The Family of Amide Molecules toward NGC 6334I

Life as we know it relies on a select number of recurring chemical structures. Among these chemical structures, the amide unit (R₁–C(=O)N–R₂R₃, see the top panel of Figure 1), is an essential component in numerous biomolecules. Notable examples are nucleobases, the molecules that encode RNA and DNA, and as the link in the peptide chains that form proteins. In order to understand the chemical origin of life, it is important to understand how molecules with an amide unit came to be on Earth. Chemical reactions on a young Earth can give rise to this class of molecules (e.g., Patel et al. 2015). However, a fascinating alternative is the formation of amides in space and subsequent delivery by, for example, meteorites to Earth (e.g., Chyba et al. 1990).

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Several small amide molecules have been detected toward star- and planet-forming regions, most of them in recent years, such as formamide (NH₂CHO; Rubin et al. 1971), acetamide (CH₃C(O)NH₂; Hollis et al. 2006), and N-methylformamide (CH₃NHCHO; Belloche et al. 2017, 2019). The first evidence for carbamide, also known as urea (NH₂C(O)NH₂), in the gas phase in the interstellar medium comes from Remijan et al. (2014), but it was securely identified by Belloche et al. (2019). Molecules that are closely related to the amide structure have been observed as well. Examples are cyanamide ⁶ (NH₂CN; Turner et al. 1975), isocyanic acid (HNCO; Snyder & Buhl 1972), and methyl isocyanate (CH₃NCO; Halfen et al. 2015; Cernicharo et al. 2016; Ligterink et al. 2017). These molecules have structural similarities with biomolecules (see the bottom panel of Figure 1) and can act as precursor molecules to their formation.

To assess the importance of interstellar amides for biochemistry, it is important to know if they commonly occur in star- and planet-forming regions. The simplest amide NH₂CHO and related molecule NH₂CN have been detected toward a number of inter- and circumstellar sources (Belloche et al. 2020), including the Sun-like protobinary IRAS 16293-2422 (Kahane et al. 2013; Coutens et al. 2016, 2018 hereafter IRAS 16293). NH₂CHO was also detected in comet 67P/Churyumov-Gerasimenko (Goesmann et al. 2015; Altwegg et al. 2017, hereafter 67P/C-G). However, larger amides have almost exclusively been observed toward the Galactic center source Sagittarius B2 (hereafter Sgr B2, Halfen et al. 2011; Cernicharo et al. 2016; Belloche et al. 2017, 2019; Ligterink et al. 2018a).

An expanded sample of amide detections can help elucidate both their interstellar chemistry and the interplay of that chemistry with physical processes during star formation. For example, a large sample of HNCO and NH₂CHO observations clearly shows an abundance correlation between these two species over a variety of sources (López-Sepulcre et al. 2019, and references therein). Hydrogenation (the addition of hydrogen atoms) of HNCO to form NH₂CHO has been proposed as a chemical explanation for the link between the two molecules (López-Sepulcre et al. 2015). However, recent modeling work indicates that the relation between the two molecules is instead due to similar responses of the formation pathways of each molecule to the physical environment (Quénard et al. 2018). Analogous analytical approaches could provide valuable insight into other amide species. But while chemical reactions for such molecules have recently been included in chemical networks (Belloche et al. 2017, 2019), observational constraints for these networks are sparse. To unravel interstellar amide chemistry and determine its relevance to biochemistry, it is important to expand the number of amide detections and to understand their formation. An ideal interstellar laboratory to study amides is found in the high-mass star-forming region (HMSFR) NGC 6334I. First, since the solar system is thought to have formed in a massive cluster (e.g., Dukes & Krumholz 2012), NGC 6334I observations present a "direct" look at the conditions of early solar system formation. Second, the chemistry of this region has been studied in great detail in the past decades, for example with the Herschel Space Observatory (Zernickel et al. 2012). In recent years, NGC 6334I has been extensively studied with the Atacama Large Millimeter/submillimeter Array (ALMA). This has not only resulted in a better understanding of the physical structure of this source (Brogan et al. 2016, 2018), but also a better understanding of its molecular inventory. Various oxygen-bearing molecules have been investigated, including the C₂H₄O₂ isomers methyl formate (CH₃OCHO), glycolaldehyde (HC(O)CH₂OH), and acetic acid (CH₃COOH; McGuire et al. 2018; El-Abd et al. 2019; Xue et al. 2019), deuterated methanol (Bøgelund et al. 2018), and even the first ever interstellar detection of methoxymethanol (CH₃OCH₂OH; McGuire et al. 2017) present toward this source. Additionally, a number of complex nitrogen-bearing molecules were investigated by Bøgelund et al. (2019b), including NH₂CHO.

In this work, the inventory of amides toward NGC 6334I is investigated in detail. The first detections toward this object of NH₂CN, CH₃C(O)NH₂, and CH₃NHCHO are presented and combined with analyses of HNCO and NH₂CHO, and the tentative detection of NH₂C(O)NH₂. These observations and results are presented in Sections 2 and 3. The implications of the results are discussed in Section 4, before conclusions are given in Section 5.

2.1. Observational Data of NGC 6334I

In this work, ALMA observations of NGC 6334I in bands 4, 7, and 10 are analyzed. The details of the individual data sets and their reductions have been published in detail elsewhere (McGuire et al. 2017, 2018; Brogan et al. 2018) and will only be discussed briefly here. The pertinent observing parameters for each set of observations are provided in Table 2. The data were self-calibrated and continuum-subtracted following the procedures detailed in Hunter et al. (2017) and Brogan et al. (2018). Prior to analysis, all of the observations were convolved to a common synthesized beam size of 026 × 026. From the data cubes, background temperatures (T_BG) were determined based on the continuum level temperature and spectra were extracted for analysis at the same positions as used in El-Abd et al. (2019) and at an additional position called MM1-nmf. These positions are chosen to give an accurate overall sample of the chemistry toward NGC 6334I MM1 and MM2. The parameters of these positions are listed in Table 1 and are indicated on moment 0 maps shown in Figure 2. In the case of Band 10 observations, robust analysis was only possible toward positions in MM1, as the MM2 source falls at the edge of the primary beam.

