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A&A
Volume 659, March 2022
Article Number A29
Number of page(s) 11
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361/202142981
Published online 08 March 2022

© ESO 2022

1 Introduction

Complex organic molecules (COMs) are the precursors of prebiotic molecules, and thus understanding their formation and evolution will help us gain more insight into how life originated in our own Solar System (Caselli & Ceccarelli 2012).With new facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA) and the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) on the Rosetta mission, we are now able to compare chemistry across a range of astronomical environments and get a better understanding of the chemical history of COMs throughout the entire star and planet-formation process, including comets (e.g. Drozdovskaya et al. 2019). It is crucial to study COMs in planet-forming disks in order to understand how the material in the disk is incorporated into planets and what degree of complexity is present at the epoch of planet formation (Öberg & Bergin 2021).

The formation of most of these COMs is thought to occur in cold molecular clouds (Boogert et al. 2015). During this time, atoms and simple molecules such as CO will stick to the dust grains forming an ice layer and undergo chemical reactions (e.g. Herbst & van Dishoeck 2009; Chuang et al. 2018; Ioppolo et al. 2021). The products are subsequently released back into the gas phase if there is an increase in temperature resulting in thermal desorption. Additionally, molecules will also return to the gas phase via other processes such as UV photodesorption, but this can lead to the fragmentation of the molecule when it enters the gas phase (Garrod et al. 2006; Cruz-Diaz et al. 2016). COMs are therefore especially abundant in the gas phase in young warm systems where they are easily detected because of thermal sublimation (Tdust > 100 K) (e.g., Bergner et al. 2017; Jørgensen et al. 2018; van Gelder et al. 2020; Mercimek et al. 2022; Belloche et al. 2020). This is in contrast to older protoplanetary disks which are colder and thus the COMs remain frozen on dust grains in the bulk of the disk and are therefore more often undetectable in the gas phase with ALMA (van ’t Hoff et al. 2020). However, COMs are expected to be abundant in protoplanetary disk ices and there is some evidence for this in the outbursting protostellar source V883 Ori which is rich in COMs (van ’t Hoff et al. 2018; Lee et al. 2019).

The current situation is that, in protoplanetary disks of more than 1 Myr old, even the most abundant COM, methanol (CH3OH), is difficult to detect. CH3OH is a cornerstone in the chemistry leading to many larger complex organic molecules (Öberg et al. 2009). Walsh et al. (2016) presented the first detection of CH3OH in the TW Hya protoplanetary disk. However, the fractional abundance relative to H2 is very low (3 × 10−12 − 4 × 10−11), indicating a chemical origin in the gas phase via inefficient and fragmenting non-thermal desorption of the ices rather than thermal sublimation (Walsh et al. 2017). Carney et al. (2019) also provided an upper limit on the abundance of CH3OH in the Herbig Ae disk HD 163296 of <1.6 × 10−12 relative to H2. For comparison, CH3OH abundances in hot protostellar cores are typically of order 10−6, comparable to those in ices (Boogert et al. 2015).

More recently, Booth et al. (2021b) detected CH3OH for the firsttime in a warm Herbig transition disk. In comparison to the ringed CH3OH emission in TW Hya the CH3OH in the HD 100546 disk originates from the inner 50 au of the disk and its likely origin is thermal desorption. Because Herbig Ae/Be sources like this one are inherently warm, which prevents freeze-out of the precursor CO, in situ formation of the CH3OH is unlikely. Instead the presenceof CH3OH in the disk can be explained via the inheritance of COM-rich ices from colder parent molecular clouds.

Also, another Herbig source was revealed to have a rich observable chemistry: the IRS 48 transition disk (van der Marel et al. 2014, 2021b; Booth et al. 2021a). What makes this disk particularly interesting is the fact that it contains a highly asymmetric dust trap of large grains (≳0.1 mm) on thesouthern side of the star, making it the most asymmetric disk detected to date (van der Marel et al. 2013, 2021a). van der Marel et al. (2021b) report the detection of CH3OH and formaldehyde (H2CO) in this disk. The emissions have the same crescent shape as the dust continuum, showing for the first time the direct link between a dust trap and COMs. This coincidence was hinted at with low signal-to-noise H2CO observations(van der Marel et al. 2014) but is now confirmed. The bulk of the ice reservoir of the IRS48 disk is constrained to the larger dust grains, and because of UV irradiation from the central star, the dust temperature increases enough to liberate the CH3OH from the ices. Booth et al. (2021a) additionally report the detection of SO2 in the IRS 48 dust trap, the first detection of this molecule in a protoplanetary disk, along with detection of SO. The detection of these molecules supports the presence of oxygen-rich gas where the C/O < 1 because of sublimated ices.

