molecules are reached in the cold cloud stage and remain high well into the warm-up stage.

From Fig. 10 we see that the temperature derived for c-C₃H₂ is about 8 K and fairly uniform across the map. For H₂CO, the temperature is about 80 K and also fairly uniform. We used these two temperatures to calculate the respective H₂ column density maps and found that, for *T* = 8 K, the c-C₃H₂ abundance varies from 4 × 10⁻¹² in the cloud core and eastern edges to 10⁻¹¹ along the western edges. For *T* = 80 K, the H₂CO abundance ranges from 2 × 10⁻¹⁰ around the cloud edges to 10⁻⁹ in the cloud core. The chemical models are able to produce these observed abundances as well. For c-C₃H₂ at 8 K, the models match the observations early in the cold cloud stage. For H₂CO at 80 K, the model matches the observations early in the 80 K warm-up stage – although, the lowest abundances can be reached in the cold cloud stage as well. Therefore, as with AFGL 2591, no shock liberation of either molecule off the grain surfaces is required to produce the observed abundances.

## 6. Summary

This paper covers a targeted survey of carbon chain molecules in two high-mass star-forming regions – AFGL 2591 and IRAS 20126 – towards the Cygnus X star-forming complex. With observations from the IRAM 30 m telescope and the GBT 100 m dish, we present:

1. Detections of numerous rotational transition lines of two carbon chain molecules, CH₃CCH and c-C₃H₂, and two molecules related to the complex organic family, CH₃OH and H₂CO, in both sources;

2. An LTE model, developed in python to loop over several spectra in a map, that determines the physical environment the detected molecular species arise from;

3. Findings of carbon chain and complex organic molecules originating from different gases. The excitation temperature of carbon chains is typically 20-30 K while for complex organics can range from 8-85 K at these large scales (smoothing out the hot core). The velocity structures of each molecular type vary, and trace known structures in the regions;

4. Chemical evolution modelling with NAUTILUS that demonstrates that the observed abundances of CH₃CCH can be produced in the warm-up stage, where the embedded protostar has warmed the cloud to temperatures of 20–30 K. This is consistent with Warm Carbon Chain Chemistry. CH₃OH, on the other hand, requires a non-thermal desorption method to produce the observed abundances. A shock, sputtering the molecules off the dust grain surface, adequately does this. These mechanisms are found for both high-mass star-forming regions.

These results expand on current knowledge of carbon chain chemistry in high-mass star-forming regions. Warm carbon chain chemistry, and other envelope scale chemical mechanisms, can readily be studied with single dish telescopes. For an investigation of hot core chemistry, higher spatial resolution observations are required. These results also demonstrate the use of LTE models for mapping the physical environment using wide-band data cubes – the type of data many modern telescopes are capable of providing.

*Acknowledgements.* This work is based on observations carried out under project numbers 021-20 and 122-20 with the 30m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). The Green Bank Observatory is a facility of the National Science Foundation operated under

cooperative agreement by Associated Universities, Inc. P.F. would like to thank Larry Morgan and Dave Frayer at the GBT/NRAO for their help developing observing scripts as well as GBTIDL scripts for ARGUS calibration and reduction. P.F. would also like to thank Pablo Torne and Monica Rodriguez at IRAM for their help observing with the IRAM telescope. P.F. and R.P. acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), through the Canada Graduate Scholarships – Doctoral, Michael Smith Foreign Study Supplement, and Discovery Grant programs. As researchers at the University of Calgary, P.F. and R.P. acknowledge and pay tribute to the traditional territories of the peoples of Treaty 7, which include the Blackfoot Confederacy (comprised of the Siksika, the Piikani, and the Kainai First Nations), the Tsuut'ina First Nation, and the Stoney Nakoda (including Chiniki, Bearspaw, and Goodstoney First Nations). The City of Calgary is also home to the Métis Nation of Alberta Region 3.