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Table 1. Physical Parameters for Spectra Analyzed toward NGC 6334I

Position	R.A.	Decl.	${V}_{\mathrm{LSR}}$ a	${\rm{\Delta }}V$	${T}_{\mathrm{ex}}$	${T}_{\mathrm{BG}}$ b
						(a)	(b)	(c)	(d)
	(hh:mm:ss)	(dd:mm:ss)	(km s−1)	(km s−1)	(K)	(K)
MM1-i	17:20:53.373	−35.46.58.341	−7.0	3.25	135	10.1	26.9	31.3	31.3
MM1-ii	17:20:53.386	−35.46.57.112	−4.0 to −4.5	3.25	175	38.9	79.7	94.6	85.1
MM1-iii	17:20:53.387	−35.46.57.533	−5.2	3.00	225	50.8	108.2	129.7	120.3
MM1-iv	17:20:53.420	−35.46.59.088	−8.2	3.50	150	15.7	38.4	44.8	73.6
MM1-v	17:20:53.434	−35.46.57.856	−4.4	3.25	285	69.0	159.3	192.7	176
MM1-vi	17:20:53.435	−35.46.58.731	−7.0 to −7.5	3.25	190	30.2	77.6	88.6	94.4
MM1-vii	17:20:53.459	−35.46.57.661	−4.0	3.00	185	43.4	96.5	112.4	106.7
MM1-viii	17:20:53.469	−35.46.58.724	−6.8	3.00	150	25.8	57.1	64.7	81.9
MM1-ix	17:20:53.475	−35.46.57.156	−5.0	2.50	150	16.9	36.8	45.2	65.7
MM1-nmf	17:20:53.461	−35.46.57.284	−4.5	2.50	150	31.1	66.8	80.9	88.1
MM2-i	17:20:53.152	−35.46.59.416	−9.0	2.80	150	9.2	21.8	27.8	57.5
MM2-ii	17:20:53.178	−35.46.59.494	−9.0	2.80	200	17.4	44.4	58.6	98.4
MM2-iii	17:20:53.202	−35.46.59.175	−7.8	2.80	180	18.9	44.2	57.7	76.1

Notes.

^a For some positions certain molecules are fit with a different ${V}_{\mathrm{LSR}}$. For these cases a ${V}_{\mathrm{LSR}}$ range is given. ^b Four different background temperature values are given for: (a) band 4, (b) band 7 $\leqslant 300\,\mathrm{GHz}$, (c) band 7 $\geqslant 300\,\mathrm{GHz}$, and (d) band 10 observations, respectively.

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Table 2. Summary of Pertinent ALMA Observing Parameters

Parameter	Band 4	Band 7 Low	Band 7 High	Band 10
Project code	#2017.1.00661.S	#2015.A.00022.T	#2015.A.00022.T	#2017.1.00717.S
Configuration(s)	C43-6	C36-4, C36-5	C36-4, C36-5	C43-3
Primary beam FWHM (${}^{{\prime\prime} }$)	41	20	17	6.6
Angular resolution (${}^{{\prime\prime} }$) a	$0.23\times 0.16$	$0.25\times 0.19$	$0.22\times 0.17$	$0.21\times 0.15$
Frequency (GHz)	130.03–132.02	279.17–282.94	336.18–339.94	873.88–881.33
	144.00–145.87	291.18–294.94	348.18–351.94	890.25–897.70
Spectral resolution (km s−1)	1.1	1.1	1.1	0.5
rms (mJy beam−1 km s−1)	0.8	2.0	3.3	44

Note.

^a Note that all the observations were convolved to a common synthesized beam size of $0\buildrel{\prime\prime}\over{.} 26\times 0\buildrel{\prime\prime}\over{.} 26$.

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2.2. Molecule Identification and Analysis

Except for position MM2-iii, molecular line emissions of NH₂CN, HN¹²CO, HN¹³CO, NH${}_{2}^{12}$CHO, NH${}_{2}^{13}$CHO, and CH₃C(O)NH₂ are detected toward all positions. The detected lines are listed in Table C1. Many of the lines of the main isotopologues of HNCO and NH₂CHO are optically thick and therefore not used for further analysis. Following the routine described in Section 2.2, the spectra of the remaining molecules were simulated and column densities derived. NH₂CN and CH₃C(O)NH₂ are detected for the first time toward NGC 6334I and their detected spectral lines toward position MM1-i are shown in Figures 3 and 4. The spectra of the other molecules toward MM1-i and all detections toward MM2-i are presented in Appendix D. The derived column densities toward each position are listed in Table 3. For further analysis, the column densities of HN¹²CO and NH${}_{2}^{12}$CHO are determined from their optically thin ¹³C counterparts. To derive the main isotopologue column density, the column densities of the ¹³C isotopologues are multiplied with the ${}^{12/13}{\rm{C}}$ ratio of 62 for NGC 6334I, as determined by Bøgelund et al. (2018), based on equations given by Milam et al. (2005). Moment 0 maps of lines of NH₂CN, HN¹²CO, NH${}_{2}^{12}$CHO, and CH₃C(O)NH₂ overplotted on dust-continuum contours are shown in Figure 2.

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Table 3. Column Densities and Upper Limits of Amides Analyzed in This Work

Position	${T}_{\mathrm{ex}}$	N(NH2CN)	N(HN13CO)	N(NH${}_{2}^{13}$CHO)	N(CH3C(O)NH2)	N(CH3NHCHO)	N(NH2C(O)NH2)
	(K)	(cm−2)	(cm−2)	(cm−2)	(cm−2)	(cm−2)	(cm−2)
MM1-i	135	(5.9 ± 0.9) × 1015	(9.9 ± 3.0) × 1015	(8.6 ± 1.4) × 1015	(4.7 ± 0.7) × 1016	$\unicode{x02A89}$(7.5 ± 2.8) × 1016	$\leqslant$2.0 × 1015
MM1-ii	175	(1.8 ± 0.3) × 1016	(4.5 ± 1.4) × 1016	(2.2 ± 0.6) × 1016	(7.4 ± 1.1) × 1016	$\leqslant$1.5 × 1017	$\leqslant$5.0 × 1015
MM1-iii	225	(3.5 ± 0.5) × 1016	(6.0 ± 1.8) × 1016	(3.2 ± 0.5) × 1016	(1.7 ± 0.4) × 1017	$\leqslant$2.5 × 1017	$\leqslant$1.0 × 1016
MM1-iv	150	(7.8 ± 1.6) × 1015	(1.3 ± 0.2) × 1016	(1.3 ± 0.2) × 1016	(4.0 ± 1.2) × 1017	$\unicode{x02A89}$(1.0 ± 0.4) × 1017	$\leqslant$1.0 × 1016
MM1-v	285	(4.0 ± 0.4) × 1016	(1.2 ± 0.1) × 1017	(2.7 ± 0.3) × 1016	(2.3 ± 0.2) × 1017	(3.6 ± 0.7) × 1017	$\unicode{x02A89}$(5.0 ± 2.0) × 1016
MM1-vi	190	(2.6 ± 0.6) × 1016	(3.7 ± 0.5) × 1016	(2.8 ± 0.3) × 1016	(1.6 ± 0.2) × 1017	(2.0 ± 0.6) × 1017	$\leqslant$1.0 × 1016
MM1-vii	185	(1.6 ± 0.3) × 1016	(4.4 ± 0.5) × 1016	(3.8 ± 0.4) × 1016	(1.6 ± 0.2) × 1017	(2.0 ± 0.6) × 1017	$\leqslant$3.0 × 1016
MM1-viii	150	(7.8 ± 0.8) × 1016	(2.0 ± 0.3) × 1016	(1.6 ± 0.3) × 1016	(7.1 ± 1.1) × 1016	(1.4 ± 0.5) × 1017	$\leqslant$2.0 × 1016
MM1-ix	150	(2.3 ± 0.4) × 1015	(6.4 ± 1.3) × 1015	(4.8 ± 0.5) × 1015	(1.6 ± 0.3) × 1016	⪉(4.8 ± 1.5) × 1016	$\leqslant$1.5 × 1016
MM1-nmf	150	(1.1 ± 0.2) × 1016	(1.9 ± 0.3) × 1016	(1.6 ± 0.3) × 1016	(6.8 ± 1.3) × 1016	(1.0 ± 0.5) × 1017	$\leqslant$1.0 × 1016
MM2-i	150	(3.5 ± 0.6) × 1015	(1.4 ± 0.5) × 1016	(3.3 ± 1.0) × 1015	(5.9 ± 1.6) × 1016	$\leqslant$5.0 × 1016	$\leqslant$3.0 × 1015
MM2-ii	200	(4.0 ± 0.5) × 1015	(2.3 ± 0.7) × 1016	(3.8 ± 1.1) × 1015	(8.8 ± 1.6) × 1016	$\leqslant$6.0 × 1016	$\leqslant$4.0 × 1015

