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Detection of Deuterated Methylcyanoacetylene, CH $ 2 Astronomy & Astrophysics manuscript no. AA202141371_Cabezas ©ESO 2021 June 8, 2021 Detection of deuterated methylcyanoacetylene, CH2DC3N, in TMC-1 ? C. Cabezas1, E. Roueff2, B. Tercero3; 4, M. Agúndez1, N. Marcelino1, P. de Vicente3 and J. Cernicharo1 1 Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain. e-mail: [email protected]; [email protected] 2 LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 92190 Meudon, France 3 Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, 28014, Madrid, Spain. 4 Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain. Received; accepted ABSTRACT We report the first detection in space of the single deuterated isotopologue of methylcyanoacetylene, CH2DC3N. A total of fifteen rotational transitions, with J = 8-12 and Ka = 0 and 1, were identified for this species in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. The observed frequencies were used to derive for the first time the spectroscopic parameters of this 10 −2 deuterated isotopologue. We derive a column density of (8:0±0:4)×10 cm . The abundance ratio between CH3C3N and CH2DC3N 13 is ∼22. We also theoretically computed the principal spectroscopic constants of C isotopologues of CH3C3N and CH3C4H and those of the deuterated isotopologues of CH3C4H for which we could expect a similar degree of deuteration enhancement. However, we 13 have not detected either CH2DC4H nor CH3C4D nor any C isotopologue. The different observed deuterium ratios in TMC-1 are reasonably accounted for by a gas phase chemical model where the low temperature conditions favor deuteron transfer through + reactions with H2D . Key words. Astrochemistry — ISM: molecules — ISM: individual (TMC-1) — line: identification — molecular data 1. Introduction species. Hence, discovering spectral features of new molecules requires a previous detailed analysis of the spectral contribution Deuterium fractionation is a well-known process in the dense in- of known species. terstellar medium which can occur both in the gas phase and on the surfaces of dust particles. This process allows deuterated iso- Methylcyanoacetylene, CH3C3N, also known as topic species of interstellar molecules to reach abundances much cyanopropyne or methylpropionitrile, has been detected higher than the D/H elemental abundance ratio (1:5 × 10−5 Lin- with high abundance in the cold dark cloud TMC-1 (Broten sky 2003). The high efficiency of deuterium fractionation allows 1984) and more recently by Marcelino et al. (2021) using a high deuterated species to achieve abundances as high as 30-40 % sensitivity line survey on TMC-1 gathered with the Yebes 40m relative to the parent species, as occurs with HDCS (Marcelino radio telescope (see, e.g., Cernicharo et al. 2021d). Hence, the deuterated isotopologues of CH C N are good candidates to et al. 2005) and CH2DOH (Parise et al. 2006). Hence, deuter- 3 3 ated isotopologues of abundant interstellar molecules make a be observed in this source using the same line survey. In fact, significant contribution to the spectral richness of line surveys. we have already detected other singly deuterated isotopologues This fact makes the astronomical identification of these isotopo- of species such as CH3CN, CH3CCH, c-C3H2,C4H, H2C4, logues of utmost importance, not only to gain knowledge on their H2CCN, HC3N and HC5N (Cabezas et al. 2021). molecular formation pathways or how deuterium fractionation In this Letter we report the identification of spectral lines of works, but also to assign unidentified features in line surveys. the deuterated