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Discovery of CH2CHCCH and detection of HCCN, HC4N, CH3CH2CN, and, tentatively, CH3CH2CCH in TMC-1

We present the discovery in TMC-1 of vinyl acetylene, CH2CHCCH, and the detection, for the first time in a cold dark cloud, of HCCN, HC4N, and CH3CH2CN. A tentative detection of CH3CH2CCH is also reported. The column density of vinyl acetylene is (1.2 +/- 0.2)e13 cm-2, which makes it one of the most abundant closed-shell hydrocarbons detected in TMC-1. Its abundance is only three times lower than that of propylene, CH3CHCH2. The column densities derived for HCCN and HC4N are (4.4 +/- 0.4)e11 cm-2 and (3.7 +/- 0.4)e11 cm-2, respectively. Hence, the HCCN/HC4N abundance ratio is 1.2 +/- 0.3. For ethyl cyanide we derive a column density of (1.1 +/- 0.3)e11 cm-2. These results are compared with a state-of-the-art chemical model of TMC-1, which is able to account for the observed abundances of these molecules through gas-phase chemical routes.


Introduction
The chemical complexity of the interstellar medium has been demonstrated by the detection of more than 200 different chemical species. In this context, the cold dark core TMC-1 presents an interesting carbon-rich chemistry that led to the formation of long neutral carbon-chain radicals and their anions (see Cernicharo et al. 2020a, and references therein). Cyanopolyynes, which are stable molecules, are also particularly abundant in TMC-1 (see Cernicharo et al. 2020b,c, and references therein). The chemistry of this peculiar object produces a large abundance of the nearly saturated species CH 3 CHCH 2 , which could mostly be a typical molecule of hot cores (Marcelino et al. 2007). The polar benzenic ring C 6 H 5 CN has also been detected for the first time in space in this object (McGuire et al. 2018), while benzene itself has only so far been been detected toward post-asymptotic giant branch objects .
Sensitive line surveys are the best tools for unveiling the molecular content of astronomical sources and searching for new molecules. A key element for carrying out a detailed analysis of line surveys is the availability of exhaustive spectroscopic information of the already-known species, their isotopologues, and their vibrationally excited states. The ability to identify the maximum possible number of spectral features leaves the cleanest possible forest of unidentified ones, therefore opening up a chance to discover new molecules and get insights into the chemistry and chemical evolution of the observed objects. In this context we have recently succeeded in discovering, prior Based on observations carried out with the Yebes 40m telescope (projects 19A003, 19A010, 20A014, 20D023). The 40m radio telescope at Yebes Observatory is operated by the Spanish Geographic Institute (IGN, Ministerio de Transportes, Movilidad y Agenda Urbana).
to any spectroscopic information from the laboratory, several new molecular species (see, e.g., Cernicharo et al. 2020aCernicharo et al. ,b,c, 2021aMarcelino et al. 2020). Consequently, the astronomical object under study has become a real spectroscopic laboratory. Experience shows that, if the sensitivity of a line survey is sufficiently high, many unknown species will be discovered, providing key information on the ongoing chemical processes in the cloud.
In this letter we report on the discovery of vinyl acetylene, CH 2 CHCCH, a species that has been spectroscopically characterized but never detected in space. We also present the detection, for the first time in a cold dark cloud, of HCCN, HC 4 N, and CH 3 CH 2 CN. Ethyl acetylene, CH 3 CH 2 CCH, has also been tentatively detected.

Observations
New receivers, built within the Nanocosmos project 1 and installed at the Yebes 40m radio telescope, were used for the observations of TMC-1. The Q-band receiver consists of two high electron mobility transistor cold amplifiers covering the 31.0-50.3 GHz range with horizontal and vertical polarizations. Receiver temperatures vary from 17 K at 32 GHz to 25 K at 50 GHz. Eight 2.5 GHz wide fast Fourier transform spectrometers, with a spectral resolution of 38.15 kHz, provide the whole coverage of the Q-band in each polarization. The main beam efficiency varies from 0.6 at 32 GHz to 0.47 at 50 GHz. A detailed description of the system is given in Tercero et al. (2021).