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Zhu, H., Tian, W., Li, A., & Zhang, M. 2017, MNRAS, 471, 3494
## Appendix A: Additional LTE model description

<table><caption>Table A.1: Parameter inputs for the LTE model.</caption><thead><tr><th>Species</th><th>N<sub>tot</sub><br>(cm<sup>-2</sup>)</th><th>T<sub>ex</sub><br>(K)</th><th>v<sub>lsr</sub><br>(km s<sup>-1</sup>)</th><th>FWHM<br>(km s<sup>-1</sup>)</th></tr></thead><tbody><tr><td colspan="5"><strong>AFGL 2591</strong></td></tr><tr><td>CH<sub>3</sub>CCH</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 100, 40</td><td>-10, -2, -5.7</td><td>0.5, 4, 2</td></tr><tr><td>c-C<sub>3</sub>H<sub>2</sub></td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 100, 40</td><td>-10, -2, -5.7</td><td>CH<sub>3</sub>CCH FWHM</td></tr><tr><td>H<sub>2</sub>CO</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 150, 90</td><td>-10, -2, -6</td><td>1, 7, 4</td></tr><tr><td>CH<sub>3</sub>OH C1</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>18</td><td>-10, -1, -6</td><td>1, 7, 3</td></tr><tr><td>CH<sub>3</sub>OH C2</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>8</td><td>-10, -2, -6</td><td>3.9</td></tr><tr><td>CH<sub>3</sub>OH C3</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>8</td><td>-9.4</td><td>2.5</td></tr><tr><td colspan="5"><strong>IRAS 20126</strong></td></tr><tr><td>CH<sub>3</sub>CCH</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 100, 30</td><td>-5, -1, -3.1</td><td>0.5, 4, 2</td></tr><tr><td>c-C<sub>3</sub>H<sub>2</sub> C1</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 100, 10</td><td>-5, -1, -3.1</td><td>0.5, 4, 2.4</td></tr><tr><td>c-C<sub>3</sub>H<sub>2</sub> C2</td><td>1.0 &times; 10<sup>13</sup></td><td>3, 100, 10</td><td>C1 v<sub>lsr</sub></td><td>0.5</td></tr><tr><td>H<sub>2</sub>CO</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 150, 50</td><td>-6, -1, -3.6</td><td>1, 8, 6.9</td></tr><tr><td>CH<sub>3</sub>OH C1</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 150, 50</td><td>-6, -1, -3.6</td><td>1, 8, 6.9</td></tr><tr><td>CH<sub>3</sub>OH C2</td><td>2.1 &times; 10<sup>13</sup></td><td>10</td><td>C1 v<sub>lsr</sub>-0.2</td><td>1.2</td></tr></tbody></table>

**Notes.** Where the parameter is variable, the inputs are minimum, maximum, and starting value, and where the parameter is fixed, the input is the starting value. If a parameter is linked to another, the reference parameter is noted in the table. The final parameter, source size, is always fixed at 20".

<figure><img src="image_19.png" /></figure>

Fig. A.1: Model spectrum, in colour, over the observed data, in black, of AFGL 2591 for transition lines of (a) CH<sub>3</sub>CCH on the brightest pixel of the CH<sub>3</sub>CCH 5<sub>0</sub>-4<sub>0</sub> (E<sub>up</sub> = 12 K), line, and (b) c-C<sub>3</sub>H<sub>2</sub> on the brightest pixel of the c-C<sub>3</sub>H<sub>2</sub> 2<sub>1,2</sub>-1<sub>0,1</sub> (E<sub>up</sub> = 6 K), line. One component is used to fit the two carbon chain molecules together. The dashed grey line represents the observed transition frequency, with the energy level of this transition labelled.
## Appendix A: Additional LTE model description