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Toward all positions, spectral features are identified that can be assigned to CH₃NHCHO, although many of these lines are blended in the wings of other spectral features or on locations where the baseline dips. Despite this, a number of unblended CH₃NHCHO lines can be identified in the spectra of positions MM1-v, vi, vii, and viii, and in particular MM1-nmf, which makes it possible to claim the first detection of CH₃NHCHO toward NGC 6334I. The detected CH₃NHCHO lines toward position MM1-nmf are shown in Figure 5. Figure D5 in Appendix D shows the CH₃NHCHO lines in the MM1-nmf spectrum that are present on locations where the baseline dips and thus are not well reproduced by the synthetic spectrum. The spectral features identified toward the other positions in MM1 and in MM2 are insufficient to claim a detection and therefore CH₃NHCHO is tentatively identified toward these positions. Column densities of this species are presented in Table 3.

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At a number of positions a handful of spectral features are identified that can be assigned to NH₂C(O)NH₂. The most and clearest lines are identified toward position MM1-v, see Figure 6, with at least three unblended lines. While these lines are not enough for a secure detection, a tentative identification can be claimed. The tentative and upper-limit column densities of NH₂C(O)NH₂ are presented in Table 3.

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4.1. Chemical Comparisons

To get a better understanding of the chemical processes leading to amides, it is useful to compare molecular ratios of the positions within NGC 6334I, but also with other inter- and circumstellar sources. In the following subsections molecular ratios with respect to NH₂CHO are discussed. Comparisons are made with the following works: IRAM 30 m and Green Bank Telescope (GBT) single-dish observations of G + 0.693-0.027 (hereafter G + 0.693), a quiescent giant molecular cloud (Zeng et al. 2018; Jiménez-Serra et al. 2020); ALMA observations of the incipient hot corino Barnard B1b-S (Marcelino et al. 2018); Northern Extended Millimeter Array observations of the low-mass protostars SVS13A, NGC 1333-IRAS4A2, NGC 1333-IRAS2A(1) (Coutens et al. 2018; Belloche et al. 2020); ALMA Protostellar Interferometric Line Survey (Jørgensen et al. 2016) observations of the low-mass protostar IRAS 16293B (Coutens et al. 2016, 2018; Ligterink et al. 2018a); GBT and IRAM 30 m single-dish observations of the Galactic center source Sgr B2 (Hollis et al. 2006; Belloche et al. 2013); ALMA observations of the HMSFR Orion KL (Cernicharo et al. 2016); the high-mass star AFGL 4176 (Bøgelund et al. 2019a); and of Sgr B2 (Belloche et al. 2017, 2019). One has to be aware that precise knowledge of the source size or emitting area of a certain molecule is essential for ratio comparisons. In particular, single-dish telescopes do not have the spatial resolution to determine the source size and these are often assumed or estimated, potentially resulting in large column density errors. Finally, mass spectrometric measurements of the Rosetta mission to comet 67P/C-G are used to compare with a solar system object (Goesmann et al. 2015; Altwegg et al. 2017). The sources are placed in the following order: comet 67P, quiescent molecular cloud, low-mass protostars/hot corinos, NGC 6334I and other HMSFRs, high-mass protostars, and hot cores. The derived ratios are presented in Figure 7.

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4.1.1. Variations in NH₂CN Abundance

Within NGC 6334I, the NH₂CN over NH₂CHO ratio shows only minor variations and in general is uniform at ∼1%–2%. More noticeable is the variation between other sources, which span almost three orders of magnitude. Interestingly, the highest ratios are found for the quiescent molecular cloud and low-mass protostars, while lower ratios are seen in the high-mass sources NGC 6334I and Sgr B2. An important deviation in this pattern is seen in Orion KL, where [NH₂CN]/[NH₂CHO] = 0.4–1.4 (White et al. 2003, data point not included in Figure 7). However, it is important to note that the column density of NH₂CHO in Orion KL was determined using a single spectral line and there is some uncertainty in the assumed excitation temperatures. If this trend is in fact dependent on the source type, it indicates that the source influences either the formation pathway or desorption from grain surfaces of one of the two species.

Recently, Coutens et al. (2018) used chemical models to explain the presence of NH₂CN toward IRAS 16293-2422B. Chemical networks for this molecule are very sparse, and no plausible gas-phase mechanism for its production is currently evident. Coutens et al. (2018) introduced a formation route for NH₂CN on grain surfaces through the reaction between NH₂ and CN radicals. In their chemical models they determined that this reaction is sensitive to the gas-phase H and H₂ density: at high gas densities ($\gt {10}^{9}$ cm⁻³), hydrogenation of the NH₂ and CN radicals on the ice mantle surface becomes important and the production of NH₂CN stagnates. Since HMSFRs generally speaking have higher gas densities, this could explain the trend in the observational data if particular sources enter this density regime for long periods when NH₂CN might otherwise be forming.