The line survey of TMC-1 (α J2000 = 4 h 41 m 41.9 s and δ J2000 = +25 • 41 27.0 ) in the Q-band was performed over several ses-sions. Previous results obtained from the line survey were based on two observing runs, one performed in November 2019 and one in February 2020. They concerned the detection of C 3 N − and C 5 N − (Cernicharo et al. 2020b), HC 5 NH + (Marcelino et al. 2020), HC 4 NC (Cernicharo et al. 2020c), andHC 3 O + (Cernicharo et al. 2020a). Additional data were taken in October 2020, December 2020, and January 2021 to improve the line survey and to further check the consistency of all observed spectral features. These new data allowed the detection of HC 3 S + (Cernicharo et al. 2021a) along with the acetyl cation, CH 3 CO + (Cernicharo et al. 2021b), the isomers of C 4 H 3 N (Marcelino et al. 2021), and HDCCN (Cabezas et al. 2021).
Two different frequency coverages were used in the line survey, 31.08-49.52  GHz, to ensure that no spurious spectral ghosts were produced in the down-conversion chain, which down-converts the signal from the receiver to 1-19.5 GHz and then splits it into eight 2.5 GHz bands, which are finally analyzed by the FFTs. The observing procedure was frequency switching with a frequency throw of 10 MHz for the first two runs and 8 MHz for the later ones. The intensity scale (i.e., the antenna temperature, T * A ) was calibrated using two absorbers at different temperatures and the atmospheric transmission model (ATM ;Cernicharo 1985;Pardo et al. 2001). Calibration uncertainties were adopted to be 10 %. The nominal spectral resolution of 38.15 kHz was kept for the final spectra. The sensitivity varied across the Q-band from 0.5 to 2.0 mK. All data were analyzed using the GILDAS package 2 .

Results and discussion
The sensitivity of our observations toward TMC-1 (see Sect. 2) is a factor of 10-20 better than previously published line surveys of this source at the same frequencies (Kaifu et al. 2004). This large improvement allowed us to detect a forest of weak lines, most of which arise from the isotopologues of abundant species such as HC 3 N, HC 5 N, and HC 7 N (Cernicharo et al. 2020c). In fact, it was possible to detect many individual lines (Marcelino et al. 2021) from molecular species that had previously only been reported using stacking techniques. Taking into account, on one hand, the large abundances found in TMC-1 for cyanide derivatives of abundant species and, on the other, the presence of nearly saturated hydrocarbons such as CH 3 CHCH 2 (Marcelino et al. 2007), we searched for species such as CH 2 CHCCH and CH 3 CH 2 CCH, as well as for cyanides found previously only in carbon-rich stars (HCCN and HC 4 N) or in warm molecular clouds (CH 3 CH 2 CN). Line identifications in this TMC-1 survey were performed using the MADEX catalogue (Cernicharo 2012), the Cologne Database of Molecular Spectroscopy (CDMS) catalogue (Müller et al. 2005), and the JPL.

Vinyl acetylene, CH 2 CHCCH
Spectroscopic constants for CH 2 CHCCH were derived from a fit to the lines reported by Thorwirth et al. (2004) and implemented in the MADEX code (Cernicharo 2012). We detected six lines with K a =0 and 1 (see Fig. 1). Although the observed line intensities (see Table A.1) are low (∼2-3 mK), the column density for this molecule should be particularly large as its dipole moment, µ a , is only 0.43 D (Sobolev et al. 1962;Thorwirth & Lichau 2003). Only a K a =2 line was marginally detected, corresponding to the 4 22 − 3 21 rotational transition. Assuming a uniform source with a radius of 40 (Fossé et al. 2001) and through 2 http://www.iram.fr/IRAMFR/GILDAS Fig. 1. Observed transitions of CH 2 CHCCH toward TMC-1. The abscissa corresponds to the rest frequency of the lines assuming a v LS R for the source of 5.83 km s −1 . Frequencies and intensities for the observed lines are given in Table A.1. The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in millikelvins (mK). The purple line shows the synthetic spectrum obtained for this species. a standard rotational diagram, we derived a rotational temperature of 5.0±0.5 K and a column density for vinyl acetylene of (1.2±0.2)×10 13 cm −2 . Figure 1 shows the synthetic spectrum of CH 2 CHCCH computed for the derived rotational temperature and column density (see Appendix A). The comparison with the data shows a very good agreement for all lines, with the exception of the 5 05 -4 04 transition, for which the predicted intensity is nearly a factor of two larger than what is observed. This line is probably affected by a negative spectral feature resulting from the folding of the frequency switching data.
