sub>OH</td><td>...</td><td>...</td><td>...</td></tr><tr><td rowspan="10">MM9</td><td><sup>13</sup>CO</td><td>3.01 ± 0.06</td><td>16.3 ± 14.4</td><td>-6.1 ± -8.0</td></tr><tr><td>HCO<sup>+</sup></td><td>6.54 ± 0.18</td><td>12.6 ± 11.3</td><td>-9.8 ± -11.1</td></tr><tr><td>HCN</td><td>≤5.70 ± 0.65</td><td>≤12.9 ± 11.3</td><td>≤-9.5 ± -11.1</td></tr><tr><td>CS</td><td>≤1.60 ± 0.07</td><td>≤12.8 ± 11.1</td><td>≤-9.6 ± -11.3</td></tr><tr><td>HNC</td><td>...</td><td>12.0 ± 10.3</td><td>-10.4 ± -12.1</td></tr><tr><td>SiO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>SO</td><td>...</td><td>...</td><td>...</td></tr><tr><td>H<sub>2</sub>CO</td><td>...</td><td>13.4 ± 12.2</td><td>-9.0 ± -9.9</td></tr><tr><td>CH<sub>3</sub>OH</td><td>...</td><td>13.9 ± 12.5</td><td>-8.5 ± -9.5</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Example of the rotational diagram analysis for CH<sub>3</sub>OH (top panel) and p-H<sub>2</sub>CO (bottom panel). The data points correspond to the observations of the two species toward the clump MM1. The errors are hidden in the symbols. The linear fit is indicated in the dashed line, yielding the T<sub>rot</sub> and N parameters displayed in the upper left corner of each panel.</figcaption></figure>

account given the close relation in abundance between the species Y and their isotopologues by a constant factor.

By definition, the abundance of species Y relies not only on N(Y) but also on N(H<sub>2</sub>). It is therefore crucial to determine N(H<sub>2</sub>) as accurately as possible. To this end, we recomputed N(H<sub>2</sub>) for the nine clumps with the published integrated 1.2 mm fluxes over the matching clump sizes, F<sub>1.2 mm</sub><sup>int</sup>, and the source-averaged temperatures, T<sub>d</sub> (see Table 1). Moreover, more than half of the clumps investigated here have sizes (FWHM<sub>clump</sub>) not comparable to the common size 28.5 (see

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Velocity correlation between CH<sub>3</sub>OH and the other eight species whose lines were derived to be optically thin (see Section 3.2). The thick and thin dashed lines indicate unity and 0.5 km s<sup>-1</sup> offsets, respectively. The velocities between CH<sub>3</sub>OH and the other species have a very tight correlation within the thin dashed lines.</figcaption></figure>

Table 1) over which the molecular column density has been computed (see Section 3.1). We therefore scaled the F<sub>1.2 mm</sub><sup>int</sup> via the relation F<sub>1.2 mm</sub><sup>scaled</sup> = F<sub>1.2 mm</sub><sup>int</sup> × (28.5/FWHM<sub>clump</sub>) assuming that the flux nearly varies linearly with the size (see Nguyen Luong et al. 2011; Liu et al. 2017). In turn, the scaled fluxes allowed us to recompute the H<sub>2</sub> column density via Equation (1) of Liu et al. (2016). The recomputed N(H<sub>2</sub>) can be seen in Table 1, ranging from 2.0 × 10<sup>22</sup> cm<sup>-2</sup> to 2.2 × 10<sup>23</sup> cm<sup>-2</sup> with a median value of 3.8 × 10<sup>22</sup> cm<sup>-2</sup>.

Regardless of possible uncertainties of the H<sub>2</sub> column density, the uncertainties of the abundance of species Y could arise from the propagation of the measurement errors of T<sub>mb</sub> and ∫T<sub>mb</sub>dV. Particularly for the species SiO, HNC, and SO, the uncertainties could also come from the optically thin approximation (see Section 3.2). This assumption would mistreat an actually optically thick line as an optically thin line. Quantitatively, for moderately optically thick lines (e.g., τ ~ 2.0–5.0), the true column densities of the species would be higher by a factor of a few (e.g., 1.6–5.0) and so would be the true abundances.

