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A&A
Volume 649, May 2021
Article Number L4
Number of page(s) 6
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202140978
Published online 05 May 2021

© ESO 2021

1. Introduction

Complex organic molecules (COMs) such as methyl formate (HCOOCH3) and dimethyl ether (CH3OCH3) have traditionally been observed in the warm gas around protostars, the so-called hot cores and corinos, where they are thought to form upon thermal desorption of ice mantles on grains (Herbst & van Dishoeck 2009). In the last decade, these molecules have also been observed in a few cold sources, such as the dense cores B1-b (Öberg et al. 2010; Cernicharo et al. 2012) and L483 (Agúndez et al. 2019), the dark cloud Barnard 5 (Taquet et al. 2017), the pre-stellar cores L1689B (Bacmann et al. 2012) and L1544 (Jiménez-Serra et al. 2016), and the starless core TMC-1 (Soma et al. 2018). The low temperatures in these environments inhibit thermal desorption, and how these molecules are formed, whether in the gas phase or on grain surfaces followed by some nonthermal desorption process, is still an active subject of debate (Vasyunin & Herbst 2013; Ruaud et al. 2015; Balucani et al. 2015; Chang & Herbst 2016; Vasyunin et al. 2017; Shingledecker et al. 2018; Jin & Garrod 2020).

The cyanopolyyne peak of TMC-1, TMC-1(CP), is characterized by a carbon-rich chemistry, with high abundances of carbon chains and a low content of O-bearing COMs (e.g., Agúndez & Wakelam 2013). Here we report the detection of four O-bearing COMs toward TMC-1(CP). Propenal (C2H3CHO) had previously been reported toward massive star-forming regions in the Galactic center (Hollis et al. 2004; Requena-Torres et al. 2008) and in the hot corino IRAS 16293-2422 B (Manigand et al. 2021). Vinyl alcohol (C2H3OH) had only been seen toward Sagittarius B2(N), where the high spectral density complicates its identification (Turner & Apponi 2001). Therefore, this is the first clear detection of C2H3OH in space and the first time that C2H3CHO and C2H3OH have been detected in a cold environment. We also report the detection of HCOOCH3 and CH3OCH3, recently detected (the latter tentatively) toward the methanol peak of TMC-1 (Soma et al. 2018) but not toward TMC-1(CP).

2. Astronomical observations

The data presented here belong to a Q-band line survey of TMC-1(CP), αJ2000 = 4h 41m 41.9s and δJ2000 = +25° 41′ 27.0″, performed with the Yebes 40m telescope. The cryogenic receiver for the Q band, which was built within the Nanocosmos project1 and covers the 31.0–50.4 GHz frequency range with horizontal and vertical polarizations, was used connected to Fast Fourier Transform spectrometers, which cover a bandwidth of 8 × 2.5 GHz in each polarization with a spectral resolution of 38.15 kHz. The system is described in Tercero et al. (2021). The half power beam width (HPBW) of the Yebes 40m telescope ranges from 36.4″ to 54.4″ in the Q band. The intensity scale is antenna temperature, T A * $ T_A^* $, for which we estimate an uncertainty of 10%; the antenna temperature can be converted to main beam brightness temperature, Tmb, by dividing by Beff/Feff (see Table 1). The line survey was carried out over several observing runs, and various results have already been published. Data taken in November 2019 and in February 2020 resulted in the detection of the negative ions C3N− and C5N− (Cernicharo et al. 2020a) and the discovery of HC4NC (Cernicharo et al. 2020b), HC3O+ (Cernicharo et al. 2020c), and HC5NH+ (Marcelino et al. 2020). A further observing run, carried out in October 2020, resulted in the detection of HDCCN (Cabezas et al. 2021), HC3S+ (Cernicharo et al. 2021a), CH3CO+ (Cernicharo et al. 2021b), and various C4H3N isomers (Marcelino et al. 2021). Additional observations were taken in December 2020 and January 2021, which led to the discovery of vinyl acetylene (CH2CHCCH; Cernicharo et al. 2021c), allenyl acetylene (CH2CCHCCH; Cernicharo et al. 2021d), and propargyl (CH2CCH; Agúndez et al. 2021), and a final run was carried out in March 2021. All observations were conducted using the frequency switching technique, with a frequency throw of 10 MHz during the first two observing runs and 8 MHz in the later ones. All data were reduced with the program CLASS of the GILDAS software (Pety 2005)2.