In this paper, we report the analysis of ALMA data of IRS 48 including the first detection of dimethyl ether (CH3OCH3) in a protoplanetary disk and a tentative detection of methyl formate (CH3OCHO). CH3OCH3 is the largest complex organic molecule that has been detected in a protoplanetary disk to date. We also report the first detection of nitric oxide (NO) in a protoplanetary disk, which will be analysed in a future paper. Our paper is structured as follows: in Sect. 2 we describe our observational methods and in Sect. 3 we show our data analysis and provide the values for the derived column densities. In Sect. 4 we discuss the chemistry of the detected species, compare abundances to other astronomical environments, and determine upper limits for other molecules covered in the data. Finally, in Sect. 5 we give a short summary and provide conclusions.

2 Observations

Our data were taken with ALMA. The Band 7 line data (~0.8 mm) were taken on August 18 2018 (2017.1. 00834.S, PI: Adriana Pohl), and the continuum data presented in Fig. 1 were taken in June and August 2015 (2013.1.00100.S, PI: Nienke van der Marel). Ohashi et al. (2020) provide a full description of the line data calibration. In papers I and II, we cover the detections of CH3OH, H2CO, SO, and SO2 (van der Marel et al. 2021b; Booth et al. 2021a) and in this paper we present the detection of CH3OCH3 and NO, and investigate other tentative detections and upper limits.

Data reduction was done using the Common Astronomy Software Applications (CASA)1 version 5.7.0. The spectral windows have channel widths of ~1.7 km s-1 and a beam size of 0′′ 55 × 0′′42 (PA = 80°). The spectral windows have central frequencies of 349.7, 351.5, 361.6, and 363.5 GHz, respectively, with SPW1 from 349.79 to 350.66 GHz,SPW2 from 350.60 to 352.47 GHz, SPW3 from 360.68 to 362.55 GHz, and SPW4 from 362.61 to 364.47 GHz.We imaged the data with the tclean function in CASA using a Briggs weighting with a robust value of 0.5. The image was recentred to the star position using the phase centre parameter in CASA and was set to ICRS 16:27:37.17 −24:30:35.55. We used a Keplerian mask over the region of emission at a distance of 136 pc (Gaia Collaboration 2021), an inclination angle of 50°, and a position angle of 100° (van der Marel et al. 2021a).

The cleaned images were subsequently stacked using GoFish version 1.3.6 (Teague 2019) in order to increase the signal-to-noise ratio. This method makes it possible to identify potential weak lines and also distinguish between lines that are blended (very close in frequency). We extract spectra over the whole azimuth of the disk although the lines are co-spatial with the dust trap. This is done because the observations are not well spatially resolved. The spectra for the four spectral windows are shown in Figs. B.1–B.4.

3 Analysis

3.1 Spectral analysis

The stacked, continuum-subtracted spectra were analysed using the CASSIS2 spectral analysis tool version 5.1.1 (Vastel et al. 2015) in a similar way to that used by Nazari et al. (2021). The flux densities were first converted to brightness temperature units and local thermodynamic equilibrium (LTE) conditions were assumed in order to derive the column densities and excitation temperatures. We made use of the Cologne Database for Molecular Spectroscopy (CDMS) (Müller et al. 2001, 2005) and the Jet Propulsion Laboratory (JPL) database (Pickett et al. 1998) for molecular information. In Table A.1 we list the transitions of the detected species. The integrated intensity maps of selected lines are presented in Fig. 1. The search for other potential features in the spectra was carried out by making a selection of commonly detected COMs in other environments and only taking into account lines with Eup ≤ 400 K and Aul ≥ 1 × 10−6 s−1. For this selection of molecules, we modelled the spectra in CASSIS by assuming an excitation temperature of 100 K motivated by the rotational temperature derived by van der Marel et al. (2021b) for the CH3OH. We also calculated the best-fit column density at both 70 and 250 K to have an estimate of the column density error for the detected species, which is typically a factor of two. The absolute calibration error is much smaller, of order 10%, and this will cancel out in abundance ratios. We used a FWHM of ~7 km s−1 based on the line width of a strong CH3OH line and a source size of 1.4 × 10−11 sr based on the 5σ emission continuum of the disk (Fig. 1), the same as in van der Marel et al. (2021b); Booth et al. (2021a). Using these variables we derived column densities and upper limits. We note that the inferred column density, N, scales inversely with the assumed emitting area, Ωsource (Goldsmith & Langer 1999), that is, (1)