The formation of NH₂CHO is still a topic of debate, with proponents of both gas-phase and solid-state ice mantle formation routes (see López-Sepulcre et al. 2019 for an overview of this discussion). Several computational studies have shown that NH₂CHO can be formed in the gas phase by a near barrierless reaction between NH₂ and H₂CO (e.g., Barone et al. 2015; Skouteris et al. 2017):

$$\begin{eqnarray}&&{\mathrm{NH}}_{2}+{{\rm{H}}}_{2}\mathrm{CO}\longrightarrow {\mathrm{NH}}_{2}\mathrm{CHO}+{\rm{H}}.\end{eqnarray} \tag{ 1 }$$

Many studies have been dedicated to understanding the formation of NH₂CHO in ice mantles, with the main proposed formation routes being the radical reaction between NH₂ and CHO directly yielding NH₂CHO,

$$\begin{eqnarray}&&{\mathrm{NH}}_{2}+\mathrm{CHO}\longrightarrow {\mathrm{NH}}_{2}\mathrm{CHO}\end{eqnarray} \tag{ 2 }$$

and the successive hydrogenation of HNCO (Raunier et al. 2004; Garrod et al. 2008; Jones et al. 2011; Rimola et al. 2018; Enrique-Romero et al. 2019; Haupa et al. 2019):

$$\begin{eqnarray}\begin{array}{rcl}{\rm{H}} & + & \mathrm{HNCO}\longrightarrow {\mathrm{NH}}_{2}\mathrm{CO}\\ {\rm{H}} & + & {\mathrm{NH}}_{2}\mathrm{CO}\longrightarrow {\mathrm{NH}}_{2}\mathrm{CHO},\end{array}\end{eqnarray} \tag{ 3 }$$

although experimental work by Noble et al. (2015) indicates that HNCO cannot be hydrogenated. A third option is the reaction of CN radicals with water ice as proposed in theoretical work by Rimola et al. (2018):

$$\begin{eqnarray}\begin{array}{rcl}\mathrm{CN} & + & {{\rm{H}}}_{2}{\rm{O}}\longrightarrow {\mathrm{NH}}_{2}\mathrm{CO}\\ {\mathrm{NH}}_{2}\mathrm{CO} & + & {\rm{H}}\longrightarrow {\mathrm{NH}}_{2}\mathrm{CHO}.\end{array}\end{eqnarray} \tag{ 4 }$$

The aforementioned influence of the hydrogen density on the NH₂CN formation reaction would also be expected to influence NH₂CHO formation in the ice. Reaction (2) would become less important, instead favoring the hydrogenation of NH₂ to NH₃:

$$\begin{eqnarray}&&{\mathrm{NH}}_{2}+{\rm{H}}\longrightarrow {\mathrm{NH}}_{3},\end{eqnarray} \tag{ 5 }$$

while at the same time the increased availability of atomic hydrogen could increase the efficiency of Reaction (3), potentially allowing it to dominate. On the other hand, gas-phase production of NH₂CHO can increase under the condition that gas-phase NH₂ and H₂CO densities increase. This can happen as the gas-phase density increases from low-mass to high-mass sources. Finally, top-down reaction pathways can contribute to an increased NH₂CHO column density. For example, radiation interacting with gas-phase CH₃C(O)NH₂ can produce the NH₂CO radical, which in turn can produce NH₂CHO (e.g., Spall & Steacie 1957). If radiation fields increase in strength from low-mass to high-mass sources, such a channel can become prominent and result in a lower [NH₂CN]/[NH₂CHO] ratio. However, for larger molecules, such as dipeptides, experiments show that photolysis of these species primarily leads to the breaking of the peptide bond itself, instead of its surrounding bonds (Johns et al. 1968; Neubacher & Schnepel 1977, at conditions not relevant to the interstellar medium). It is therefore questionable whether top-down chemistry can efficiently produce NH₂CHO (see also Section 4.1.2), but dedicated experiments under interstellar conditions are needed to answer this question. In general, care has to be taken with these chemical explanations, as they will influence formation pathways of, and ratios between, other species as well. Yet, trends like [NH₂CN]/[NH₂CHO] are not observed for most other species, such as HNCO and CH₃C(O)NH₂, see Figure 7.

Alternatively, the ice mantle desorption characteristics can explain the observed trend. Thermal desorption depends on the binding energy of the molecule. Recent laboratory studies using temperature-programmed desorption investigated the binding energy of NH₂CHO on graphite and on amorphous water deposited onto graphite (Chaabouni et al. 2018). In the latter case, NH₂CHO remained on the surface until after the water had already desorbed. Those authors found binding energies of 4810 and 5056–6990 K for amorphous water and for formamide, respectively (assuming a desorption-rate prefactor of 10¹² s⁻¹); the binding energy for NH₂CHO corresponds simply to binding to the graphite left behind following the desorption of the water. The desorption energy of NH₂CN is not experimentally determined, but chemical models commonly use binding energies of 5556 K for both species, based on the interpolation method described by Garrod & Herbst (2006). If the true value for NH₂CN were close to or less than the binding energy of water, or otherwise much less than that of NH₂CHO, it could influence the [NH₂CN]/[NH₂CHO] trend. When a source is sufficiently warm over a large area, such as a HMSFR, most volatile organic species will desorb from the grains. However, a more compact source, such as a single low-mass protostar, will have a decreasing radial temperature profile. Around such sources, species with a low desorption energy desorb over a larger area than those with a high desorption energy. Depending on the spatial resolution of the observations, the beam may simply cover a larger emitting area of NH₂CN than of NH₂CHO, thus explaining the decreasing ratio. To better understand how desorption characteristics influence the [NH₂CN]/[NH₂CHO] ratio, laboratory measurements of the NH₂CN desorption energy are required, but nonthermal desorption processes of NH₂CN and NH₂CHO, such as reactive desorption and photodesorption, need to be studied as well.

4.1.2. HNCO Chemistry

For many years, an abundance correlation between HNCO and NH₂CHO has been observed toward a large range of interstellar sources, which has led to the belief that there is a direct chemical relationship between these two species (see López-Sepulcre et al. 2019 for a recent review). In Figure 7, the [HNCO]/[NH₂CHO] ratios found toward NGC 6334I are compared with ratios from a select number of other sources. We note that the HNCO column density is determined from its ¹³C isotope, multiplied by a ${}^{12/13}{\rm{C}}$ ratio of 62. In NGC 6334I MM1 the [HNCO]/[NH₂CHO] is slightly above one, while in MM2 it is somewhat higher at a ratio of ∼5. Therefore, the ratios in NGC 6334I follow the average interstellar trend and strengthen the probable link between HNCO and NH₂CHO. Whether this is a direct chemical link (e.g., Raunier et al. 2004; Haupa et al. 2019) or an effect caused by similar chemical responses to physical conditions (Quénard et al. 2018) cannot be inferred from these ratios. Since HNCO and CH₃C(O)NH₂ are found cospatially toward NGC 6334I, gas-phase ion–molecule destruction of CH₃C(O)NH₂ may be to some extent responsible for the observed abundances of HNCO, as was suggested by Garrod et al. (2008) and Tideswell et al. (2010). In the aforementioned works, CH₃C(O)NH₂ forms by radical–radical reactions on grain surfaces, together with NH₂CHO, where afterwards it is released to the gas phase and can be destroyed by ions. This pathway therefore provides an indirect link between HNCO and NH₂CHO, by way of CH₃C(O)NH₂ and other larger amides, assuming that NH₂CHO and larger amides form in related reactions in ice mantles. This mechanism can help explain the higher [HNCO]/[NH₂CHO] ratio in MM2, if more ions are present in MM2, for example due to a stronger radiation field, which results in enhanced HNCO production.

