Discovery of CH2CHCCH and detection of HCCN, HC4N, CH3CH2CN, and, tentatively, CH3CH2CCH in TMC-1
J. Cernicharo, M. Agundez, C. Cabezas, N. Marcelino, B. Tercero, J. R. Pardo, J. D. Gallego, F. Tercero, J. A. Lopez-Perez, P. de Vicente
AAstronomy & Astrophysics manuscript no. ms © ESO 2021February 23, 2021 L etter to the E ditor Discovery of CH CHCCH and detection of HCCN, HC N, CH CH CN,and, tentatively, CH CH CCH in TMC-1 (cid:63)
J. Cernicharo , M. Agúndez , C. Cabezas , N. Marcelino , B. Tercero , , J. R. Pardo , J. D. Gallego , F. Tercero ,J. A. López-Pérez , and P. de Vicente Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C / Serrano 121, 28006 Madrid, Spain. e-mail: j [email protected] Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain. Observatorio Astronómico Nacional (IGN), C / Alfonso XII, 3, 28014, Madrid, Spain.Received; accepted
ABSTRACT
We present the discovery in TMC-1 of vinyl acetylene, CH CHCCH, and the detection, for the first time in a cold dark cloud, ofHCCN, HC N, and CH CH CN. A tentative detection of CH CH CCH is also reported. The column density of vinyl acetylene is(1.2 ± × cm − , which makes it one of the most abundant closed-shell hydrocarbons detected in TMC-1. Its abundance is onlythree times lower than that of propylene, CH CHCH . The column densities derived for HCCN and HC N are (4.4 ± × cm − and (3.7 ± × cm − , respectively. Hence, the HCCN / HC N abundance ratio is 1.2 ± ± × cm − . These results are compared with a state-of-the-art chemical model of TMC-1, which is able to accountfor the observed abundances of these molecules through gas-phase chemical routes. Key words. molecular data — line: identification — ISM: molecules — ISM: individual (TMC-1) — — astrochemistry
1. Introduction
The chemical complexity of the interstellar medium has beendemonstrated by the detection of more than 200 di ff erent chem-ical species. In this context, the cold dark core TMC-1 presentsan interesting carbon-rich chemistry that led to the formationof long neutral carbon-chain radicals and their anions (see Cer-nicharo et al. 2020a, and references therein). Cyanopolyynes,which are stable molecules, are also particularly abundant inTMC-1 (see Cernicharo et al. 2020b,c, and references therein).The chemistry of this peculiar object produces a large abundanceof the nearly saturated species CH CHCH , which could mostlybe a typical molecule of hot cores (Marcelino et al. 2007). Thepolar benzenic ring C H CN has also been detected for the firsttime in space in this object (McGuire et al. 2018), while benzeneitself has only so far been been detected toward post-asymptoticgiant branch objects (Cernicharo et al. 2001).Sensitive line surveys are the best tools for unveiling themolecular content of astronomical sources and searching for newmolecules. A key element for carrying out a detailed analysisof line surveys is the availability of exhaustive spectroscopicinformation of the already-known species, their isotopologues,and their vibrationally excited states. The ability to identify themaximum possible number of spectral features leaves the clean-est possible forest of unidentified ones, therefore opening upa chance to discover new molecules and get insights into thechemistry and chemical evolution of the observed objects. Inthis context we have recently succeeded in discovering, prior (cid:63)
Based on observations carried out with the Yebes 40m telescope(projects 19A003, 19A010, 20A014, 20D023). The 40m radio telescopeat Yebes Observatory is operated by the Spanish Geographic Institute(IGN, Ministerio de Transportes, Movilidad y Agenda Urbana). to any spectroscopic information from the laboratory, severalnew molecular species (see, e.g., Cernicharo et al. 2020a,b,c,2021a,b; Marcelino et al. 2020). Consequently, the astronomi-cal object under study has become a real spectroscopic labora-tory. Experience shows that, if the sensitivity of a line surveyis su ffi ciently high, many unknown species will be discovered,providing key information on the ongoing chemical processes inthe cloud.In this letter we report on the discovery of vinyl acetylene,CH CHCCH, a species that has been spectroscopically charac-terized but never detected in space. We also present the detec-tion, for the first time in a cold dark cloud, of HCCN, HC N,and CH CH CN. Ethyl acetylene, CH CH CCH, has also beententatively detected.