Table 1.

Observed line parameters of the target O-bearing COMs of this study in TMC-1.

3. Molecular spectroscopy

C2H3CHO has two conformers. The most stable, and the only one reported in space (Hollis et al. 2004; Requena-Torres et al. 2008; Manigand et al. 2021), is the trans form, which is the one reported here as well. Level energies and transition frequencies were obtained from the rotational constants derived by Daly et al. (2015). The dipole moment along the a axis (all transitions observed here are a-type) is 3.052 D (Blom et al. 1984).

C2H3OH also has two conformers, named syn and anti. Turner & Apponi (2001) assigned various emission features to the two conformers in the crowded spectra of Sgr B2(N). Here we report an unambiguous detection of the syn form, which is the most stable one, in a colder source that is much less affected by line confusion. Level energies and transition frequencies were computed from the rotational constants given by Melosso et al. (2019). The components of the dipole moment along the a and b axes are 0.616 D and 0.807 D, respectively (Saito 1976). Both a- and b-type transitions are observed here.

HCOOCH3 is a well-known interstellar molecule in which the internal rotation, or torsion, of the methyl group splits each rotational level into A and E substates, with statistical weights A:E = 1:1. We used the spectroscopy from a fit to the lines measured by Ogata et al. (2004) and implemented in MADEX (Cernicharo 2012). Here all observed transitions are a-type, and thus we adopted μa = 1.63 D (Curl 1959).

CH3OCH3 is an asymmetric rotor in which the large amplitude internal motion of the two equivalent methyl groups leads to level splitting into AA, EE, EA, and AE substates, with nontrivial statistical weights (Endres et al. 2009). We adopted the spectroscopy from the Cologne Database for Molecular Spectroscopy (Müller et al. 2005)3. Due to the geometry of the molecule, it only has nonzero dipole moment along the b axis, with a measured value of 1.302 D (Blukis et al. 1963).

4. Results

We detected various lines for each of the O-bearing COMs targeted in this study (see Table 1), with signal-to-noise ratios (S/N) well above 3σ. The position of the lines is consistent with the calculated frequencies based on laboratory data (see Sect. 3) and the systemic velocity of the source, VLSR = 5.83 km s−1 (Cernicharo et al. 2020b). Moreover, the relative intensities of the lines are those expected for rotational temperatures in the range 3–10 K, which are typical of TMC-1 (Gratier et al. 2016), and there are no missing lines. We thus consider the detection of the four O-bearing COMs in TMC-1 to be secure. Hereafter we discuss the particularities of each molecule.

The detection of the trans form of C2H3CHO is very solid since the six observed lines are detected with S/N between 6.5σ and 19.4σ and the VLSR of the lines are fully consistent with the systemic velocity of TMC-1 (see Table 1 and Fig. 1). The rotational temperature (Trot) of C2H3CHO is not very precisely determined, 7.5 ± 3.5 K from a rotation diagram, but synthetic spectra computed under thermodynamic equilibrium indicate that values in the range 5–10 K are consistent with the relative intensities observed. We thus adopted Trot = 7.5 K to compute the synthetic spectra and derive the column density. The arithmetic mean of the observed C2H3CHO line widths, 0.71 km s−1, was adopted when computing the synthetic spectra (see Table 1). We also assumed a circular emission distribution with a diameter θs = 80″, as observed for various hydrocarbons in TMC-1 (Fossé et al. 2001). For the other three molecules, we followed the same convention, adopting as the line width the average of the observed values and assuming the same emission distribution. The column density derived for C2H3CHO is (2.2 ± 0.3) × 1011 cm−2.

thumbnail Fig. 1.

Lines of C2H3CHO observed in TMC-1 (see the parameters in Table 1). Shown in red are the computed synthetic spectra for N = 2.2 × 1011 cm−2, Trot = 7.5 K, FWHM = 0.71 km s−1, and θs = 80″.