# 4. Analysis

## 4.1. Velocity Comparison between Different Species

Figure 5 shows the velocity comparison between CH<sub>3</sub>OH and other species whose lines were derived to be optically thin (see Section 3.2). The optically thin lines are used since their overall simple (single-peak) spectra make the determination of the line velocity centroid more accurate than the optically thick lines. In addition, CH<sub>3</sub>OH is chosen as the reference for the comparison since it has both at least one significant detection toward each of the clumps investigated here and the simplest spectra among all species. The velocities between CH<sub>3</sub>OH and other species have a very tight correlation (Pearson's coefficient ρ is 0.87 and the P value for the null hypothesis is 6 × 10<sup>-17</sup>) for all measurements. The correlation will be much tighter (ρ = 0.95 and P = 3 × 10<sup>-24</sup>) if the four measurements of C<sup>18</sup>O and SiO more than 0.5 km s<sup>-1</sup> away from the the unity relation (i.e., outside the thin dashed lines) are
ignored. This suggests that all of these lines do trace the same
source with a matching velocity. In particular, these four
measurements consist of one in MM2 where the single-
Gaussian model does not fit the ¹³CO spectrum well and the
other three in MM3 and MM4 where the true velocities are
indeed >0.5 km s⁻¹ from that of CH₃OH.

## 4.2. Line Width Comparison

The line widths can provide turbulence information in star-
forming clumps. Even though we cannot rule out whether the
clumps have organized velocity structures, such as rotation and
large-scale velocity gradients, the turbulence is presumed in
this work to dominate the measured FWHMs at the resolution
of our observations (~28''/5 corresponding to ~0.5 pc).

Figure 6 presents the FWHM distribution for the nine optically thin species in the top panel. In the figure, we have organized the species from broad to narrow line widths, with the median values ranging from 9.8 to 2.1 km s⁻¹. This trend suggests that different species are sensitive to different turbulent motions. In more detail, SiO and SO show the largest FWHMs among the nine species regardless of H¹³CN, having a median of FWHM = 9.8 km s⁻¹ within an interquartile range (IQR) of 6.9–12.6 km s⁻¹ and a median of FWHM = 4.9 km s⁻¹ within an IQR of 4.5–5.1 km s⁻¹, respectively. These largest FWHMs are in large part attributed to the shock turbulence from outflows since SiO and SO have been detected only in the MM1–MM4 clumps associated with outflows (see Section 1), and on the other hand they are believed to be good tracers of shocked molecular gas (e.g., Sanhueza et al. 2012). In addition, CH₃OH, p-H₂CO, and C³⁴S have comparable FWHMs with median values around 4.1 km s⁻¹. In contrast, HNC, H¹³CO⁺, and C¹⁸O have the smallest FWHMs among all species. We suggest that these smallest FWHMs could be related to the turbulent motions of the envelope of protostellar clumps, which could be kinematically colder than the inside of the clumps as traced by CH₃OH, p-H₂CO, and C³⁴S. Particularly, the FWHMs of H¹³CN should be treated with caution, as the line blending of the hyperfine components of this species cannot be ruled out in current analysis.

The bottom panel of Figure 6 displays the FWHM distribution for the nine sources. We find that the median values of FWHM (i.e., 3.6–4.9 km s⁻¹) in the high-luminosity protostellar clumps are higher than those (i.e., 2.1–3.1 km s⁻¹) in the low-luminosity protostellar clumps, indicating different turbulent degrees between these two classes of protostellar clumps. We suggest that the high degree of turbulence in the high-luminosity protostellar clumps is mainly ascribed to the associated outflows and other feedback (e.g., stellar winds, UC H II region), while such turbulence (if any) in the low-luminosity protostellar clumps is not as strong as in the high-luminosity counterparts.