<table><caption>Table A.1: Parameter inputs for the LTE model.</caption><thead><tr><th>Species</th><th>N<sub>tot</sub><br>(cm<sup>-2</sup>)</th><th>T<sub>ex</sub><br>(K)</th><th>v<sub>lsr</sub><br>(km s<sup>-1</sup>)</th><th>FWHM<br>(km s<sup>-1</sup>)</th></tr></thead><tbody><tr><td colspan="5"><strong>AFGL 2591</strong></td></tr><tr><td>CH<sub>3</sub>CCH</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 100, 40</td><td>-10, -2, -5.7</td><td>0.5, 4, 2</td></tr><tr><td>c-C<sub>3</sub>H<sub>2</sub></td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 100, 40</td><td>-10, -2, -5.7</td><td>CH<sub>3</sub>CCH FWHM</td></tr><tr><td>H<sub>2</sub>CO</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 150, 90</td><td>-10, -2, -6</td><td>1, 7, 4</td></tr><tr><td>CH<sub>3</sub>OH C1</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>18</td><td>-10, -1, -6</td><td>1, 7, 3</td></tr><tr><td>CH<sub>3</sub>OH C2</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>8</td><td>-10, -2, -6</td><td>3.9</td></tr><tr><td>CH<sub>3</sub>OH C3</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>8</td><td>-9.4</td><td>2.5</td></tr><tr><td colspan="5"><strong>IRAS 20126</strong></td></tr><tr><td>CH<sub>3</sub>CCH</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 100, 30</td><td>-5, -1, -3.1</td><td>0.5, 4, 2</td></tr><tr><td>c-C<sub>3</sub>H<sub>2</sub> C1</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 100, 10</td><td>-5, -1, -3.1</td><td>0.5, 4, 2.4</td></tr><tr><td>c-C<sub>3</sub>H<sub>2</sub> C2</td><td>1.0 &times; 10<sup>13</sup></td><td>3, 100, 10</td><td>C1 v<sub>lsr</sub></td><td>0.5</td></tr><tr><td>H<sub>2</sub>CO</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 150, 50</td><td>-6, -1, -3.6</td><td>1, 8, 6.9</td></tr><tr><td>CH<sub>3</sub>OH C1</td><td>1 &times; 10<sup>11</sup>, 1 &times; 10<sup>17</sup>, 1 &times; 10<sup>14</sup></td><td>3, 150, 50</td><td>-6, -1, -3.6</td><td>1, 8, 6.9</td></tr><tr><td>CH<sub>3</sub>OH C2</td><td>2.1 &times; 10<sup>13</sup></td><td>10</td><td>C1 v<sub>lsr</sub>-0.2</td><td>1.2</td></tr></tbody></table>

Notes. Where the parameter is variable, the inputs are minimum, maximum, and starting value, and where the parameter is fixed, the input is the starting value. If a parameter is linked to another, the reference parameter is noted in the table. The final parameter, source size, is always fixed at 20".

<figure><img src="image_18.png" /></figure>

Fig. A.1: Model spectrum, in colour, over the observed data, in black, of AFGL 2591 for transition lines of (a) CH<sub>3</sub>CCH on the brightest pixel of the CH<sub>3</sub>CCH 5<sub>0</sub>-4<sub>0</sub> (E<sub>up</sub> = 12 K), line, and (b) c-C<sub>3</sub>H<sub>2</sub> on the brightest pixel of the c-C<sub>3</sub>H<sub>2</sub> 2<sub>1,2</sub>-1<sub>0,1</sub> (E<sub>up</sub> = 6 K), line. One component is used to fit the two carbon chain molecules together. The dashed grey line represents the observed transition frequency, with the energy level of this transition labelled.
<figure><img src="image_20.png" /><figcaption>Fig. A.2: Model spectrum, in colour, over the observed data, in black, of AFGL 2591 for transition lines of (a) CH<sub>3</sub>OH on the brightest pixel of the CH<sub>3</sub>OH on the brightest pixel of the CH<sub>3</sub>OH 6<sub>0.60</sub>-5<sub>0.50</sub> (E<sub>up</sub> = 48 K) line, and (b) H<sub>2</sub>CO on the brightest pixel of the H<sub>2</sub>CO 4<sub>0.4</sub>-3<sub>0.3</sub> (E<sub>up</sub> = 35 K) line. Three components are used to fit CH<sub>3</sub>OH while one component is used to fit H<sub>2</sub>CO. The dashed grey line represents the observed transition frequency, with the energy level of this transition labelled.</figcaption></figure>
<figure><img src="image_19.png" /></figure>