In the case where the source does not fill the beam, the column density will be underestimated by a dilution factor: (2)

where Ωbeam is the beam size (e.g. van Gelder et al. 2020).

thumbnail Fig. 1

Integrated intensity maps of the 0.9 mm continuum emission and a subset of the detected molecular lines listed in Table A.1. Top right: CH3OH 161,15 –160,16. Bottom left: CH3OCH3 113,8 –102,9, and bottom right: CH3OCHO 322,30 –322,29 and 323,30–323,29 blend. The beam is shown in the bottom left corner and a scale bar is shown in the bottom right corner.

3.2 Revising the CH3OH column density

We first modelled the CH3OH lines in our spectra based on the parameters derived in van der Marel et al. (2021b). Using CASSIS we find models consistent with the data using a column density for CH3OH of 5 ×1014 cm−2 and an excitation temperature of 100 K which is in agreement with the rotational diagram analysis of van der Marel et al. (2021b). These results are shown in Fig. 2. However, in this work, additional CH3OH transitions are detected and these are highlighted in Table A.1. Figure 1 shows the integrated intensity maps of a CH3OH line with an upper energy level of 333 K. We also detect two weaker lines in the stacked spectra that are better fit at a higher column density of 2 × 1015 cm−2 and still at a temperature of 100 K (see Fig. 2 for a comparison of the two models). These two lines are the 9−5,4 –9−4,6 and 31,2 –42,2 at 351.236 GHz (Eup =  241 K, log10 (EA) = −4.44 s−1) and 361.236 GHz (Eup =  339 K, log10(EA) = −3.58 s−1) respectively. Both lines were visible in our stacked spectra at the 2.5–3σ level but neither one was detected in the channel maps above the 3σ level. These weak lines are reproduced at a different column density, which likely indicates that the emission traced by the stronger lines is optically thick. van der Marel et al. (2021b) calculate the optical depth of these lines and show they are optically thin for the assumed emitting area. The difference between this and our result can be resolved if the lines are optically thick and beam diluted, because the column density is inversely proportional to the assumed emitting area. We also derived a 3σ upper limit for the column density of 13CH3OH of < 5.5 × 1014 cm−2. This gives a strict upper limit on the CH3OH column density of ≈3.3 × 1016 cm−2 assuming a 12C/13C ratio of 60. This upper-limit is consistent with the column density found via the weakest CH3OH lines. We use N(CH3OH) = 2 × 1015 cm−2 as a reference for comparisons.

3.3 Detection of dimethyl ether and methyl formate

We detect two sets of blended lines for CH3OCH3. See Table A.1 for the transition information, Fig. 1 for an intensity-integrated map of one of the sets of blended lines, and Fig. C.1 for the channel maps of both. We derive a column density of 1.5 × 1015 cm−2 at an assumed excitation temperature of 100 K; see Fig. 2 for the CASSIS model fits. The excitation temperature of CH3OCH3 may be lower than that of CH3OH (e.g. Jørgensen et al. 2018) but given that only a few transitions are detected, we calculate the column densities over a range of excitation temperatures from 80 to 250 K (listed in Table 1). We also found a tentative detection of CH3OCHO after modelling several features in our spectra. One such emission feature with a S/N above 3σ noise level can be seen at 363.48 and 363.49 GHz (Fig. 2). We show a clear > 5σ detection ofa line in the channel maps at this frequency range (Fig. C.2) and the integrated intensity map is shown in Fig. 1. While the modelled spectrum of CH3OCHO does not provide an exact fit for the emission feature, it is the closest fit found after considering other possible candidates. Furthermore, the model spectrum for CH3OCHO fits several other smaller features in the spectra (see Figs. B.1–B.4). From these models we derive a best-fit column density of 1.3 × 1015 cm−2 at an excitation temperature of 100 K.