2. Observations
New receivers, built within the Nanocosmos project and in-stalled at the Yebes 40m radio telescope, were used for the ob-servations of TMC-1. The Q-band receiver consists of two highelectron mobility transistor cold amplifiers covering the 31.0-50.3 GHz range with horizontal and vertical polarizations. Re-ceiver temperatures vary from 17 K at 32 GHz to 25 K at 50GHz. Eight 2.5 GHz wide fast Fourier transform spectrometers,with a spectral resolution of 38.15 kHz, provide the whole cov-erage of the Q-band in each polarization. The main beam e ffi -ciency varies from 0.6 at 32 GHz to 0.47 at 50 GHz. A detaileddescription of the system is given in Tercero et al. (2021).The line survey of TMC-1 ( α J = h m . s and δ J =+ ◦ (cid:48) . (cid:48)(cid:48) ) in the Q-band was performed over several ses- https://nanocosmos.iff.csic.es/ Article number, page 1 of 11 a r X i v : . [ a s t r o - ph . GA ] F e b & A proofs: manuscript no. ms sions. Previous results obtained from the line survey were basedon two observing runs, one performed in November 2019 andone in February 2020. They concerned the detection of C N − and C N − (Cernicharo et al. 2020b), HC NH + (Marcelino etal. 2020), HC NC (Cernicharo et al. 2020c), and HC O + (Cer-nicharo et al. 2020a). Additional data were taken in October2020, December 2020, and January 2021 to improve the linesurvey and to further check the consistency of all observed spec-tral features. These new data allowed the detection of HC S + (Cernicharo et al. 2021a) along with the acetyl cation, CH CO + (Cernicharo et al. 2021b), the isomers of C H N (Marcelino etal. 2021), and HDCCN (Cabezas et al. 2021).Two di ff erent frequency coverages were used in the line sur-vey, 31.08-49.52 GHz and 31.98-50.42 GHz, to ensure that nospurious spectral ghosts were produced in the down-conversionchain, which down-converts the signal from the receiver to 1-19.5 GHz and then splits it into eight 2.5 GHz bands, whichare finally analyzed by the FFTs. The observing procedure wasfrequency switching with a frequency throw of 10 MHz for thefirst two runs and 8 MHz for the later ones. The intensity scale(i.e., the antenna temperature, T ∗ A ) was calibrated using two ab-sorbers at di ff erent temperatures and the atmospheric transmis-sion model (ATM; Cernicharo 1985; Pardo et al. 2001). Calibra-tion uncertainties were adopted to be 10 %. The nominal spectralresolution of 38.15 kHz was kept for the final spectra. The sen-sitivity varied across the Q-band from 0.5 to 2.0 mK. All datawere analyzed using the GILDAS package .
3. 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 surveysof this source at the same frequencies (Kaifu et al. 2004). Thislarge improvement allowed us to detect a forest of weak lines,most of which arise from the isotopologues of abundant speciessuch as HC N, HC N, and HC N (Cernicharo et al. 2020c). Infact, it was possible to detect many individual lines (Marcelinoet al. 2021) from molecular species that had previously onlybeen reported using stacking techniques. Taking into account,on one hand, the large abundances found in TMC-1 for cyanidederivatives of abundant species and, on the other, the presence ofnearly saturated hydrocarbons such as CH CHCH (Marcelinoet al. 2007), we searched for species such as CH CHCCH andCH CH CCH, as well as for cyanides found previously onlyin carbon-rich stars (HCCN and HC N) or in warm molec-ular clouds (CH CH CN). Line identifications in this TMC-1 survey were performed using the MADEX catalogue (Cer-nicharo 2012), the Cologne Database of Molecular Spectroscopy(CDMS) catalogue (Müller et al. 2005), and the JPL. CHCCH
Spectroscopic constants for CH CHCCH were derived from afit to the lines reported by Thorwirth et al. (2004) and imple-mented in the MADEX code (Cernicharo 2012). We detected sixlines with K a = ∼ µ a , is only 0.43 D (Sobolev et al. 1962; Thorwirth &Lichau 2003). Only a K a = − rotational transition. Assuming a uni-form source with a radius of 40 (cid:48)(cid:48) (Fossé et al. 2001) and through Fig. 1.