4.1.3. The Large Amides

Of the large amides, CH₃C(O)NH₂ is detected toward most positions. A relatively small abundance difference is seen between MM1 and MM2, but in general ratios are quite similar throughout the region. This suggests that production of CH₃C(O)NH₂ is occurring through a similar mechanism throughout the region.

Various gas-phase formation routes of CH₃C(O)NH₂ have been proposed and computationally tested (Hollis et al. 2006; Quan & Herbst 2007; Halfen et al. 2011; Redondo et al. 2014), with the reaction

$$\begin{eqnarray}&&\begin{array}{l}{\mathrm{NH}}_{2}\mathrm{CHO}+{\mathrm{CH}}_{5}^{\,+}\longrightarrow {\mathrm{CH}}_{3}{\rm{C}}({\rm{O}}){\mathrm{NH}}_{3}^{+}+{{\rm{H}}}_{2}\\ {\mathrm{CH}}_{3}{\rm{C}}({\rm{O}}){\mathrm{NH}}_{3}^{+}+{{\rm{e}}}^{-}\longrightarrow {\mathrm{CH}}_{3}{\rm{C}}({\rm{O}}){\mathrm{NH}}_{2}+{\rm{H}}\end{array}\end{eqnarray} \tag{ 6 }$$

considered to be the most efficient. In ice mantles, however, CH₃C(O)NH₂ is thought (e.g., Agarwal et al. 1985; Ligterink et al. 2018a) to be primarily produced by Reaction (7):

$$\begin{eqnarray}&&{\mathrm{CH}}_{3}+{\mathrm{NH}}_{2}\mathrm{CO}\longrightarrow {\mathrm{CH}}_{3}{\rm{C}}({\rm{O}}){\mathrm{NH}}_{2}.\end{eqnarray} \tag{ 7 }$$

Laboratory simulations of chemical reactions in the ice mantles of interstellar dust grains found a [CH₃C(O)NH₂]/[NH₂CHO] ratio of 0.4 upon desorption from the surface (Ligterink et al. 2018a), close to the observed average of ∼0.15 in NGC 6334I. Ice formation of CH₃C(O)NH₂, in particular during the dense cloud stage of star formation, is a plausible mechanism to explain its uniform ratio derived throughout NGC 6334I. While abundances also seem to be similar between NGC 6334I and other sources, it is too early to say if the general interstellar CH₃C(O)NH₂ chemistry is the result of the same process(es). A larger sample with confirmed detections and better constraints on the source sizes of the emitting regions toward these objects is required.

Both CH₃C(O)NH₂ and CH₃NHCHO are present at ∼10% with respect to NH₂CHO toward NGC 6334I. This similarity in abundances supports the conclusion of Belloche et al. (2017) that kinetic processes are at the basis of the formation of these species rather than thermal equilibrium. This species can form by energetically processing frozen CH₃NH₂:CO mixtures (Frigge et al. 2018) and thus can be formed on interstellar dust grains. We note that CH₃NH₂ is abundantly present toward NGC 6334I (Bøgelund et al. 2019b). CH₃NHCHO abundances are similar between NGC 6334I and Sgr B2(N1S), the only other source where this molecule is securely detected (Belloche et al. 2019). However, compared to a tentative identification toward Sgr B2(N2), a difference of almost an order of magnitude is seen. At this point it not possible to draw any conclusions on the overall interstellar chemistry of CH₃NHCHO and more observations of this molecule are required.

The ratios of the tentative detection and upper limits of NH₂C(O)NH₂ vary by about an order of magnitude within NGC 6334I. It is possible that high temperatures are needed to desorb NH₂C(O)NH₂ from dust grains, thus causing this variation. We note that this molecule is tentatively detected toward position MM1-v, which has the highest excitation temperature, and perhaps physical temperature, of 285 K. Compared to secure detections of NH₂C(O)NH₂ toward Sgr B2(N1S) and G + 0.693 (Belloche et al. 2019; Jiménez-Serra et al. 2020), abundances seem to be similar at the few percent level. However, as with the case of CH₃NHCHO, it is too early to draw conclusions about the formation of NH₂C(O)NH₂ at this stage. The tentative identification of NH₂C(O)NH₂ warrants future searches for this molecule toward NGC 6334I in order to securely detect this species, better constrain its abundance, and refine and constrain chemical models to elucidate its chemistry.

4.2. Implications for Biomolecule Formation

The detection of CH₃C(O)NH₂ and tentative identification of CH₃NHCHO in NGC 6334I strengthens the case that these molecules can contribute to the formation of biomolecules. It is possible that, in the interstellar medium, these molecules act as precursor species to, or form in parallel with, larger biomolecules. Multiple laboratory experiments show that in UV irradiated simulated ice mantles, consisting of simple molecules such as H₂O, NH₃, CH₃OH, and CH₄, simple amides and molecules of greater complexity, including biomolecules, form simultaneously (e.g., Agarwal et al. 1985; Caro et al. 2002; Caro & Schutte 2003; Nuevo et al. 2010; Ligterink et al. 2018a; Oba et al. 2019). If the formation of amides in the interstellar medium primarily occurs on interstellar dust grains, it is not unlikely that biomolecules are present as well, at least on grain surfaces. Their presence and abundance in the gas phase depends on the local physical conditions and parameters such as the binding energy and photodissociation rate.

Alternatively, amides can also be involved in the earliest stages of biochemistry after delivery to a newly formed planet. Under early Earth conditions and catalyzed by minerals, metals, or clays, NH₂CHO can form a variety of biomolecules, most notably nucleobases and amino acids (Saladino et al. 2012, and references therein). Cyanamide and carbamide are known to act as condensing agents in amino acids and nucleotide polymerization reactions (Ibanez et al. 1971; Sakurai & Yanagawa 1984). It is not unlikely that CH₃C(O)NH₂ and CH₃NHCHO engage in similar reactions, but laboratory investigations on this topic are limited.