thumbnail Fig. 2

Stacked continuum-subtracted spectra (black lines) and CASSIS models (coloured lines) for the molecules detected in this work. Dashed lines mark the frequencies of the transitions as listed in Table 1 and the grey bar marks the ± 1σ error calculated from the line-free channels in each spectral window. Panels A and B: two weak CH3OH lines 9−5,4–9−4,6 and 31,2 –42,2 with the 100 K CASSIS models at 5 × 1014 cm−1 (blue) and 2 × 1015 cm−1 (green). Panels C and D: best-fit models for the CH3OCH3 201,20 –190,19 and 113,8–102,9 transitions and the CH3OCHO 323,30 –313,29 and 323,30–312,29 transitions. In panel D the negative dip in the spectrum at ≈363.5 GHz may be an atmospheric absorption feature (https://almascience.eso.org/about-alma/atmosphere-model). Panels E and F: best-fit models for NO covering the 41–43 and 41 –44 transitions. Panels C, D, and E: CH3OH model for the strong lines.

3.4 Other line detections and upper limits

We detect a total of five transition lines for nitric oxide (Table A.1). This is the first detection of NO in a protoplanetary disk. We were alerted to the possible presence of NO in our disk when we encountered a difficulty in fitting the bright CH3OH line at 350.68 GHz (Fig. 2). NO has two transitions at this frequency, but a single line was not enough to confirm a definitive detection of the molecule because this line is also blended with CH3OH. We are able to prove the presence of NO in the disk after successfully fitting an additional double feature at 351.04 and 351.05 GHz (Fig. 2). From the CASSIS spectral analysis models, we derive a best-fit total column density for NO of 3 × 1015 cm−2 at an excitation temperature of 40 K. The NO lines have a low excitation temperature (36 K) compared to many of the COM lines detected and the lower temperature best fits the multiple lines. The NO lines will be more quantitatively analysed in a future paper.

We also detect an additional SO2 line at 363.16 GHz (Fig. B.1) that was not reported in Booth et al. (2021a). The column densities from our spectral analysis are in agreement with their value.

Finally, we also derive upper limits for species that remain undetected in the IRS 48 disk but that have been observed in younger sources and other older disks such as formic acid (t-HCOOH), acetaldehyde (CH3CHO), and methyl cyanide (CH3CN) (e.g., Bergner et al. 2017; Favre et al. 2018; van Gelder et al. 2020; Ilee et al. 2021). These upper limits are listed in Table 1.

4 Discussion

We find a wealth of molecular complexity in the IRS 48 disk, including the first detections of multiple molecules in disks. In this section, we discuss the chemical origin of the COMs, compare relative abundances to other environments, and consider the prospects for further complexity in the disk.

4.1 Chemical origin of the COMs

The observed CH3OH emission in the IRS 48 disk first presented by van der Marel et al. (2021b) is azimuthally co-spatial with the dust trap and peaking at slightly smaller radius. van der Marel et al. (2021b) proposed that the presence of CH3OH in the disk is due to thermal ice sublimation and that the ice reservoir is constrained to the larger millimetre-sized grains. Vertical mixing in the vortex may also help in lifting icy dust grains to the warm surface. CH3OH forms on CO ice via a sequence of H-addition reactions with key intermediates HCO and H2CO (Fuchs et al. 2009; Chuang et al. 2017). Because the grain surface chemistry of CH3OH is related to both CH3OCH3 and CH3OCHO, particularly in the presence of UV radiation, we expect that both of these COMs also originate from the sublimating ices (Öberg et al. 2009), Garrod & Herbst (2006) and Garrod et al. (2008) provide a theoretical model in which complex organic molecules, including CH3OCH3 and CH3OCHO, can form via cold grain-surface reactions (≤50 K) involving radicals: (3) (4)

This model shows a common formation route from the methoxy precursor CH3O. These pathways have also been shown to be present in laboratory experiments (Chuang et al. 2016). These COMs could nevertheless be further enhanced due to UV irradiation of the ices from the central star resulting in photodissociation of CH3OH (Öberg et al. 2009; Walsh et al. 2014), (5)

where the dissociation products can then recombine via reactions (1) and (2).

thumbnail Fig. 3

Abundances of commonly detected COMs relative to CH3OH. Solid squares show the detections in IRS 48 and arrows show the non-detected species for which upper limits on the column density are derived. The error bars on the IRS 48 points come from calculating the column densities over a range of excitation temperatures. For the other sources, see references in Sect. 4.