Observed transitions of CH CHCCH toward TMC-1. The ab-scissa corresponds to the rest frequency of the lines assuming a v
LSR forthe source of 5.83 km s − . Frequencies and intensities for the observedlines 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 temper-ature of 5.0 ± ± × cm − . Figure 1 shows the synthetic spectrumof CH CHCCH computed for the derived rotational temperatureand column density (see Appendix A). The comparison with thedata shows a very good agreement for all lines, with the excep-tion of the 5 -4 transition, for which the predicted intensityis nearly a factor of two larger than what is observed. This lineis probably a ff ected by a negative spectral feature resulting fromthe folding of the frequency switching data.Using the column density of H derived by Cernicharo &Guélin (1987), the abundance of CH CHCCH relative to H to-ward TMC-1 is 1.2 × − . This abundance is only three times Article number, page 2 of 11ernicharo et al.: CH2CHCCH and exotic cyanides in TMC-1
Fig. 2.
Line profile for CH CH CCH toward TMC-1 from stacking thesignal from the transitions of this species in the 31-50 GHz frequencyrange. The abscissa corresponds to the local standard of rest velocity.The ordinate is the antenna temperature, corrected for atmospheric andtelescope losses, in mK. The observed spectra from the individual linesare shown in Fig. B.1. lower than that of propylene (Marcelino et al. 2007), and tentimes lower than that of methyl acetylene (Cabezas et al.2021). Hence, vinyl acetylene is one of the most abundant hy-drocarbons in TMC-1 and probably the most abundant com-pound containing four carbon atoms. It is interesting to com-pare the abundance of vinyl acetylene with that of vinyl cyanide.We analyzed the lines of CH CHCN covered by our Q-banddata. We derived a rotational temperature of 4.5 ± N (CH CHCN) = (6.5 ± × cm − (see Appendix C). Hence,the abundance ratio between the acetylenic and cyanide deriva-tives of ethylene (CH CH ) is 1.8 ± CHCCH and CH CHCNthrough reactions with CCH and CN, respectively (see Sect. 3.4for more details).We searched for CH CH CCH in our survey. Figure 2 showsthe resulting spectrum from stacking the lines of this species cov-ered in our data (see details in Appendix B). The whole set of in-dividual lines is shown in Fig. B.1. We consider this molecule tobe tentatively detected with a column density of 9 × cm − .We also detected ethyl cyanide, CH CH CN, in TMC-1 (seeSect. 3.3) with a column density of 10 cm − . Hence, assum-ing that CH CH CCH in TMC-1 is detected, the abundance ratioCH CH CCH / CH CH CN is ∼ N The cyano methylene radical, HCCN, was detected toward thecarbon-rich star IRC + CHCN (4.5 ± Fig. 3.
Observed lines of HCCN in our Q-band survey toward TMC-1. The abscissa corresponds to the rest frequency assuming a v
LSR of the source of 5.83 km s − . The ordinate is the antenna tempera-ture, corrected for atmospheric and telescope losses, in mK. The vi-olet line shows the synthetic spectrum computed for T r = = × cm − . For the sake of clarity, only the upper quan-tum numbers F and F are provided in panel (c). see Sect. 3.1) and computed a synthetic spectrum with the col-umn density as a free parameter. The best fit was obtained for acolumn density of (4.4 ± × cm − . As shown in Fig. 3, thematch between the model and the observations is very good.In addition to HCCN, the linear cyano ethynyl-methyleneradical, HC N, was also detected in IRC + ± ± × cm − . Figure 4 showsthe synthetic spectrum corresponding to these parameters. Theagreement with the observations is very good. This is the firsttime that this species has been detected in a cold dark cloud.We searched for the bent and cyclic isomers of HC N(McCarthy et al. 1999a,b) without success. We derived 3 σ upper limits to their column densities of 1.5 × cm − and1.0 × cm − , respectively. It is interesting to note that thesetwo isomers have a very di ff erent molecular structure relative to Article number, page 3 of 11 & A proofs: manuscript no. ms
Fig. 4.