Fig. A.2: Model spectrum, in colour, over the observed data, in black, of AFGL 2591 for transition lines of (a) CH<sub>3</sub>OH on the brightest pixel of the CH<sub>3</sub>OH on the brightest pixel of the CH<sub>3</sub>OH 6<sub>0.60</sub>-5<sub>0.50</sub> (E<sub>up</sub> = 48 K) line, and (b) H<sub>2</sub>CO on the brightest pixel of the H<sub>2</sub>CO 4<sub>0.4</sub>-3<sub>0.3</sub> (E<sub>up</sub> = 35 K) line. Three components are used to fit CH<sub>3</sub>OH while one component is used to fit H<sub>2</sub>CO. The dashed grey line represents the observed transition frequency, with the energy level of this transition labelled.
<figure><img src="image_21.png" /><figcaption>Fig. A.3: Parameters maps of AFGL 2951 for components two and three of CH₃OH in AFGL 2591 from the LTE model. The black or white ‘x’ in each plot represents the source VLA 3. For both components, T<sub>ex</sub> is fixed at 8 K. For component two, the FWHM is fixed at 3.9 km s<sup>-1</sup>. For component three, the v<sub>lsr</sub> is fixed at -9.4 km s<sup>-1</sup> and the FWHM at 2.5 km s<sup>-1</sup></figcaption></figure>

<figure><img src="image_22.png" /><figcaption>Fig. A.4: Same as Fig. A.1 for (a) CH<sub>3</sub>CCH and (b) c-C<sub>3</sub>H<sub>2</sub> in IRAS 20126. One component is used to fit CH<sub>3</sub>CCH while two components are used for c-C<sub>3</sub>H<sub>2</sub>.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>Fig. A.3: Parameters maps of AFGL 2951 for components two and three of CH₃OH in AFGL 2591 from the LTE model. The black or white ‘x’ in each plot represents the source VLA 3. For both components, T<sub>ex</sub> is fixed at 8 K. For component two, the FWHM is fixed at 3.9 km s<sup>-1</sup>. For component three, the v<sub>lsr</sub> is fixed at -9.4 km s<sup>-1</sup> and the FWHM at 2.5 km s<sup>-1</sup></figcaption></figure>

<figure><img src="image_21.png" /><figcaption>Fig. A.4: Same as Fig. A.1 for (a) CH<sub>3</sub>CCH and (b) c-C<sub>3</sub>H<sub>2</sub> in IRAS 20126. One component is used to fit CH<sub>3</sub>CCH while two components are used for c-C<sub>3</sub>H<sub>2</sub>.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>Fig. A.5: Same as Fig. A.2 for (a) CH₃OH and (b) H₂CO in IRAS 20126. Two components are used to fit CH₃OH while one component is used to fit H₂CO.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>Fig. A.5: Same as Fig. A.2 for (a) CH₃OH and (b) H₂CO in IRAS 20126. Two components are used to fit CH₃OH while one component is used to fit H₂CO.</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>Fig. A.6: Parameters maps for component two of *c*-C~3~H~2~ and CH~3~OH in IRAS 20126 from the LTE model. The black or white ‘x’ in each plot represents the protostar. For *c*-C~3~H~2~, N~tot~ is fixed at 1 × 10~13~ cm~−2~ and the FWHM at 0.5 km s~−1~. For CH~3~OH, N~tot~ is fixed at 2.1 × 10~13~ cm~−2~, T~ex~ at 10 K the FWHM at 1.2 km s~−1~.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>Fig. A.6: Parameters maps for component two of *c*-C₃H₂ and CH₃OH in IRAS 20126 from the LTE model. The black or white ‘x’ in each plot represents the protostar. For *c*-C₃H₂, Nᵢₜₐ is fixed at 1 × 10¹³ cm⁻² and the FWHM at 0.5 km s⁻¹. For CH₃OH, Nᵢₜₐ is fixed at 2.1 × 10¹³ cm⁻², Tₑₓ at 10 K the FWHM at 1.2 km s⁻¹.</figcaption></figure>