4.2 Comparisons to other environments

CH3OCH3 is the largest complex organic molecule detected in a protoplanetary disk. It has been detected in several other, younger sources (e.g. Taquet et al. 2015; Soma et al. 2018; Bergner et al. 2018). We compare our results as summarised in Table 1 (100 K column) and Fig. 3 with the observed abundances in other astronomical environments including the class 0 protostellar binary IRAS 16293 A and B (Jørgensen et al. 2018; Manigand et al. 2020), the outbursting source V883 Ori (Lee et al. 2019), and the comet 67P (Drozdovskaya et al. 2019). The high observed CH3OCH3/CH3OH ratio in the IRS 48 disk, of order unity, is different from that in the other sources by a factor of 5–10 (Fig. 3). The abundance derived for CH3OCHO also shows a similar trend to CH3OCH3 in that CH3OCHO seems to be more abundant compared to the other sources. The high derived column-density ratios of CH3OCH3/CH3OH and CH3OCHO/CH3OH in IRS 48 (Fig. 3) compared to other environments may be due to optically thick CH3OH emission that is beam diluted resulting in over-estimated abundance ratios. To increase the optical depth in CH3OH to the amount that would make the ratio consistent with other sources, an area of order 10−12 sr would be needed. If the emission is constrained to the inner edge of the dust cavity, this would require a crescent shape for the emitting area with a length of ≈1′′0, corresponding to a width of ≈0′′1 in COM emission, which could be resolved in future higher resolution data. Detections of CH3OH isotopologues are needed to determine whether optical depth is indeed the cause of the difference or chemical processing in the disk relative to ices in dark clouds and young stars could be responsible for the enhanced chemical complexity in the UV-irradiated ice trap.

The CH3OCH3/CH3OCHO ratio in our disk is approximately 0.9 and this is consistent with what is observed in other sources across a full range of environments from star and disk formation to comets (Coletta et al. 2020). This adds further evidence that these two species are likely chemically related to one another and points towards ice formation of both molecules and therefore the inheritance of ices in the IRS 48 disk.

Table 1

Derived column densities and upper limits.

4.3 Upper limits

We also derived upper limits for the column densities of COMs that were previously detected in other sources. These molecules are listed in Table 1; see also Fig. 3. Although CH3CHO, t-HCOOH, and CH3CN have been detected in several sources, we note that they remain absent in our disk despite having formation routes via grain-surface chemistry (Walsh et al. 2014).

In particular, HCOOH can form via the HCO or HOCO radicals and CH3CHO via the CH3 and HCO radicals. The non-detection of formic acid is potentially interesting, as in disk chemical models it is predicted to have a similar fractional abundance to CH3OH in the gas phase (Walsh et al. 2014). From the upper limit, we constrain this ratio to <10%. However, Walsh et al. (2014) predict that the CH3OH ice column density is approximately ten times higher than HCOOH. This could explain the non-detection of HCOOH in our data if the sublimating ice reservoir is the primary origin of both molecules. TW Hya also has a detection of formic acid (Favre et al. 2018) and in this disk the t-HCOOH/CH3OH abundance ratio is approximately unity. This is at least an order of magnitude higher than the 1–10% seen in young stars and comets (e.g. Drozdovskaya et al. 2019). In comparison to IRS 48, where the observable chemistry appears to be dominated by ice sublimation, TW Hya is a cold disk where small amounts of COMs in the gas phase are due to non-thermal desorption and/or gas-phase chemistry.

Similarly, with the detection of CH3OCH3 we might also expect to have detected CH3CHO. The abundance of CH3CHO with respect to CH3OH is found to be about ten times lower thanthat of CH3OCH3. This difference is consistent with the results of van Gelder et al. (2020) who find lower abundances of CH3CHO compared to CH3OCH3 in young protostellar envelopes.