Selected lines of HC N in the 31-50 GHz frequency range to-ward TMC-1. The abscissa corresponds to the rest frequency assuminga v
LSR for TMC-1 of 5.83 km s − . The ordinate is the antenna tem-perature, corrected for atmospheric and telescope losses, in mK. Theviolet line shows the synthetic spectrum computed for T r = N) = × cm − . linear HC N (Cernicharo et al. 2004). Hence, they are proba-bly not formed in the reaction between CH and HC N, which isthe main pathway to the linear isomer. Finally, we searched forthe related radicals CCN (Kakimoto & kasuya 1982; Ohshima& Endo 1995) and C N (McCarthy et al. 2003). However, dueto their low permanent dipole moment (Pd et al. 2001; Fiser &Polák 2013), we obtained very conservative 3 σ upper limits totheir abundances of 1.8 × cm − and 4.0 × cm − , respec-tively.The abundance ratio HCCN / HC N derived in TMC-1 is1.2 ± ff erent than that derived in the carbon-star envelope IRC + ∼
9; Cernicharo et al. 2004). InIRC + n + N / HC n + N decrement observed for cyanopolyynes(Cernicharo et al. 2004), while in TMC-1 the HCCN / HC N ra-tio is two to three times lower than that of the cyanopolyyne decrement. The di ff erent behavior in TMC-1 compared toIRC + ff erences in the abundancesof the precursors of HCCN and HC N (see Sect. 3.4). CH CN This molecule is typical of warm molecular clouds, where it pro-duces a forest of lines arising from all its isotopologues and low-energy vibrational excited states (Demyk et al. 2007; Daly etal. 2013). It was searched for in TMC-1 by Minh & Irvine(1991) without success. More recently, Lee et al. (2021) usedstacking techniques, providing an upper limit to its column den-sity in TMC-1 of 4 × cm − . We searched for the lines of thismolecule in our TMC-1 Q-band survey. Figure B.2 shows thesix lines with K a = σ levels. The K a = CHCN (4.5 K; see AppendixC) and a column density of N (CH CH CN) = × cm − (with an estimated error of 30%). The match between theobservations and the model is very reasonable. We de-rived abundance ratios CH CHCN / CH CH CN = ±
20 andCH CHCCH / CH CH CCH = ±
40. It is interesting to com-pare the abundance ratio between vinyl cyanide and ethylcyanide in TMC-1 and that in Orion KL. In the latter source it is0.06 (López et al. 2014), which is a factor of ∼ ff erence in this abundance ratio tells us about the di ff er-ent chemical processes prevailing in cold and warm molecularclouds. While the chemistry is dominated by the contribution ofthe evaporating ices covering dust grains in objects such as OrionKL, reactions between radicals and neutrals in cold dark cloudsplay a key role in producing these molecules. After the discovery of abundant propylene in TMC-1 (Marcelinoet al. 2007), the detection of an even larger, partially saturated,and abundant hydrocarbon, such as vinyl acetylene, in the samecloud brings to light the existence of a rich organic chemistryin cold dark clouds, going beyond the long-known presence ofunsaturated carbon chains. Moreover, just as the hydrocarbonsCH CCH and CH CCH act as precursors of the various nitrilesC H N found in TMC-1 (Marcelino et al. 2021), CH CHCCHemerges as a very likely candidate precursor of the large nitrileswith molecular formula C H N that have recently been claimedin TMC-1 (Lee et al. 2021).To get insight into the formation mechanism of CH CHCCHand the other molecules covered in this study, we ran a pseudo-time-dependent gas-phase chemical model that adopts typicalparameters of cold dark clouds (see, e.g., Agúndez & Wakelam2013). We used the chemical network
RATE12 from the UMISTdatabase (McElroy et al. 2013), augmented with reactions rel-evant for the molecules of interest in this work, which are dis-cussed below.Vinyl acetylene is formed in the model through the neutral-neutral gas-phase reactions C + C H , CH + C H (where C H stands for the two isomers CH CCH and CH CCH ), and C H + C H . These reactions have been found to be rapid at low tem-peratures (Canosa et al. 2007; Daugey et al. 2005; Bowman et Article number, page 4 of 11ernicharo et al.: CH2CHCCH and exotic cyanides in TMC-1 time (yr) a b un d a n c e r e l a t i v e t o H CH CHCCHC H NCH CH CNHCCNHC N Fig. 5.