## 4.3. Temperature Comparison

Figure 7 gives the distributions of rotational and dust temperatures. CH₃OH has the temperature (i.e., ~30.8 K) slightly higher than the dust temperature (i.e., ~26.0 K) on average over eight of nine clumps that have significant detection (i.e., the spectrum peak ≥3 rms) in at least two CH₃OH transitions (see Table 3, and Section 3.3). This suggests that the populations of CH₃OH are overall thermally

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Box-and-whisker plot showing the distributions of FWHM for the optically thin species (top panel) and for the clumps of IRDC G34 (bottom panel). In the top panel each box and whisker unit refers to the statistics of the FWHMs of a given species over all of the clumps where the species has significant detection, while in the bottom panel it refers to the statistics of the FWHMs of a given source over all of the species that have been significantly detected in that given source. The numbers associated with the boxes represent the upper quartile, median, and lower quartile, respectively. The red crosses indicate the outliers outside 1.5 times the interquartile range either above the upper quartile or below the lower quartile.</figcaption></figure>

excited in the clump environment of the IRDC G34. In contrast, p-H₂CO has a rotational temperature (~61 K) much higher than the dust temperature (i.e., ~35 K) on average over a subsample of clumps (MM1–MM4) that have significant detection in at least two p-H₂CO transitions. Actually, this difference between the two temperatures has been found toward the Galactic central molecular zone with generally T<sub>rot</sub> of p-H₂CO greater than T<sub>d</sub> (Ginsburg et al. 2016) and also toward several high-mass protostellar clumps associated with outflows or shocks in the Galactic plane (Tang et al. 2017, 2018a, 2018b). In theory, the line strength ratios of three K<sub>a</sub>-ladders of p-H₂CO, 3<sub>21</sub>–2<sub>20</sub>, 3<sub>22</sub>–2<sub>21</sub>, and 3<sub>03</sub>–2<sub>02</sub>, are
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Box-and-whisker plot showing the distributions of the rotational and dust temperatures. The rotational temperatures refer to the species CH<sub>3</sub>OH and p-H<sub>2</sub>CO (see Section 3.3). Each box-and-whisker unit corresponds to the statistics of the temperatures of a given species over a subsample of the nine clumps (see Section 4.3). The number near the box represents the median temperature of a given species, and the red crosses are the same outliers as in Figure 6.</figcaption></figure>

thought to be a sensitive thermometer because the relative populations of the three ladders are governed by the collisions that are tightly related to the kinetic temperature (Mangum & Wootten 1993; Mühle et al. 2007). Moreover, we find that p-H<sub>2</sub>CO has been detected in the four high-luminosity protostellar clumps only, MM1-MM4, which can indeed reach the high gas temperatures as derived from p-H<sub>2</sub>CO owing to the high luminosities of the clumps. From an observational point of view, the gas and dust temperatures are coupled in small, dense scales (i.e., >10<sup>5</sup> cm<sup>-3</sup>), whereas the gas and dust are decoupled in larger, more diffuse scales (e.g., di Francesco et al. 2007). In our case, the gas temperature traced by p-H<sub>2</sub>CO could be from small and dense scales owing to its high critical density, while the dust temperature would be an average of the temperatures from both the small (warm), dense and relatively large (colder), diffuse scales, resulting in a large difference between the gas and dust temperatures. This is consistent with the result obtained in Orion-OMC1 by Tang et al. (2018a) that p-H<sub>2</sub>CO traces dense and warm gas.

### 4.4. Evidence for Active Star Formation

The high-luminosity protostellar clumps of IRDC G34 have been found to be sites of active star formation as characterized by the associated outflows (e.g., Rathborne et al. 2005; Sanhueza et al. 2010). These four outflows appear in the IRAC three-color 3.5, 4.6, and 8.0 µm image to be slightly extended emission with a relative enhancement at 4.5 µm (see Figure 1 of Rathborne et al. 2005). This enhanced emission at 4.5 µm is suggested to be an indicator of shocked gas (e.g., by outflows) in star-forming regions (e.g., Marston et al. 2004; Cyganowski et al. 2011; Chen et al. 2013). On top of this, the outflows are also robustly characterized by broad line widths (e.g., ~10 km s<sup>-1</sup>) of CS (3–2) and HCN (4–3) and substantial SiO emission (Rathborne et al. 2005).