Another molecule that is particularly interesting to look at is CH3CN as it has been detected in multiple protoplanetary disks (Loomis et al. 2018; Bergner et al. 2018; Ilee et al. 2021). The formation of CH3CN seems to be dominated by gas-phase chemistry but grain surface processes cannot be neglected (Loomis et al. 2018). In particular, gas-phase CH3CN is enhanced in environments with high C/O ratio. In TW Hya, the only disk with detections of both CH3OH and CH3CN, the CH3CN/CH3OH column density ratio is approximately unity. Unlike what we expect for IRS 48, in TW Hya the observable CH3CN and CH3OH likely do not have the same chemical origin. The CH3CN is primarily formed via gas-phase routes whereas the CH3OH most likely originates from the ices (Loomis et al. 2018; Walsh et al. 2016, 2017). In IRS 48 we have an upper limit of ≈10%. This is in better agreement with the 1–10% seen in comets and young stars (see Fig. 3 and e.g. Bergner et al. 2017).

We also obtained upper limits on the deuterated form of methanol CH2DOH. The upper limit on this column density at an excitation temperature of 100 K is 6.0 × 1014 cm−2. This then gives an upper limit on the D/H of 10% and this is consistent with the ratios seen in protostellar cores, young low-mass stars, and comet 67P (≈1–10%, e.g. van Gelder et al. 2020; Drozdovskaya et al. 2021).

4.4 Prospects for further complexity in the IRS 48 ice trap

Confirmation of the CH3OCHO detection is needed because our models do not fit the emission feature very well and we only have one significant feature to fit given the frequency coverage of the observations. There are several other COMs that remain undetected in our disk and which should be the focus of future work. The molecules listed in Table 1 are examples of species that should be searched for in future studies due to the fact that most have been detected in multiple protostellar sources. The detection of CH3OCH3 and CH3OCHO alongside CH3OH implies a rich ice chemistry in the IRS 48 dust trap. Other COMs which have related formation routes via the radials HCO, CH3O, and CH2OH, including ethylene-glycol, acetaldehyde, ethanol, and glycolaldehyde, should be subject of follow-up observations.

5 Conclusions

We analyzed ALMA data of the IRS 48 transition disk, revealing a wealth of molecular complexity.

  • We report the first detections of dimethy ether (CH3OCH3), nitric oxide (NO), and a tentative detection of methyl formate (CH3OCHO) in a protoplanetary disk.

  • We report an additional detection of a SO2 transition in the disk with an upper energy level of ≈250 K.

  • The emissions of the detected species show a direct link with the asymmetric dust trap in the southern region of the disk, further suggesting that molecular complexity in this disk is due to ice sublimation.

  • The abundance ratios of CH3OCH3 and CH3OCHO compared with CH3OH are high relative to other environments. This either means that these molecules are enhanced relative to CH3OH in this disk or that the CH3OH column density we derive is underestimated. The latter situation could be due to the lines being optically thick but beam diluted. A higher CH3OH column density would mean a COM-emitting area smaller than the assumed area, the 5σ extent of the millimetre dust trap. With further high-angular-resolution observations, we will be able to determine whether or not the emitting area is truly just the thin inner edge of the dust trap.

  • The detection of CH3OCH3 and CH3OCHO in such a warm disk and the agreement in the CH3OCH3/CH3OCHO column density ratio with other environments strengthens the case for an origin inherited from the cold cloud phase, but the abundances with respect to CH3OH may be enhanced because of UV irradiation.

Hopefully future observations of the IRS 48 icy dust trap will allow for the detection of other COMs and more robust constraints on the column density and excitation conditions. This work is an important puzzle piece in tracing the full interstellar journey of COMs across the different evolutionary stages of star, disk, and planet formation.

Acknowledgements

Astrochemistry in Leiden is supported by the Netherlands Research School for Astronomy (NOVA). 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. This paper makes use of the following ALMA data: 2013.1.00100.S, 2017.1.00834.S.

Appendix A Table of molecular lines detected

Table A.1

Properties of the molecular lines analysed in this work.