Calculated fractional abundances of assorted molecules relevantto this study as a function of time. Horizontal dashed lines correspondto observed values. al. 2012), and branching ratios have been measured for someof them (Loison & Bergeat 2009; Goulay et al. 2009; Bowmanet al. 2012). Vinyl acetylene is predicted to form with a peakabundance of ∼ − relative to H (see Fig. 5), in good agree-ment with the value derived from observations. We also investi-gated whether cyanide derivatives of CH CHCCH can be formedthrough the reaction CN + CH CHCCH, which has been foundto be rapid for temperatures down to 174 K (Yang et al. 1992).The peak abundance calculated for the generic species C H N,which accounts for di ff erent possible isomers, is a few times10 − relative to H , somewhat above the observed abundance,which means that the reaction between CN and vinyl acetyleneis a viable route to the C H N isomers found in TMC-1 (Lee etal. 2021). Information on the kinetics of this reaction down tovery low temperatures would be highly valuable to validate thismechanism.Ethyl cyanide can also be formed e ffi ciently in the gasphase in cold dark cloud conditions. In our chemical modelthe main route involves the radiative association between CH + and CH CN and the dissociative recombination of the ionC H CNH + with electrons, both of which have measured rateconstants (Anicich 1993; Vigren et al. 2010). The calculatedpeak abundance is ∼ − relative to H (see Fig. 5), whichis around ten times higher than the observed value. The chem-ical network probably misses some important reactions of de-struction of CH CH CN (e.g., with neutral atoms or ions) whichcould explain the abundance overestimation.We finally discuss the formation of the allenic moleculesHCCN and HC N in TMC-1. The chemical network that in-volves HCCN is mostly taken from Loison et al. (2015), whilethat for HC N is assumed to be similar. The chemical modelpredicts HCCN to be around ten times more abundant thanHC N (see Fig. 5), in contrast with observations that find bothmolecules to have similar abundances. The reason for this isthat the main routes to HCCN and HC N are the reactionsCH + HCN / HNC and CH + HC N, respectively (with rateconstants based on Zabarnick et al. 1991), and the chemicalmodel predicts that HCN and HNC together are around tentimes more abundant than HC N. However, observations indi-cate that HC N is as abundant as HCN and HNC in TMC-1(e.g., Agúndez & Wakelam 2013), and therefore if the reactionsCH + HCN / HNC and CH + HC N are the main pathways toHCCN and HC N, respectively, one would expect similar abun-dances for HCCN and HC N, in agreement with observations.The higher HCCN / HC N observed in IRC + Nin this source.
Acknowledgements.
We thank Ministerio de Ciencia e Innovación of Spain (MI-CIU) for funding support through projects AYA2016-75066-C2-1-P, PID2019-106110GB-I00, PID2019-107115GB-C21 / AEI / / References
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Appendix A: Line parameters for CH CHCCH and model fitting procedure
Line parameters were derived for CH CHCCH by fitting a Gaussian line profile to the observed lines. A velocity range of ±
20 kms − around each feature was considered for the fit after a polynomial baseline was removed. The derived line parameters are givenin Table A.1.For the other species we show the synthetic spectrum resulting from the best fit model to all the lines computed using theMADEX code (Cernicharo 2012). We assumed a homogeneous rotational temperature for all rotational levels, a source of uniformbrightness with a radius of 40 (cid:48)(cid:48) (Fossé et al. 2001), and a full linewidth at half power intensity of 0.6 km s − , which represents agood averaged value to the linewidth of all observed lines. A fit to the observed line profiles and intensities provide the rotationaltemperature and the column density for the observed species. The final parameters derived in this way should be very similar tothose derived from a standard rotational diagram. However, this model fit allows us to compare the modeled line profiles with thoseof the observations, which is particularly interesting when the rotational transitions exhibit hyperfine structure, as is the case forHCCN, HC N, and, to a lesser extent, CH CH CN.
Table A.1.