In the case of C2H3OH (see Table 1 and Fig. 2), three lines are clearly detected, with S/N of 6.7σ and 9.0σ. The frequency of the 21, 2  −  11, 1 transition, 37 459.184 MHz, coincides with a hyperfine component of CH2CCH, which was recently reported in TMC-1 (Agúndez et al. 2021). We predict T A * $ T_A^* $ = 1.1 mK for the 21, 2  −  11, 1 transition, which indicates that the observed line (see Agúndez et al. 2021) has contributions from both CH2CCH and C2H3OH. As a result of the high detection significance of the three lines, the overlap of a fourth line with CH2CCH, and the fact that there are no missing lines, we consider the detection of C2H3OH to be secure. The rotational temperature is not well constrained for C2H3OH, although the observed relative intensities indicate that it must be in the high range of the values typically observed in TMC-1. We thus adopted Trot = 10 K and a line width of 0.87 km s−1 to compute the synthetic spectra. We derive a column density of (2.5 ± 0.5) × 1012 cm−2 for C2H3OH.

thumbnail Fig. 2.

Lines of C2H3OH observed in TMC-1 (see the parameters and note on line 21, 1  −  11, 0 in Table 1). Shown in red are the computed synthetic spectra for N = 2.5 × 1012 cm−2, Trot = 10 K, FWHM = 0.87 km s−1, and θs = 80″.

We detected five A/E doublets of HCOOCH3 with S/N in the range 5.8–14.6σ (see Table 1 and Fig. 3). The only problematic line was the 30, 3  −  20, 2 transition of the E substate, which happened to lie close to a negative frequency-switching artifact, making it appear less intense and slightly shifted from the correct position. Therefore, we did not fit this line. The derived rotational temperature is 5.1 ± 2.5 K, and we thus adopted Trot = 5 K and a line width of 0.67 km s−1 to compute the synthetic spectra. The total column density obtained for HCOOCH3, including both A and E substates, is (1.1 ± 0.2) × 1012 cm−2.

thumbnail Fig. 3.

Lines of HCOOCH3 observed in TMC-1 (see the parameters and note on line 30, 3 − 20, 2E in Table 1). Shown in red are the computed synthetic spectra for a N = 1.1 × 1012 cm−2, Trot = 5 K, FWHM = 0.67 km s−1, and θs = 80″.

For CH3OCH3, we observed four triplets with the characteristic structure of an intense component corresponding to the EE substate lying between two equally intense components corresponding to the AE+EA and AA substates. The EE component of the four triplets is detected with good confidence levels, between 5.0σ and 9.6σ (see Table 1 and Fig. 4). The weaker components corresponding to the AE+EA and AA substates are sometimes found to lie within the noise, although the computed synthetic spectra are consistent with this fact. We consider the detection of CH3OCH3 to be secure. From a rotation diagram we derive a low rotational temperature of 3.6 ± 0.6 K, which is well constrained by the availability of transitions covering upper level energies from 2.3 K to 10.8 K. We thus adopted Trot = 3.6 K and a line width of 0.72 km s−1 to compute the synthetic spectra, which implies a total column density, including the four substates, of (2.5 ± 0.7) × 1012 cm−2 for CH3OCH3.

thumbnail Fig. 4.

Lines of CH3OCH3 observed in TMC-1 (see the parameters in Table 1). Shown in red are the computed synthetic spectra for N = 2.5 × 1012 cm−2, Trot = 3.6 K, FWHM = 0.72 km s−1, and θs = 80″.

The variation in the column densities due to the uncertainty in Trot is small, ∼15%, for C2H3CHO and C2H3OH, and higher, by a factor of two, for HCOOCH3 and CH3OCH3.

5. Discussion

The abundances derived for C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3 are 2.2 × 10−11, 2.5 × 10−10, 1.1 × 10−10, and 2.5 × 10−10, respectively, relative to H2, if we adopt a column density of H2 of 1022 cm−2 (Cernicharo & Guélin 1987). Next we wish to discuss how these four O-bearing COMs are formed in TMC-1.