Appendix B Spectra

thumbnail Fig. B.1

Stacked spectra with CASSIS model fits with Tex = 100 K for all species aside from NO which is modelled at 40 K. The grey region shows the +/- 1σ error. The vertical dashed lines denote the rest frequency of the lines. The CH3OH model is with a column density of 5 × 1014 cm−2. In Fig. 2 we show how a higher column density better fits weaker CH3OH lines covered in the observations.
thumbnail Fig. B.2

Same as Figure B.1
thumbnail Fig. B.3

Same as Figure B.1
thumbnail Fig. B.4

Same as Figure B.1

Appendix C Channel maps

thumbnail Fig. C.1

Channel maps of blended dimethyl ether and methanol lines. The first two rows show the two sets of dimethyl ether transitions while the bottom row shows emission coming from the methanol. The beam is shown in the bottom left corner and the scale bar is shown in the bottom right corner. Contours show the [3,5,7,9] × σ levels.
thumbnail Fig. C.2

Channel maps of the methyl formate detection. The beam is shown in the bottom left corner and the scale bar is shown in the bottom right corner. Contours show the [3,5] × σ levels.

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All Tables

Table 1

Derived column densities and upper limits.

In the text
Table A.1

Properties of the molecular lines analysed in this work.

In the text

All Figures

thumbnail Fig. 1

Integrated intensity maps of the 0.9 mm continuum emission and a subset of the detected molecular lines listed in Table A.1. Top right: CH3OH 161,15 –160,16. Bottom left: CH3OCH3 113,8 –102,9, and bottom right: CH3OCHO 322,30 –322,29 and 323,30–323,29 blend. The beam is shown in the bottom left corner and a scale bar is shown in the bottom right corner.
In the text
thumbnail Fig. 2

Stacked continuum-subtracted spectra (black lines) and CASSIS models (coloured lines) for the molecules detected in this work. Dashed lines mark the frequencies of the transitions as listed in Table 1 and the grey bar marks the ± 1σ error calculated from the line-free channels in each spectral window. Panels A and B: two weak CH3OH lines 9−5,4–9−4,6 and 31,2 –42,2 with the 100 K CASSIS models at 5 × 1014 cm−1 (blue) and 2 × 1015 cm−1 (green). Panels C and D: best-fit models for the CH3OCH3 201,20 –190,19 and 113,8–102,9 transitions and the CH3OCHO 323,30 –313,29 and 323,30–312,29 transitions. In panel D the negative dip in the spectrum at ≈363.5 GHz may be an atmospheric absorption feature (https://almascience.eso.org/about-alma/atmosphere-model). Panels E and F: best-fit models for NO covering the 41–43 and 41 –44 transitions. Panels C, D, and E: CH3OH model for the strong lines.
In the text
thumbnail Fig. 3

Abundances of commonly detected COMs relative to CH3OH. Solid squares show the detections in IRS 48 and arrows show the non-detected species for which upper limits on the column density are derived. The error bars on the IRS 48 points come from calculating the column densities over a range of excitation temperatures. For the other sources, see references in Sect. 4.
In the text
thumbnail Fig. B.1

Stacked spectra with CASSIS model fits with Tex = 100 K for all species aside from NO which is modelled at 40 K. The grey region shows the +/- 1σ error. The vertical dashed lines denote the rest frequency of the lines. The CH3OH model is with a column density of 5 × 1014 cm−2. In Fig. 2 we show how a higher column density better fits weaker CH3OH lines covered in the observations.
In the text
thumbnail Fig. B.2

Same as Figure B.1
In the text
thumbnail Fig. B.3

Same as Figure B.1
In the text
thumbnail Fig. B.4

Same as Figure B.1
In the text
thumbnail Fig. C.1

Channel maps of blended dimethyl ether and methanol lines. The first two rows show the two sets of dimethyl ether transitions while the bottom row shows emission coming from the methanol. The beam is shown in the bottom left corner and the scale bar is shown in the bottom right corner. Contours show the [3,5,7,9] × σ levels.
In the text
thumbnail Fig. C.2

Channel maps of the methyl formate detection. The beam is shown in the bottom left corner and the scale bar is shown in the bottom right corner. Contours show the [3,5] × σ levels.
In the text

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