Our observations with significant detection in HCO<sup>+</sup> (3–2), CS (5–4), and SiO (6–5) also confirm the associated outflows in the four high-luminosity protostellar clumps. We present the full width of 1 rms intensity (FWRI) of the three species instead of the traditional FWHM, as the former can better reflect the velocities of the line wings associated with outflows than the

<figure><img src="image_9.png" /><figcaption>**Figure 8.** Abundance of different species (not counting the isotopologues) for each source. The parameters on the top of the figure represent the first quartile, median, and third quartile, respectively.</figcaption></figure>

latter. The FWRIs of HCO<sup>+</sup>, CS, and SiO are observed to be ~[21.0, 29.8, 33.2] km s<sup>-1</sup> in MM1, ~[18.8, 28.4, 19.2] km s<sup>-1</sup> in MM2, ~[19.4, 26.4, 19.2] km s<sup>-1</sup> in MM3, and ~[20.0, 17.2, 9.6] km s<sup>-1</sup> in MM4. The average value of FWRIs of both HCO<sup>+</sup> and CS is 22.6 km s<sup>-1</sup>, which agrees with the typical FWRI value of 26.7 km s<sup>-1</sup> derived from the CS (3–2) observations toward four massive clumps associated with outflows by Garay et al. (2010). Therefore, the observed large FWRIs of both HCO<sup>+</sup> and CS, along with the significant SiO detection, demonstrate that active star formation is indeed occurring in the four high-luminosity protostellar clumps MM1-MM4. In contrast, the other five low-luminosity protostellar clumps do not have detectable SiO (6–5) emission, and the FWRIs of HCO<sup>+</sup> (i.e., ~6.0–11.8 km s<sup>-1</sup>) and CS (~3.8–8.6 km s<sup>-1</sup>) in these clumps are much lower.

## 5. Discussion

### 5.1. Abundance Trend in Two Classes of Protostellar Clumps

The abundance, X(species), is commonly evaluated to reveal the chemical composition of a molecular cloud. Figure 8 presents the abundance distribution of the species (not counting the isotopes) for all clumps investigated here. In general, X<sup>(13)CO</sup> distribution of all clumps shows a nearly constant trend with not much difference between the high- and low-luminosity protostellar clumps (see more explanation in Section 5.2). In contrast, a difference of the abundances between the two classes of protostellar clumps can be found for other species. For the species HCO<sup>+</sup>, HCN, HNC, CS, and CH<sub>3</sub>OH, their abundances in the high-luminosity protostellar clumps are around three times higher on average than those in the low-luminosity protostellar clumps. For p-H<sub>2</sub>CO, SiO, and SO, their abundance distributions show that the three species can be produced with significant detection only in the high-luminosity protostellar clumps.

In general, the heating of protostellar clumps due to central, newly formed stars can enhance the abundance of molecules
through a substantial evaporation from ice mantles into gas phase in the innermost regions of the clumps. In addition, the shocks related to the outflows can liberate ice mantles and trigger similar effects (see van Dishoeck & Blake 1998 for a review; see also Jørgensen et al. 2004). Therefore, the high abundance of the species discussed here in the high-luminosity protostellar clumps is mainly due to both the high luminosities and associated outflows of the clumps. In what follows, the abundance of the individual species will be discussed.

## 5.2. Abundance of Individual Species

**Carbon Monoxide (¹³CO).** It is one of the CO isotopologues (e.g., ¹³CO, C¹⁸O, and ¹⁷CO). CO, ¹³CO, and C¹⁸O are highly detectable in molecular clouds and thus are routinely used to determine the column density of molecular hydrogen (H₂) by assuming a [CO/H₂] abundance ratio. Nevertheless, this ratio is highly sensitive to chemical effects such as the CO depletion in very cold and dense molecular clouds (e.g., Tafalla et al. 2002; Liu et al. 2013). Likewise, ¹³CO can also reach the depletion in an extremely cold and dense environment (e.g., Ripple et al. 2013). As shown in Figure 8, the ¹³CO abundance, X(¹³CO), of the nine clumps has a median value of 1.0 × 10⁻⁶ within an IQR of (0.5–1.2) × 10⁻⁶. These X(¹³CO) values are typical (~~10⁻⁶) of both nearby low- and high-mass star-forming clouds (e.g., Pineda et al. 2008; Ripple et al. 2013). In addition, the abundance of ¹³CO shows a nearly constant distribution independent of clumps. This suggests that the depletion effect is not severe in the clump environment of the IRDC G34 cloud with a typical dust temperature of 23.0 K and column density of 3.8 × 10²² cm⁻² (i.e., A$_{V}$ ~ 40 mag). Furthermore, following Fontani et al. (2012), we compared the expected ¹³CO abundance (i.e., 2.4 × 10⁻⁶) at the distance of G34 with the measured values for the nine clumps and thus obtained the depletion factor of ¹³CO (f$_{D}$), which describes the degree of the gas depletion, to be ~[9.3, 2.3, 4.8, 1.1, 4.5, 2.4, 1.8, 2.0, 2.8] for the clumps MM1–MM9. These values indicate almost no depletion effect of ¹³CO (f$_{D}$ < 3) in MM2, MM4, and MM6–MM9 and a rather low depletion effect (4 < f$_{D}$ < 10) in MM1, MM3, and MM5.