C2H3CHO and C2H3OH could be formed by gas-phase neutral-neutral reactions between reactive radicals such as OH, CH, or C2H and abundant closed-shell molecules. However, among the potential sources of C2H3OH, the reactions OH + C2H4 and OH + CH2CHCH3 seem to have activation barriers (Zhu et al. 2005; Zádor et al. 2009) and the reaction CH + CH3OH seems to yield H2CO and CH3 as products (Zhang et al. 2002). In the case of C2H3CHO, a potential formation reaction is OH + allene, but the main products are H2CCO + CH3 (Daranlot et al. 2012). Two more promising routes to C2H3CHO are the reactions CH + CH3CHO, which has been found to produce C2H3CHO (Goulay et al. 2012), and C2H + CH3OH, which to our knowledge has not been studied experimentally or theoretically.

C2H3CHO and C2H3OH are not specifically considered in the chemical networks UMIST RATE12 (McElroy et al. 2013) or KIDA uva.kida.2014 (Wakelam et al. 2015), but acetaldehyde (CH3CHO), which is an isomer of C2H3OH, is included. Since astrochemical databases often do not distinguish between different isomers because information is not available, it is conceivable that some of the reactions that are considered to produce CH3CHO could also form C2H3OH. According to a standard pseudo-time-dependent gas-phase chemical model of a cold dark cloud (e.g., Agúndez & Wakelam 2013), there are two main formation reactions for CH3CHO. The first is O + C2H5, for which the formation of C2H3OH does not seem to be an important channel, according to experiments (Slagle et al. 1988) and theory (Jung et al. 2011; Vazart et al. 2020). The second is the dissociative recombination of CH3CHOH+ with electrons, in which case experiments show that the CCO backbone is preserved with a branching ratio of 23% (Hamberg et al. 2010); as such, it is possible that both CH3CHO and C2H3OH are formed. A different route to CH3CHO starting from abundant ethanol (C2H5OH) has been proposed (Skouteris et al. 2018; Vazart et al. 2020), but in L483, the only cold environment where C2H5OH has been detected, its abundance is half that of CH3CHO (Agúndez et al. 2019). The column density of CH3CHO at TMC-1(CP) is (2.7–3.5) × 1012 cm−2 (Gratier et al. 2016; Cernicharo et al. 2020c), which implies a C2H3OH/CH3CHO ratio of ∼1. Therefore, if the two isomers are formed by the same reaction, then the branching ratios should be similar. The reaction CH3+ + H2CO produces CH4 + HCO+ (Smith & Adams 1978), and thus it is unlikely to form C2H3OH, unlike what was suggested by Turner & Apponi (2001).

Grain-surface processes could also form C2H3CHO and C2H3OH in TMC-1. Experiments show that C2H3OH is formed upon the proton irradiation of H2O/C2H2 ices (Hudson & Moore 2003) as well as the electron irradiation of CO/CH4 and H2O/CH4 ices (Abplanalp et al. 2016; Bergantini et al. 2017), while C2H3CHO is produced after the electron irradiation of CO/C2H4 ices (Abplanalp et al. 2015). Non-energetic processing of C2H2 ices, in which reactions with H atoms and OH radicals occur on the surface, also produces C2H3OH (Chuang et al. 2020). It remains uncertain, however, whether these experimental setups (e.g., in terms of irradiation fluxes and ice composition) resemble those of cold dark clouds. Abplanalp et al. (2016) attempted to resolve this uncertainty by incorporating the results of electron irradiation experiments in a chemical model of a cold dark cloud; they found that cosmic rays could drive the formation of C2H3OH on grain surfaces. Recently, Shingledecker et al. (2019) proposed that C2H3CHO can be efficiently formed on grain surfaces by successive reactions of addition of an H atom to HC3O. This process would also produce propynal (HCCCHO), which led these authors to propose a chemical connection, and thus a potential correlation, between HCCCHO and C2H3CHO. If this mechanism is correct, then C2H3CHO would more likely be detected in sources with intense HCCCHO emission (see Loison et al. 2016).