The cocktail of molecules found toward NGC 6334I, but also other sources such as the Sun-like protostar IRAS 16293B, presents more options to directly form biomolecules on a young Earth. This is highlighted by the reaction between NH₂CN and glycolaldehyde (HC(O)CH₂OH), resulting in either 2-aminooxazole or 2-aminoimidazole, a precursor to nucleotide formation and a catalyst for RNA copying, respectively (see Figure 8, Patel et al. 2015; Fahrenbach et al. 2017). In the aforementioned works, both NH₂CN and HC(O)CH₂OH are part of in situ chemistry on a planetary body (Earth in this case) deriving from HCN and involve multiple reaction steps to produce these species. Formation of these two species in space and subsequent delivery to a young Earth directly supplies the in situ chemical network with these compounds, leading to a more rapid emergence of biomolecules.

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In this work, a group of amides is analyzed toward the high-mass star-forming region NGC 6334I. Secure detections of NH₂CN, HNCO, NH₂CHO, CH₃C(O)NH₂, and CH₃NHCHO are presented along with a tentative identification of NH₂C(O)NH₂. Abundances of amides are determined with respect to NH₂CHO and are generally found to be similar within NGC 6334I, often varying less than a factor of five, hinting that the same chemical pathways and physical conditions are at the basis of this chemistry, despite the large size and physical differences of this HMSFR. It is plausible that production of these amides occurred on surfaces of interstellar dust grains during the dense cloud stage of the star formation process in NGC 6334I. The relatively minor abundance variations between MM1 and MM2 can be the result of slightly different physical conditions, such as higher radiation fields, resulting in the efficient photodestruction of one or more of the amides, either in the ice mantles or gas phase.

The comparison of (tentative) abundances of CH₃C(O)NH₂, CH₃NHCHO, and NH₂C(O)NH₂ in NGC 6334I with abundances of these species toward other sources shows that these are quite similar. This hints that similar processes may be responsible for the formation of these species, but due to the limited number of detections and different observing parameters, it is too early to draw any strong conclusion. A different trend is observed for [NH₂CN]/[NH₂CHO], for which abundances vary by three orders of magnitude over a large number of observed sources. This variation seems to depend on the source type, with high ratios toward low-mass and prestellar sources and low ratios toward high-mass sources. The origin of this variation is not easily explained and could depend on different reaction pathways and physical conditions, and is perhaps affected by observational parameters as well. More experimental and theoretical work is needed to confirm this. However, it is likely that NH₂CN is not part of the same chemistry that produces the other amides. The rich amide chemistry toward NGC 6334I, in combination with other molecules, such as HC(O)CH₂OH, strengthens the case that interstellar molecules contribute to the rapid emergence of biomolecules on planetary objects.

We thank Dr. E. G. Bøgelund for helpful discussions and support with the data analysis. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2017.1.00717.S, #2017.1.00661.S, and #2015.A.00022.T. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Support for B.A.M. was provided by NASA through Hubble Fellowship grant #HST-HF2-51396 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. This research made use of NASA's Astrophysics Data System Bibliographic Services, Astropy, a community-developed core Python package for Astronomy, and APLpy, an open-source plotting package for Python hosted at http://aplpy.github.com.

According to the definition given by the International Union of Pure and Applied Chemistry, amides are oxoacids, which follow the general chemical formula –E(=O)(OH) (E = carbon, sulfur, or other atom that can make this bond), in which the OH group has been replaced by an NH_x group. Proper amides are therefore molecules that, for example, follow the structures R₁–C(=O)N–R₂R₃. However, several molecules that do not adhere to this structure are also called amides, such as cyanamide (NH₂CN). Despite technically not being an amide, cyanamide and the other amides covered in this work do share a distinct chemical structure by which they can be grouped together.

All amides discussed in this work have structures involving a double (C=O) or triple bonded (C≡N) group attached to a –NH₂ group. The molecular orbitals of these fragments are quite different. The double or triple bonded carbon atom and its counterpart have a flat SP2 and SP hybridized molecular orbital, respectively. Normally, a –NH₂ group will have a tetrahydral SP3 orbital, similar to, for example, methane (CH₄). Two molecular fragments that form a bond through an SP2 or SP hybridized orbital will form a rigid structure, but fragments that form a bond through an SP3 orbital can usually freely rotate around the bond axis. However, in the case of an amide, the –NH₂ group is attached to a carbon atom with an SP2 or SP orbital, causing it to take on an SP2 character, forming a so-called resonance hybrid. This effectively means that the (C=X)–NH₂ bond in an amide takes on a double-bond character and becomes a rigid and flat structure. This chemical structure is found in the proper amides (e.g., NH₂CHO), but also in cyanamide (NH₂CN), thus making this a distinct feature of this group of molecules.

Spectroscopic linelists used in the work are primarily taken from the CDMS database, but several list from the JPL spectroscopic database and literature sources are used as well. Table B1 gives an overview of the catalogs and identifiers for each molecule analyzed in this work and the main works these entries are based on.

Table B1. Linelists and Spectroscopic Literature

Molecule	Catalog	Literature
	and ID
NH2CN	JPL	Read et al. (1986)
	42003
HN12CO	CDMS	Kukolich et al. (1971)
	43511	Hocking et al. (1975)
		Niedenhoff et al. (1995)
		Lapinov et al. (2007)
HN13CO	JPL	Hocking et al. (1975)
	44008
NH${}_{2}^{12}$CHO	CDMS	Motiyenko et al. (2012)
	45512	Kryvda et al. (2009)
		Blanco et al. (2006)
		Vorob'eva & Dyubko (1994)
		Moskienko & Dyubko (1991)
		Gardner et al. (1980)
		Hirota et al. (1974)
		Kukolich & Nelson (1971)
NH${}_{2}^{13}$CHO	CDMS	Motiyenko et al. (2012)
	46512	Gardner et al. (1980)
		Blanco et al. (2006)
		Kryvda et al. (2009)
CH3C(O)NH2	Literature	Ilyushin et al. (2004)
CH3NHCHO	Literature	Belloche et al. (2017)
NH2C(O)NH2	CDMS	Remijan et al. (2014)
	60517	Brown et al. (1975)
		Kasten & Dreizler (1986)
		Kretschmer et al. (1996)

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Except for position MM2-iii, molecular line emissions of NH₂CN, HN₁₂CO, HN₁₃CO, NH${}_{2}^{12}$CHO, NH${}_{2}^{13}$CHO, and CH₃C(O)NH₂ are detected toward all positions. The detected lines are listed in Table C1.