**Formylum (HCO⁺).** It is an abundant molecule with particularly enhanced abundances around regions of high fractional ionization. The X(HCO⁺) we derived have a median value of 1.7 × 10⁻¹⁰ within an IQR of (1.6–5.4) × 10⁻¹⁰. Surveying the literature, we find that the computed HCO⁺ abundances vary in different works. For instance, based on a large sample of 159 massive IRDC clumps with different evolutionary stages from starless up to the H II region, Sanhueza et al. (2012) computed X(HCO⁺) to be in the range of 3.9 × 10⁻⁹ to 2.8 × 10⁻⁷, with a median value of 2.5 × 10⁻⁸, although the approaches in calculating the molecular abundance are almost the same in their and our works. In addition, the values derived by Miettinen (2014) from 35 IR-dark and bright massive clumps lie in the range of 6 × 10⁻¹¹ to 3.6 × 10⁻⁹, with a median value of 3.3 × 10⁻¹⁰. Moreover, X(HCO⁺) is found down to ~10⁻¹¹ on average in very cold (T$_{d}$ < 14 K), low-mass Planck clumps (Yuan et al. 2016) and can also go up to ~10⁻⁹ on average in warm, low-mass protostellar cores (Jørgensen et al. 2004). In summary, the HCO⁺ abundances of the IRDC G34 clumps match the values derived from previous work both on massive IRDC clumps and on warm low-mass protostellar cores.

**Hydrogen Cyanide (HCN) and Hydrogen Isocyanide (HNC).** The two species both trace dense molecular gas. Chemically HCN and its geometrical isomer, HNC, are thought to be closely related via the dissociative recombination reaction HCNH⁺ + e⁻ → HCN + H or HNC + H (e.g., Herbst 1978). The abundances X(HCN) and X(HNC) we derived have a median value of 0.4 × 10⁻⁹ within an IQR of (0.3–1.2) × 10⁻⁹ and a median value of 4.0 × 10⁻¹¹ within an IQR of (3.3–6.9) × 10⁻¹¹, respectively. For X(HCN), our derived values are in accordance with those measured toward both low- and high-mass star-forming regions, for example, 2.7 × 10⁻¹⁰ to 2.1 × 10⁻⁷ and 3.3 × 10⁻¹⁰ to 6.8 × 10⁻⁹ for massive IRDC clumps in Vasyunina et al. (2011) and Miettinen (2014), respectively. For X(HNC), however, the values we derived are more than one order of magnitude lower than those of both Vasyunina et al. and Miettinen. Our lower values could in part stem from the assumption of HNC as an optically thin tracer. If HNC is optically thick in reality (τ > 10; e.g., Sanhueza et al. 2012), the true abundance will be higher by at least one order of magnitude. Therefore, the absolute values of X(HNC) here should be treated with caution. Regarding the ratio of X(HNC)/X(HCN), it has been reported to be highly dependent on the temperature. For example, the ratio of unity even slightly lower has typically been observed in cold molecular cores (e.g., Ohishi et al. 1992; Jørgensen et al. 2004; Tafalla et al. 2006; Hoq et al. 2013), while the higher ratios have been measured in warm (hot), active star-forming regions, up to ~7 observed in high-mass protostellar objects (HMPOs; e.g., Jin et al. 2015) and even ~200–300 in the extremely hot regions like Orion (Goldsmith et al. 1986). The ratios of X(HNC)/X(HCN) we derived range from 8 to 34 and show a difference of a factor of ~2 between the low-luminosity (~9 on average) and high-luminosity (~19 on average) protostellar clumps. Although the absolute values of the ratio cannot be explained further owing to the aforementioned probable uncertainty, the small difference of the ratio between the two classes of protostellar clumps suggests that the X(HNC)/X(HCN) ratio could be sensitive to the temperature.