There is as of yet no consensus on how HCOOCH3 and CH3OCH3 are formed in cold sources. Models in which the synthesis relies on chemical desorption and gas-phase radiative associations usually require a chemical desorption efficiency as high as 10% (Vasyunin & Herbst 2013; Balucani et al. 2015; Chang & Herbst 2016), which can be relaxed if Eley-Rideal processes (Ruaud et al. 2015), radiation chemistry (Shingledecker et al. 2018), or non-diffusive grain-surface processes (Jin & Garrod 2020) are considered. These models can account for abundances relative to H2 of around 10−10 for HCOOCH3 and/or CH3OCH3 under certain assumptions, although they rely on still poorly constrained chemical and physical processes. Astronomical observations have shown that HCOOCH3 and CH3OCH3 could have a chemical connection with CH3OH, based on the slight abundance enhancement inferred for these O-bearing COMs at the CH3OH peak with respect to the dust peak in the pre-stellar core L1544 (Jiménez-Serra et al. 2016). The column densities derived here for HCOOCH3 and CH3OCH3 at TMC-1(CP) are similar, within a factor of two, to those reported by Soma et al. (2018) at the CH3OH peak of TMC-1. A coherent study using the same telescope and a detailed radiative transfer model is needed to see if there is a significant abundance enhancement of HCOOCH3 and CH3OCH3 at the CH3OH peak of TMC-1.

6. Conclusions

We report the detection of C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3 toward TMC-1(CP). This region, which is a prototypical cold dark cloud with abundant carbon chains, has now been revealed as a new cold source where the O-bearing COMs HCOOCH3 and CH3OCH3 are present. In addition, we provide the first evidence of two other O-bearing COMs in a cold source, C2H3CHO and C2H3OH, the latter being identified unambiguously for the first time in space here. The derived abundances relative to H2 are a few 10−11 for C2H3CHO and a few 10−10 for the three other molecules. Interestingly, C2H3OH has a similar abundance to its isomer CH3CHO, with C2H3OH/CH3CHO ∼ 1. We discuss potential formation routes to these molecules and conclude that further experimental, theoretical, and astronomical studies are needed to shed light on the origin of these COMs in cold interstellar sources.

Acknowledgments

We acknowledge funding support from Spanish MICIU through grants AYA2016-75066-C2-1-P, PID2019-106110GB-I00, PID2019-106235GB-I00, and PID2019-107115GB-C21 and from the European Research Council (ERC Grant 610256: NANOCOSMOS). M. A. also acknowledges funding support from the Ramón y Cajal programme of Spanish MICIU (grant RyC-2014-16277). We thank the anonymous referee for a constructive report that helped to improve this manuscript.

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All Tables

Table 1.

Observed line parameters of the target O-bearing COMs of this study in TMC-1.

In the text

All Figures

thumbnail Fig. 1.

Lines of C2H3CHO observed in TMC-1 (see the parameters in Table 1). Shown in red are the computed synthetic spectra for N = 2.2 × 1011 cm−2, Trot = 7.5 K, FWHM = 0.71 km s−1, and θs = 80″.
In the text
thumbnail Fig. 2.

Lines of C2H3OH observed in TMC-1 (see the parameters and note on line 21, 1  −  11, 0 in Table 1). Shown in red are the computed synthetic spectra for N = 2.5 × 1012 cm−2, Trot = 10 K, FWHM = 0.87 km s−1, and θs = 80″.
In the text
thumbnail Fig. 3.

Lines of HCOOCH3 observed in TMC-1 (see the parameters and note on line 30, 3 − 20, 2E in Table 1). Shown in red are the computed synthetic spectra for a N = 1.1 × 1012 cm−2, Trot = 5 K, FWHM = 0.67 km s−1, and θs = 80″.
In the text
thumbnail Fig. 4.

Lines of CH3OCH3 observed in TMC-1 (see the parameters in Table 1). Shown in red are the computed synthetic spectra for N = 2.5 × 1012 cm−2, Trot = 3.6 K, FWHM = 0.72 km s−1, and θs = 80″.
In the text

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