Table C1. Overview of Spectral Lines of Amides Molecules toward NGC 6334I

Molecule	Transition	Frequency a	${A}_{\mathrm{ij}}$	${E}_{\mathrm{up}}$
	$J^{\prime} ,{K}_{{\rm{a}}}^{\prime} ,{K}_{{\rm{c}}}^{\prime} -J^{\prime\prime} ,{K}_{{\rm{a}}}^{\prime\prime} ,{K}_{{\rm{c}}}^{\prime\prime}$	(MHz)	$({{\rm{s}}}^{-1})$	(K)
NH2CN	14 4 11-13 4 10	279 479.5918 (0.0066)	2.05 × 10−3	398
	14 4 10-13 4 9	279 479.5921 (0.0066)	2.05 × 10−3	398
	14 0 14-13 0 13	279 600.4593 (0.0049)	2.24 × 10−3	172
	14 4 10-13 4 9	279 763.3778 (0.0056)	2.12 × 10−3	332
	14 4 11-13 4 10	279 763.3778 (0.0056)	2.12 × 10−3	332
	14 0 14-13 0 13	279 818.8464 (0.0060)	2.30 × 10−3	101
	14 2 13-13 2 12	279 820.8787 (0.0037)	2.26 × 10−3	159
	14 2 12-13 2 11	279 900.1968 (0.0037)	2.26 × 10−3	159
HN12CO	6 1 6-5 1 5	131 394.2300 (0.0004)	2.92 × 10−5	65
	6 4 2-5 4 1	131 733.5888 (0.0029)	1.41 × 10−5	673
	6 4 3-5 4 2	131 733.5888 (0.0029)	1.41 × 10−5	673
	6 3 4-5 3 3	131 799.2971 (0.0024)	2.07 × 10−5	397
	6 3 3-5 3 2	131 799.2972 (0.0024)	2.07 × 10−5	397
	6 2 5-5 2 4	131 845.8900 (0.0014)	2.61 × 10−5	192
	6 2 4-5 2 3	131 846.5998 (0.0014)	2.61 × 10−5	192
	6 0 6-5 0 6	131 885.7341 (0.0005)	3.08 × 10−5	22
	16 1 16-15 1 15	350 333.0590 (0.0100)	5.97 × 10−4	186
	16 3 13-15 3 12	351 416.8123 (0.0052)	5.31 × 10−4	518
	16 3 14-15 3 13	351 416.7983 (0.0052)	5.31 × 10−4	518
	16 2 15-15 2 14	351 537.7954 (0.0030)	5.75 × 10−4	314
	16 2 14-15 2 13	351 551.5731 (0.0032)	5.75 × 10−4	314
	16 0 16-15 0 15	351 633.2570 (0.0100)	6.13 × 10−4	143
	40 1 40-39 1 39	875 046.1710 (0.0031)	9.50 × 10−3	905
	40 3 38-39 3 37	877 808.5933 (0.0111)	8.68 × 10−3	1239
	40 3 37-39 3 36	877 809.9771 (0.0112)	8.68 × 10−3	1239
	40 2 39-39 2 38	878 062.2368 (0.0063)	9.25 × 10−3	1035
	40 0 40-39 0 39	878 137.4392 (0.0029)	9.72 × 10−3	864
	40 2 38-39 2 37	878 276.9143 (0.0065)	9.26 × 10−3	1035
	41 1 41-40 1 40	896 873.8160 (0.0150)	1.02 × 10−2	948
HN13CO	6 1 6-5 1 5	131 397.0955 (0.0084)	2.96 × 10−5	65
	6 0 6-5 0 5	131 889.4868 (0.0128)	3.06 × 10−5	22
	16 1 16-15 1 15	350 340.3407 (0.0908)	6.18 × 10−4	186
	16 0 16-15 0 15	351 642.8746 (0.0763)	6.27 × 10−4	143
NH${}_{2}^{12}$CHO	6 1 5-5 1 4	131 617.9025 (0.0005)	1.56 × 10−4	25
	25 2 23-25 1 24	280 689.4724 (0.0009)	5.21 × 10−5	351
	12 5 8-13 4 9	281 381.6715 (0.0012)	1.04 × 10−5	154
	15 2 14-15 1 15	281 934.6250 (0.0008)	3.40 × 10−5	134
	14 3 11-14 2 12	282 822.5932 (0.0009)	5.44 × 10−5	134
	15 2 14-15 0 15	294 753.3491 (0.0008)	1.07 × 10−5	134
	14 2 13-13 2 12	294 776.9896 (0.0009)	1.84 × 10−3	118
	17 3 15-17 2 16	336 733.0134 (0.0009)	8.20 × 10−5	183
	16 13 3-15 13 2	339 746.2586 (0.0023)	9.84 × 10−4	640
	16 13 4-15 13 3	339 746.2586 (0.0023)	9.84 × 10−4	640
	16 7 10-15 7 9	339 779.5370 (0.0300)	2.34 × 10−3	284
	16 7 9-15 7 8	339 779.5370 (0.0300)	2.34 × 10−3	284
	16 2 14-15 2 13	349 478.2047 (0.0009)	3.10 × 10−3	153
	9 2 8-8 1 7	349 634.0304 (0.0012)	6.22 × 10−5	58
	41 734-40 7 33	874 836.2150 (0.0500)	4.89 × 10−2	1025
	42 2 40-41 2 39	875 818.8790 (0.0500)	5.01 × 10−2	940
	15 5 10-14 4 11	876 148.3790 (0.0500)	1.11 × 10−3	197
	41 5 37-40 5 36	876 252.7700 (0.0500)	4.98 × 10−2	956
	41 6 36-40 6 35	876 493.3340 (0.0500)	4.95 × 10−2	988
	41 6 35-40 6 34	878 706.9000 (0.0500)	4.99 × 10−2	988
	22 3 19-21 2 20	879 169.0967 (0.0022)	3.84 × 10−4	287
	42 13 29-41 13 28	891 600.4990 (0.0500)	4.82 × 10−2	1422
	42 13 30-41 13 29	891 600.4990 (0.0500)	4.82 × 10−2	1422
	43 3 41-42 3 40	893 045.4010 (0.0500)	5.32 × 10−2	983
	45 0 45-44 1 44	893 077.5610 (0.0500)	2.76 × 10−3	997
	45 1 45-44 1 44	893 083.8870 (0.0500)	5.35 × 10−2	997
	45 0 45-44 0 44	893 085.9940 (0.0500)	5.35 × 10−2	997
	45 1 45-44 0 44	893 092.3230 (0.0500)	2.76 × 10−3	997
	44 2 43-43 2 42	893 184.3320 (0.0500)	5.34 × 10−2	992
	42 9 34-41 9 33	893 415.4200 (0.0500)	5.12 × 10−2	1162
	42 9 33-41 9 32	893 415.4200 (0.0500)	5.12 × 10−2	1162
	42 8 35-41 8 34	894 533.0350 (0.0500)	5.19 × 10−2	1112
	42 8 34-41 8 33	894 550.9190 (0.0500)	5.19 × 10−2	1112
	43 2 41-42 2 40	895 080.2490 (0.0500)	5.35 × 10−2	983
NH${}_{2}^{13}$CHO	6 1 5-5 1 4	131 495.8540 (0.0100)	1.55 × 10−4	25