Using the column density of H 2 derived by Cernicharo & Guélin (1987), the abundance of CH 2 CHCCH relative to H 2 toward TMC-1 is 1.2×10 −9 . This abundance is only three times lower than that of propylene (Marcelino et al. 2007), and ten times lower than that of methyl acetylene (Cabezas et al. 2021). Hence, vinyl acetylene is one of the most abundant hydrocarbons in TMC-1 and probably the most abundant compound containing four carbon atoms. It is interesting to compare the abundance of vinyl acetylene with that of vinyl cyanide. We analyzed the lines of CH 2 CHCN covered by our Q-band data. We derived a rotational temperature of 4.5±0.3 K, and N(CH 2 CHCN)=(6.5±0.5)×10 12 cm −2 (see Appendix C). Hence, the abundance ratio between the acetylenic and cyanide derivatives of ethylene (CH 2 CH 2 ) is 1.8±0.4, which suggests that ethylene could be a likely precursor of CH 2 CHCCH and CH 2 CHCN through reactions with CCH and CN, respectively (see Sect. 3.4 for more details).
We searched for CH 3 CH 2 CCH in our survey. Figure 2 shows the resulting spectrum from stacking the lines of this species covered in our data (see details in Appendix B). The whole set of individual lines is shown in Fig. B.1. We consider this molecule to be tentatively detected with a column density of 9×10 11 cm −2 . We also detected ethyl cyanide, CH 3 CH 2 CN, in TMC-1 (see Sect. 3.3) with a column density of 10 11 cm −2 . Hence, assuming that CH 3 CH 2 CCH in TMC-1 is detected, the abundance ratio CH 3 CH 2 CCH/CH 3 CH 2 CN is ∼9.

HCCN and HC 4 N
The cyano methylene radical, HCCN, was detected toward the carbon-rich star IRC+10216 by Guélin & Cernicharo (1991). The molecule has been observed in the laboratory by several authors (Saito et al. 1984;Endo & Ohshima 1993;McCarthy et al. 1995;Allen et al. 2001). Its dipole moment is 3.04 D (Inostroza et al. 2012). Given the large abundance of cyanopolyynes in TMC-1 (e.g., Cernicharo et al. 2020c), HCCN could be expected in this source, although it was searched for by McGonagle & Irvine (1996) with no success. Several of its rotational transitions, which show significant hyperfine splitting, are within the frequency range of our TMC-1 survey. All of them were detected, as shown in Fig. 3 (see Table A.1). This is the first time HCCN has been detected in cold interstellar clouds. The range of level energies covered by the observed transitions, 1.6-3.7 K, did not allow us to build a reliable rotational diagram. We adopted the rotational temperature derived for CH 2 CHCN (4.5±0.5 K; Fig. 3. Observed lines of HCCN in our Q-band survey toward TMC-1. The abscissa corresponds to the rest frequency assuming a v LS R of the source of 5.83 km s −1 . The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in mK. The violet line shows the synthetic spectrum computed for T r =4.5 K and N(HCCN)=4.4×10 11 cm −2 . For the sake of clarity, only the upper quantum numbers F 1 and F are provided in panel (c). see Sect. 3.1) and computed a synthetic spectrum with the column density as a free parameter. The best fit was obtained for a column density of (4.4±0.4)×10 11 cm −2 . As shown in Fig. 3, the match between the model and the observations is very good.
In addition to HCCN, the linear cyano ethynyl-methylene radical, HC 4 N, was also detected in IRC+10216 (Cernicharo et al. 2004). Spectroscopic information on this molecule (Tang et al. 1999) was implemented in MADEX. Its dipole moment is 4.33 D (Ikuta et al. 2000). The observed lines toward TMC-1 are shown in Fig. 4. From those lines and the model fitting procedure, it is possible to derive a rotational temperature of 4±1 K and a column density of (3.7±0.4)×10 11 cm −2 . Figure 4 shows the synthetic spectrum corresponding to these parameters. The agreement with the observations is very good. This is the first time that this species has been detected in a cold dark cloud.
We searched for the bent and cyclic isomers of HC 4 N (McCarthy et al. 1999a,b) without success. We derived 3σ upper limits to their column densities of 1.5×10 11 cm −2 and 1.0×10 11 cm −2 , respectively. It is interesting to note that these two isomers have a very different molecular structure relative to Fig. 4. Selected lines of HC 4 N in the 31-50 GHz frequency range toward TMC-1. The abscissa corresponds to the rest frequency assuming a v LS R for TMC-1 of 5.83 km s −1 . The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in mK. The violet line shows the synthetic spectrum computed for T r =4.0 K and N(HC 4 N)=3.7×10 11 cm −2 .
linear HC 4 N (Cernicharo et al. 2004). Hence, they are probably not formed in the reaction between CH and HC 3 N, which is the main pathway to the linear isomer. Finally, we searched for the related radicals CCN (Kakimoto & kasuya 1982;Endo 1995) andC 4 N (McCarthy et al. 2003). However, due to their low permanent dipole moment (Pd et al. 2001;Fiser & Polák 2013), we obtained very conservative 3σ upper limits to their abundances of 1.8×10 12 cm −2 and 4.0×10 13 cm −2 , respectively.