Observed line parameters for CH CHCCH, HCCN, HC N, andCH CH CN.Transition ν (cid:82) T ∗ A dv v LSR ∆ v T A ∗ Notes(MHz) (mk km s − ) (km s − ) (km s − ) (mK)CH CHCCH4 − ± ± ± ± ± − ± ± ± ± ± − ± − ± ± ± ± ± − ± ± ± ± ± − ± ± ± ± ± − ± ± ± ± ± − ± ≤ − ± ≤ − ± ± ± ± ± CH CN4 , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± , − , ± , − , ± ± ± ± ± , − , ± , − , ± , − , ± ± ± ± ± , − , ± , − , ± − / / ± ± ± ± ± − / / ± ± ± ± ± − / / ± ± ± ± ± − / / ± ± ± ± ± − / / ± ± ± ± ± − / / ± − / / ± ± ± ± ± − / / ± ± ± ± ± − / / ± ± ± ± ± − / / ± ± ± ± ± − / / ± ≤ − / / ± ≤ − / / ± ± ± ± ± − / / ± ≤ − / / ± ± ± ± ± − / / ± ± ± ± ± − / / ± ± ± ± ± N7 −
6- 5 11 /
2- 9 / ± ± ± ± ± −
7- 6 13 / / ± ± ± ± ± −
8- 7 15 / / ± ± ± ± ± −
6- 5 13 / / ± ± ± ± ± Table A.1. continued.Transition ν (cid:82) T ∗ A dv v LSR ∆ v T A ∗ Notes(MHz) (mk km s − ) (km s − ) (km s − ) (mK)7 −
7- 6 15 / / ± −
8- 7 17 / / ± ± ± ± ± −
7- 6 15 / / ± ± ± ± ± −
8- 7 17 / / ± −
7- 6 13 / / ± ± ± ± ± −
9- 8 19 / / ± −
8- 7 15 / / ± ± ± ± ± −
9- 8 17 / / ± −
7- 6 13 / / ± ± ± ± ± −
8- 7 15 / / ± ± ± ± ± −
6- 5 11 /
2- 9 / ± ± ± ± ± −
7- 6 15 / / ± ± ± ± ± −
8- 7 17 / / ± −
6- 5 13 / / ± ± ± ± ± −
7- 6 13 / / ± ± ± ± ± −
8- 7 15 / / ± −
9- 8 17 / / ± −
7- 6 15 / / ± ± ± ± ± −
8- 7 17 / / ± ± ± ± ± −
9- 8 19 / / ± −
8- 7 17 / / ± ≤ −
8- 7 15 / / ± ± ± ± ± −
9- 8 19 / / ± −
10- 9 21 / / ± −
9- 8 17 / / ± ± ± ± ± −
10- 9 19 / / ± −
8- 7 15 / / ± ± ± ± ± −
9- 8 17 / / ± −
7- 6 13 / / ± ± ± ± ± −
8- 7 17 / / ± −
9- 8 19 / / ± ± ± ± ± −
7- 6 15 / / ± −
8- 7 15 / / ± −
9- 8 17 / / ± −
10- 9 19 / / ± −
8- 7 17 / / ± ± ± ± ± −
9- 8 19 / / ± −
10- 9 21 / / ± −
9- 8 19 / / ± ± ± ± ± −
9- 8 17 / / ± −
10- 9 21 / / ± − / / ± ± ± ± ± −
10- 9 19 / / ± ± ± ± ± − / / ± −
9- 8 17 / / ± −
9- 9 19 / / ± − / / ± ± ± ± ± −
10- 8 19 / / ± −
10- 9 21 / / ± − / / ± ± ± ± ± −
10- 9 21 / / ± −
10- 9 19 / / ± − / / ± ± ± ± ± − / / ± − / / ± ± ± ± ± − / / ± CHCN4 , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± & A proofs: manuscript no. ms
Table A.1. continued.Transition ν (cid:82) T ∗ A dv v LSR ∆ v T A ∗ Notes(MHz) (mk km s − ) (km s − ) (km s − ) (mK)4 , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± , − , ± ± ± ± ± , − , ± , − , ± ± ± ± ± , − , ± , − , ± ± ± ± ± , − , ± , − , ± ± ± ± ± , − , ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± ± ± ± ± , − , ± , − , ± , − , ± ± ± ± ± , − , ± Notes. ( A ) Blended with a strong feature. ( B ) Three sigma upper limit. ( NR ) Hyperfine structure line blended with other components of the same rotational transition. The line has been included in the fit to the previousor next entry in the table.