	13 1 12-12 1 11	282 107.1134 (0.0026)	1.63 × 10−3	98
	14 1 14-13 1 13	282 693.5048 (0.0027)	1.65 × 10−3	105
	16 8 8-15 8 7	339 213.4944 (0.0029)	2.16 × 10−3	324
	16 8 9-15 8 8	339 213.4944 (0.0029)	2.16 × 10−3	324
	16 2 14-15 2 13	349 308.8534 (0.0030)	3.10 × 10−3	153
CH3C(O)NH2	23 10 14-22 9 13	351 083.4533 (0.0458)	2.12 × 10−3	235
	22 10 12-21 11 11	351 097.8247 (0.0412)	1.89 × 10−3	225
	22 11 12-21 10 11	351 097.8510 (0.0412)	1.89 × 10−3	225
	24 8 16-23 9 15	351 134.2229 (0.0499)	2.32 × 10−3	244
	24 9 16-23 8 15	351 134.2229 (0.0499)	2.32 × 10−3	244
	25 7 18-24 8 17	351 214.4618 (0.0534)	2.51 × 10−3	253
	25 8 18-24 7 17	351 214.4618 (0.0534)	2.51 × 10−3	253
	26 6 20-25 7 19	351 309.0658 (0.0563)	2.69 × 10−3	260
	26 7 20-25 6 19	351 309.0658 (0.0563)	2.69 × 10−3	260
	29 3 26-28 4 25	351 621.6370 (0.0657)	3.18 × 10−3	276
	29 4 26-28 3 25	351 621.6370 (0.0657)	3.18 × 10−3	276
	31 1 30-30 2 29	351 830.6986 (0.0819)	3.47 × 10−3	282
	31 2 30-30 1 29	351 830.6986 (0.0819)	3.47 × 10−3	282
CH3NHCHO	11 3 8-10 3 $7$ b	130 190.7247 (0.0009)	9.21 × 10−5	43
	12 2 11-11 2 10	130 874.1508 (0.0010)	9.75 × 10−5	44
	11 2 9-10 2 $8$ b	131 427.7286 (0.0009)	9.86 × 10−5	40
	13 8 5-12 8 4	144 416.877 (0.0014)	8.97 × 10−5	97
	9 7 2-8 6 2	279 381.6637 (0.0035)	5.18 × 10−4	62
	25 12 13-24 12 $12$ b	280 384.9819 (0.0041)	8.20 × 10−4	274
	25 14 11-24 14 $11$ b	280 716.5012 (0.0202)	6.82 × 10−4	308
	25 10 15-24 10 14	280 743.3675 (0.0049)	8.94 × 10−4	245
	24 5 19-23 5 $18$ b	280 994.8045 (0.0021)	9.96 × 10−4	186
	25 13 13-24 13 12	281 110.7375 (0.0045)	7.76 × 10−4	291
	25 13 12-24 13 11	281 113.0259 (0.0046)	7.81 × 10−4	291
	25 11 14-24 11 13	282 584.3703 (0.0088)	8.55 × 10−4	259
	26 9 17-25 9 16	291 978.5554 (0.0027)	1.06 × 10−3	247
	26 12 14-25 12 13	292 546.3434 (0.0055)	9.46 × 10−4	288
	26 5 22-25 5 21	292 648.065 (0.0023)	1.13 × 10−3	211
	26 15 11-25 15 $10$ b	292 847.7083 (0.0108)	8.05 × 10−4	341
	25 5 20-24 5 19	293 006.2032 (0.0023)	1.14 × 10−3	200
	8 8 0-7 7 0	294 241.2632 (0.0049)	7.64 × 10−4	67
	34 0 34-33 0 33	337 907.4314 (0.0048)	1.30 × 10−3	291
	34 1 34-33 1 33	337 907.4314 (0.0048)	1.30 × 10−3	291
	34 1 34-33 0 33	337 907.4314 (0.0048)	1.30 × 10−3	291
	34 0 34-33 1 33	337 907.4314 (0.0048)	1.30 × 10−3	291
	30 10 20-29 10 $19$ b	337 993.2492 (0.0041)	1.61 × 10−3	321
	30 4 26-29 4 25	338 606.6396 (0.0028)	1.74 × 10−3	272
	29 6 23-28 6 22	348 416.9795 (0.0029)	1.89 × 10−3	268
	31 4 27-30 4 26	348 417.6722 (0.009)	1.39 × 10−3	288
	30 5 25-29 5 24	348 837.7283 (0.0030)	1.94 × 10−3	279
	31 5 27-30 4 $26$ b	350 710.9037 (0.0031)	7.43 × 10−4	288
NH2C(O)NH2	25 3 23-24 2 22	279 472.8671 (0.0468)	2.55 × 10−4	195
	25 2 23-24 2 22	279 472.8671 (0.0468)	1.25 × 10−3	195
	25 3 23-24 3 22	279 472.8671 (0.0468)	1.25 × 10−3	195
	25 2 23-24 3 22	279 472.8671 (0.0468)	2.55 × 10−4	195
	24 5 20-23 4 19	282 190.8684 (0.0399)	1.20 × 10−3	199
	24 5 20-23 5 19	282 190.8684 (0.0399)	2.16 × 10−4	199
	24 4 20-23 4 19	282 190.8684 (0.0399)	2.16 × 10−4	199
	24 4 20-23 5 19	282 190.8684 (0.0399)	1.20 × 10−3	199
	25 4 21-24 4 20	290 862.3844 (0.0473)	1.48 × 10−3	213
	25 4 21-24 5 20	290 862.3844 (0.0473)	9.83 × 10−5	213
	25 5 21-24 5 20	290 862.3844 (0.0473)	9.83 × 10−5	213
	25 5 21-24 4 20	290 862.3844 (0.0473)	1.48 × 10−3	213
	22 7 16-21 6 15	293 236.5715 (0.0337)	1.18 × 10−3	186
	22 6 16-21 7 15	293 236.5646 (0.0337)	1.18 × 10−3	186
	21 7 14-20 8 13	293 292.5803 (0.0297)	1.05 × 10−3	179
	21 8 14-20 7 13	293 293.5513 (0.0297)	1.05 × 10−3	179
	24 5 19-23 5 18	293 642.0215 (0.0385)	1.01 × 10−4	208
	24 6 19-23 5 18	293 642.0215 (0.0385)	1.39 × 10−3	208
	24 5 19-23 6 18	293 642.0215 (0.0385)	1.39 × 10−3	208
	24 6 19-23 6 18	293 642.0215 (0.0385)	1.01 × 10−4	208

Notes. 

^a The uncertainty of each line frequency is given in parentheses. ^b CH₃NHCHO lines that are present on locations where the baseline dips.

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Figures D1–D11 present the detected transitions of the molecules that are not covered in the main text toward positions MM1-i and MM2-i.

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