The abundance ratio HCCN/HC 4 N derived in TMC-1 is 1.2±0.3, which is very different than that derived in the carbonstar envelope IRC +10216 (∼ 9; Cernicharo et al. 2004). In IRC +10216, this abundance ratio is a factor of two larger than the HC 2n+1 N/HC 2n+3 N decrement observed for cyanopolyynes (Cernicharo et al. 2004), while in TMC-1 the HCCN/HC 4 N ratio is two to three times lower than that of the cyanopolyyne decrement. The different behavior in TMC-1 compared to IRC +10216 is probably caused by differences in the abundances of the precursors of HCCN and HC 4 N (see Sect. 3.4).

CH 3 CH 2 CN
This molecule is typical of warm molecular clouds, where it produces a forest of lines arising from all its isotopologues and lowenergy vibrational excited states (Demyk et al. 2007;Daly et al. 2013). It was searched for in TMC-1 by Minh & Irvine (1991) without success. More recently, Lee et al. (2021) used stacking techniques, providing an upper limit to its column density in TMC-1 of 4×10 11 cm −2 . We searched for the lines of this molecule in our TMC-1 Q-band survey. Figure B.2 shows the six lines with K a =0 and 1 detected. All of them appear at 4-5σ levels. The K a =2 lines are too weak to be detected with the sensitivity of our survey.
We computed a synthetic spectrum using the rotational temperature derived for CH 2 CHCN (4.5 K; see Appendix C) and a column density of N(CH 3 CH 2 CN)=1.1×10 11 cm −2 (with an estimated error of 30%). The match between the observations and the model is very reasonable. We derived abundance ratios CH 2 CHCN/CH 3 CH 2 CN=65±20 and CH 2 CHCCH/CH 3 CH 2 CCH=120±40. It is interesting to compare the abundance ratio between vinyl cyanide and ethyl cyanide in TMC-1 and that in Orion KL. In the latter source it is 0.06 (López et al. 2014), which is a factor of ∼1100 smaller than in TMC-1. The spatial distribution of both molecules in Orion KL has been analyzed by Cernicharo et al. (2016). Both species arise in regions with temperatures of 100 and 350 K. The huge difference in this abundance ratio tells us about the different chemical processes prevailing in cold and warm molecular clouds. While the chemistry is dominated by the contribution of the evaporating ices covering dust grains in objects such as Orion KL, reactions between radicals and neutrals in cold dark clouds play a key role in producing these molecules.

Chemistry of detected molecules
After the discovery of abundant propylene in TMC-1 (Marcelino et al. 2007), the detection of an even larger, partially saturated, and abundant hydrocarbon, such as vinyl acetylene, in the same cloud brings to light the existence of a rich organic chemistry in cold dark clouds, going beyond the long-known presence of unsaturated carbon chains. Moreover, just as the hydrocarbons CH 3 CCH and CH 2 CCH 2 act as precursors of the various nitriles C 4 H 3 N found in TMC-1 (Marcelino et al. 2021), CH 2 CHCCH emerges as a very likely candidate precursor of the large nitriles with molecular formula C 5 H 3 N that have recently been claimed in TMC-1 (Lee et al. 2021).
To get insight into the formation mechanism of CH 2 CHCCH and the other molecules covered in this study, we ran a pseudotime-dependent gas-phase chemical model that adopts typical parameters of cold dark clouds (see, e.g., Agúndez & Wakelam 2013). We used the chemical network RATE12 from the UMIST database (McElroy et al. 2013), augmented with reactions relevant for the molecules of interest in this work, which are discussed below.
Vinyl acetylene is formed in the model through the neutralneutral gas-phase reactions C 2 + C 2 H 6 , CH + C 3 H 4 (where C 3 H 4 stands for the two isomers CH 3 CCH and CH 2 CCH 2 ), and C 2 H + C 2 H 4 . These reactions have been found to be rapid at low temperatures (Canosa et al. 2007;Daugey et al. 2005;Bowman et  al. 2012), and branching ratios have been measured for some of them (Loison & Bergeat 2009;Goulay et al. 2009;Bowman et al. 2012). Vinyl acetylene is predicted to form with a peak abundance of ∼ 10 −9 relative to H 2 (see Fig. 5), in good agreement with the value derived from observations. We also investigated whether cyanide derivatives of CH 2 CHCCH can be formed through the reaction CN + CH 2 CHCCH, which has been found to be rapid for temperatures down to 174 K (Yang et al. 1992).