Appendix B: Ethyl acetylene and ethyl cyanide
We searched for the lines of ethyl acetylene, CH CH CCH, a molecule for which accurate laboratory data are available up to317.7 GHz (Demaison et al., 1983; Landsberg & Suenram 1983; Bestmann & Dreizler 1985; Steber et al. 2012) and which hasmoderate dipole moments of µ a = µ b = A and E species. Of theten lines expected for this molecule in the Q-band, five are detected at a 3 σ level, two are blended with other weak features slightlyshifted in frequency, and three are too weak. Using stacking techniques (see below) and removing the blended lines, which couldintroduce a significant bias in the final spectrum, we detected a signal at 5 σ at the correct velocity, as shown in Fig. 2. However, wehave no explanation for the lack of emission at the frequency of the 5 − transition other than that the data are too noisy at itsfrequency (see Fig. B.1). It is expected to be the strongest feature for the parameters we used for the synthetic spectrum (T r = CH CCH) = cm − ).For CH CH CCH, all the expected strongest lines in the Q-band are shown in Fig. B.1, and Fig. 2 shows the resulting stackedprofile. The stacking procedure we used is rather simple: A velocity range of ±
20 km s − was selected for each line and thenoise, σ , outside 5.83 ± − was computed (with known and unknown lines in each individual spectrum blanked). The datawere normalized to the expected intensity of each transition computed under local thermodynamic equilibrium for a rotationaltemperature of 5 K, the same as the value derived for CH CHCCH. Finally, all the data were multiplied by the expected intensityof the strongest transition in the sample. The lines are optically thin, and hence the intensity of all lines scale in the same way withthe assumed column density. Each individual spectrum was weighted as 1 / σ N , where σ N now contains the normalization intensityfactor. All rotational transitions falling in the frequency ranges where our data have the highest sensitivity were detected (transitions4 − , − , − , − , and 5 − ). Two of the observed transitions are heavily blended (4 − and 5 − )and were excluded from the stacked spectrum. Two K a = − and 5 − ) but were included in the data stacking with the corresponding weights, as explained above. The data for the strongestrotational transition, the 5 − , have a sensitivity of 0.6 mK. The expected intensity is 1.3 mK. This rotational transition wasincluded in the stacked spectrum, although it is clearly not detected at the noise level of the data. Figure 2 shows the resultingspectrum from this procedure. A feature, at exactly the velocity of the cloud, appears at a 5 σ level ( σ = Article number, page 8 of 11ernicharo et al.: CH2CHCCH and exotic cyanides in TMC-1
Fig. B.1.
Observed lines of CH CH CCH in the Q-band toward TMC-1. The abscissa corresponds to the rest frequency assuming a local standard ofrest velocity of 5.83 km s − . The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in mK. The red line shows thesynthetic spectrum obtained using the parameters derived from a rotational diagram of the observed lines (T r = CH CCH) = × cm − ). 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 thecase of CH CH CN, which was detected in our work (see Sect. 3.3 and Fig. B.2), while only upper limits were obtained by Lee etal. (2021). The observed lines of CH CH CN, together with the best model fit to the observed emission, are shown in Fig. B.2.
Article number, page 9 of 11 & A proofs: manuscript no. ms
Fig. B.2.
Observed lines of CH CH CN in the 31-50 GHz frequency range toward TMC-1. The abscissa corresponds to the rest frequency assuminga local standard of rest velocity of 5.83 km s − . 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 = CH CN) = × cm − . Appendix C: Vinyl cyanide, CH CHCN
Vinyl cyanide produces a very rich spectrum in the 31-50 GHz range, with most of its lines exhibiting the hyperfine structure due tothe nitrogen nucleus. All the observed lines are shown in Fig. C.1. The model fitting procedure provides a rotational temperature of4.5 ± ± × cm − . Its abundance relative to vinyl acetylene and ethyl cyanide is discussed inSects. 3.1, 3.3, and 3.4. Article number, page 10 of 11ernicharo et al.: CH2CHCCH and exotic cyanides in TMC-1
Fig. C.1.
Observed lines of CH CHCN in the 31-50 GHz frequency range toward TMC-1. The abscissa corresponds to the rest frequency assuminga local standard of rest velocity of 5.83 km s − . The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in mK. Theviolet line shows the synthetic spectrum obtained using the parameters derived from a rotational diagram of the observed lines (T r = ± CHCN) = (6.5 ± × cm −2