**Carbon Monosulfide (CS).** It is a high-density tracer. The X(CS) we derived is 2.5 × 10⁻⁹ on average within an IQR of (2.3–3.3) × 10⁻⁹. These values were found to be typical both in high-mass protostellar clumps associated with hot cores and UC H II regions (e.g., van der Tak et al. 2000b; Garay et al. 2010) and in low-mass protostellar cores (e.g., Jørgensen et al. 2004).

**Silicon Monoxide (SiO) and Sulfur Monoxide (SO).** The two species are commonly used tracers of shocked molecular gas usually in active star-forming regions. As shown in Figure 8, both SiO emission and SO emission are detected in the four high-luminosity protostellar clumps but not in the low-luminosity counterparts, where no signs of active star formation (e.g., outflows and high luminosities) have been found. This high detection rate in the high-luminosity protostellar clumps is therefore mainly due to the formation mechanisms of both species triggered by shocks (e.g., from outflows). The compound SiO can form via the neutral–neutral gas-phase reactions Si + O₂ → SiO + O and Si + OH → SiO + H. The Si atoms can be sputtered out from the icy grain mantles with the shocks that do not require very high velocities (Gusdorf et al. 2008). As for SO, it may be produced through the gas reaction S + OH → SO + H, which can be facilitated by low-velocity shocks (e.g., Pineau des Forets et al. 1993;
Guzmán et al. 2018). The X(SiO) and X(SO) we derived are on average $2.5 \times 10^{-11}$ within an IQR of $(2.3–3.3) \times 10^{-11}$ and $1.6 \times 10^{-10}$ within an IQR of $(1.5–1.8) \times 10^{-10}$. These values agree with those of molecular clumps associated with hot cores or UCHii regions (e.g., Garay et al. 2010; Li et al. 2015).

Methanol (CH₃OH) and Formaldehyde (p-H₂CO). Both species are typical grain-mantle products observed in interstellar ices (Jørgensen et al. 2004). The X(CH₃OH) and X(p-H₂CO) of the clumps investigated here are $0.4 \times 10^{-8}$ on average within an IQR of $(0.2–1.2) \times 10^{-8}$ and $1.5 \times 10^{-9}$ within an IQR of $(1.3–2.0) \times 10^{-9}$, respectively. The models of gas–grain chemistry predict that the production of CH₃OH yields the abundances of ~$10^{-9}$ for the coldest sources, ~$10^{-9}$ to ~$10^{-7}$ for the warmer sources, and ~$10^{-8}$ for hot cores (van der Tak et al. 2000a). The X(CH₃OH) values we derived agree with those for the warmer sources, which may correspond to a mixing of the hot core of the protostellar clump and its cold envelope. In addition, for p-H₂CO, van der Tak et al. (2000a) found that the observed data toward 13 high-mass star-forming regions could be well reproduced using the gas–grain chemistry model with a constant abundance of ~$10^{-9}$, suggesting a stable abundance of p-H₂CO at different stages of star formation. This stable abundance is strongly related to the formation process of p-H₂CO mainly through the gas phase (van der Tak et al. 2000a). As shown in Figure 8, the p-H₂CO abundance distribution observed in IRDC G34 does appear stable.

## 6. Summary

We have reported millimeter observations toward the nine protostellar clumps of the high-mass star-forming filamentary IRDC G34, with APEX in several lines at ~1 mm. The lines include CO/$^{13}$CO/C$^{18}$O (2–1), HCO$^{+}$/H$^{13}$CO$^{+}$ (3–2), HCN/H$^{13}$CN (3–2), HNC (3–2), CS/C$^{34}$S (5–4), SiO (6–5), SO (6–5), p-H₂CO (3–2), and CH₃OH (5–4). With these lines, we have investigated the chemical composition of the nine clumps, which are grouped into two classes regarding the luminosity: high-luminosity protostellar clumps (MM1–MM4) with $L_{bol} > \sim 10^3 L_{\odot}$ and low-luminosity protostellar clumps (MM5–MM9) with $L_{bol} \ll 10^3 L_{\odot}$. The main findings are summarized below:

1. Thirty transitions of 14 molecular species and isotopologues have been observed in total toward the nine clumps. The species CO/$^{13}$CO/C$^{18}$O, HCO$^{+}$/H$^{13}$CO$^{+}$, HCN, HNC, CS, and CH₃OH have a much higher detection rate (89%–100%) for at least 3 rms signals than H$^{13}$CN, C$^{34}$S, SiO, SO, and p-H₂CO (<50%).

2. Significant SiO emission, along with broad FWRIs of HCO$^{+}$ and CS (~22.6 km s$^{-1}$ on average), the evidence for outflows, confirms active star formation in the high-luminosity protostellar clumps (Rathborne et al. 2005), while these signatures cannot be found in the low-luminosity protostellar clumps.

3. Kinematics analysis suggests that the two classes of protostellar clumps have different degrees of turbulence as characterized by the median values of FWHM of the optically thin lines (i.e., 3.6–4.9 km s$^{-1}$) in the high-luminosity protostellar clumps as opposed to those (i.e., 2.1–3.2 km s$^{-1}$) in the low-luminosity counterparts. The high degree of turbulence in the high-luminosity protostellar clumps is mainly attributed to feedback of active

star formation (e.g., outflows), which is stronger than in the low-luminosity counterparts.

4. The nearly constant level of the X($^{13}$CO) distributions for all clumps indicates either no or rather low depletion effect in the IRDC G34 cloud, which is further confirmed by depletion factors of ~1–9 for all the clumps.

5. For the species HCO$^{+}$, HCN, HNC, CS, and CH₃OH, their abundances in the high-luminosity protostellar clumps are around three times higher on average than in the low-luminosity counterparts. For p-H₂CO, SiO, and SO, their abundance distributions show that the three species can be sufficiently produced to be detected only in the high-luminosity protostellar clumps.

To conclude, we conjecture that for most, if not all, of the species investigated here (see above) the high abundance in the high-luminosity protostellar clumps mainly arises from both the high luminosities and associated outflows of the clumps, while the low abundance or even no detectable emission of the species could be attributed to the lack of these active star-forming signatures. If this conjecture is proven in future studies either on large samples or in theory, it will signify that the chemistry of a star-forming region is indeed highly sensitive to physical conditions. In addition, the abundances of the species (i.e., SiO, SO, and p-H₂CO) detectable only in the high-luminosity protostellar clumps are consistent with those typical of the clumps associated with high-mass protostellar objects (e.g., hot core or UC H II regions). However, the abundances of the species (i.e., $^{13}$CO, HCO$^{+}$, HCN, and CS) significantly detected in the two classes of protostellar clumps are found to match those measured in both low- and high-mass star-forming regions. Therefore, our chemical analysis does not allow us to tell whether the low-luminosity protostellar clumps are able to form high-mass stars or not in the future.

We thank the anonymous referee for the comments that improved much the quality of this paper. H.-L.L. acknowledges the funding from Fondecyt Postdoctoral (project code 3190161) and thanks Shanghai Astronomical Observatory for providing office facilities in preparation of this work. P.S. was partially supported by a Grant-in-Aid for Scientific Research (KAKENHI No. 18H01259) of the Japan Society for the Promotion of Science (JSPS). H.-L.L. and P.S. gratefully acknowledge the support from the NAOJ Visiting Fellow Program to visit the National Astronomical Observatory of Japan in 2019 February. X.-D.T. acknowledges support by the NSFC under grant Nos. 11903070 and 11433008 and the Heaven Lake Hundred-Talent Program of Xinjiang Uygur Autonomous Region of PR China. This research made use of Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration 2018).

## ORCID iDs

Hong-Li Liu https://orcid.org/0000-0003-3343-9645

Patricio Sanhueza https://orcid.org/0000-0002-7125-7685

Tie Liu https://orcid.org/0000-0002-5286-2564

Xin-Di Tang https://orcid.org/0000-0002-4154-4309

Yuefang Wu https://orcid.org/0000-0002-5076-7520

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