The peak abundance calculated for the generic species C 5 H 3 N, which accounts for different possible isomers, is a few times 10 −10 relative to H 2 , somewhat above the observed abundance, which means that the reaction between CN and vinyl acetylene is a viable route to the C 5 H 3 N isomers found in TMC-1 (Lee et al. 2021). Information on the kinetics of this reaction down to very low temperatures would be highly valuable to validate this mechanism. Ethyl cyanide can also be formed efficiently in the gas phase in cold dark cloud conditions. In our chemical model the main route involves the radiative association between CH + 3 and CH 3 CN and the dissociative recombination of the ion C 2 H 5 CNH + with electrons, both of which have measured rate constants (Anicich 1993;Vigren et al. 2010). The calculated peak abundance is ∼ 10 −10 relative to H 2 (see Fig. 5), which is around ten times higher than the observed value. The chemical network probably misses some important reactions of destruction of CH 3 CH 2 CN (e.g., with neutral atoms or ions) which could explain the abundance overestimation.
We finally discuss the formation of the allenic molecules HCCN and HC 4 N in TMC-1. The chemical network that involves HCCN is mostly taken from Loison et al. (2015), while that for HC 4 N is assumed to be similar. The chemical model predicts HCCN to be around ten times more abundant than HC 4 N (see Fig. 5), in contrast with observations that find both molecules to have similar abundances. The reason for this is that the main routes to HCCN and HC 4 N are the reactions CH + HCN/HNC and CH + HC 3 N, respectively (with rate constants based on Zabarnick et al. 1991), and the chemical model predicts that HCN and HNC together are around ten times more abundant than HC 3 N. However, observations indicate that HC 3 N is as abundant as HCN and HNC in TMC-1 (e.g., Agúndez & Wakelam 2013), and therefore if the reactions CH + HCN/HNC and CH + HC 3 N are the main pathways to HCCN and HC 4 N, respectively, one would expect similar abundances for HCCN and HC 4 N, in agreement with observations. The higher HCCN/HC 4 N observed in IRC +10216 is probably explained by the higher abundance of HCN compared to HC 3 N in this source.
Line parameters were derived for CH 2 CHCCH by fitting a Gaussian line profile to the observed lines. A velocity range of ±20 km s −1 around each feature was considered for the fit after a polynomial baseline was removed. The derived line parameters are given in Table A.1. For the other species we show the synthetic spectrum resulting from the best fit model to all the lines computed using the MADEX code (Cernicharo 2012). We assumed a homogeneous rotational temperature for all rotational levels, a source of uniform brightness with a radius of 40 (Fossé et al. 2001), and a full linewidth at half power intensity of 0.6 km s −1 , which represents a good averaged value to the linewidth of all observed lines. A fit to the observed line profiles and intensities provide the rotational temperature and the column density for the observed species. The final parameters derived in this way should be very similar to those derived from a standard rotational diagram. However, this model fit allows us to compare the modeled line profiles with those of the observations, which is particularly interesting when the rotational transitions exhibit hyperfine structure, as is the case for HCCN, HC 4 N, and, to a lesser extent, CH 3 CH 2 CN.  Observed lines of CH 3 CH 2 CCH in the Q-band toward TMC-1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s −1 . The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in mK. The red line shows the synthetic spectrum obtained using the parameters derived from a rotational diagram of the observed lines (T r =5 K and N(CH 3 CH 2 CCH)=9.0×10 11 cm −2 ). The rotational quantum numbers are provided in the upper right corners of each panel.
in sensitivity with our survey may provide false positive or negative results for low abundant molecular species. An example is the case of CH 3 CH 2 CN, which was detected in our work (see Sect. 3.3 and Fig. B.2), while only upper limits were obtained by Lee et al. (2021). The observed lines of CH 3 CH 2 CN, together with the best model fit to the observed emission, are shown in Fig. B.2. Observed lines of CH 2 CHCN in the 31-50 GHz frequency range toward TMC-1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s −1 . The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in mK. The violet line shows the synthetic spectrum obtained using the parameters derived from a rotational diagram of the observed lines (T r =4.5±0.3 K and N(CH 2 CHCN)=(6.5±0.5)×10 12 cm −2 ). Numbers in the upper left corners of the panels indicate the multiplicative factor applied to the synthetic spectrum to match the data. The rotational quantum numbers are provided in the upper right corners of each panel. The hyperfine structure of each rotational transition has been taken into account in the calculation of the synthetic spectra. Line parameters are given in Table A.1.