species CH2DC3N in TMC-1. Our search for this Very sensitive broadband line surveys of astronomical molecule is based on the change in the rotational parameters of sources can now be achieved thanks to new technical develop- CH3C3N produced by the H/D exchange, which have been ob- ments on radiotelescopes. These surveys have boosted up the arXiv:2106.03404v1 [astro-ph.GA] 7 Jun 2021 tained by ab initio calculations. The derived deuterium ratios are number of new molecular identifications in the last years, be- confronted to an extended chemical model including the related cause weak lines arising from low-abundance species and from deuterated compounds. low-dipole moment species can be now easily detected (Agún- dez et al. 2021a; Cernicharo et al. 2021a,b,c). The negative coun- terpart of this high sensitivity is the huge number of new lines which populate the survey, including isotopologues and vibra- 2. Observations tionally excited states, in warm environments, of well known h m s ? The Q-band observations of TMC-1 (αJ2000 = 4 41 41:9 and Based on observations carried out with the Yebes 40m telescope ◦ 0 00 (projects 19A003, 20A014, and 20D15). The 40m radiotelescope at δJ2000 = +25 41 27:0 ) described in this work were performed Yebes Observatory is operated by the Spanish Geographic Institute in several sessions between November 2019 and April 2021. (IGN, Ministerio de Transportes, Movilidad y Agenda Urbana). They were carried out using a set of new receivers, built within Article number, page 1 of 5 A&A proofs: manuscript no. AA202141371_Cabezas the Nanocosmos project1, and installed at the Yebes 40m radio Table 1. Observed line parameters for CH2DC3N in TMC-1. telescope. a R ∗ b c ∗ The Q-band receiver consists of two high electron mobil- (JKa;Kc )u-(JKa;Kc )l νobs TAdv ∆v TA −1 −1 ity transitor cold amplifiers covering the 31.0-50.4 GHz band in (MHz) (mK km s ) (km s ) (mK) − ± ± ± horizontal and vertical polarizations. The receiver temperature 81;8 71;7 31797.891 2.08 0.12 0.84 0.15 2.3 0.4 80;8 − 70;7 31830.626 2.76±0.07 0.79±0.11 3.3±0.4 varies from 22 K at 32 GHz to 42 K at 50 GHz. The spectrome- 81;7 − 71;6 31862.950 1.39±0.20 0.70±0.12 1.9±0.4 ters formed by 2×8×2:5 GHz FFTs provide a spectral resolution 91;9 − 81;8 35772.575 1.67±0.14 0.64±0.11 2.5±0.4 of 38.15 kHz and cover the whole Q-band in both polarizations. 90;9 − 80;8 35809.389 3.11±0.04 0.69±0.06 4.3±0.3 The receivers and the spectrometers are described before by Ter- 91;8 − 81;7 35845.744 1.31±0.16 0.74±0.14 3.1±0.3 101;10 − 91;9 39747.236 0.73±0.30 0.45±0.15 1.6±0.4 cero et al. (2021). 100;10 − 90;9 39788.132 2.66±0.06 0.67±0.07 3.8±0.4 Different frequency coverages were observed, 31.08-49.52 101;9 − 91;8 39828.533 1.07±0.34 0.62±0.16 1.6±0.4 GHz and 31.98-50.42 GHz, which permits to check that no spu- 111;11 − 101;10 43721.867 1.45±0.33 0.73±0.32 1.9±0.4 rious ghosts are produced in the down-conversion chain in which 110;11 − 100;10 43766.845 2.11±0.10 0.47±0.09 4.2±0.4 11 − 10 43811.297 1.77±0.24 0.98±0.35 1.7±0.4 the signal coming from the receiver is downconverted to 1-19.5 1;10 1;9 121;12 − 111;11 47696.518 0.86±0.40 0.82±0.20 2.1±0.5 GHz, and then splits into 8 bands with a coverage of 2.5 GHz, 120;12 − 110;11 47745.519 1.91±0.10 0.46±0.06 3.9±0.5 each of which being analyzed by the FFTs. 121;11 − 111;10 47794.026 0.83±0.46 0.45±0.11 1.7±0.5 The observing procedure used was the frequency switching Notes. (a) Observed frequencies towards TMC-1 for which we adopted a mode, with a frequency throw of 10 MHz or 8 MHz (see, e.g., −1 vLSR of 5.83 km s (Cernicharo et al. 2020a,b,c). The frequency uncer- Cernicharo et al. 2021d,e,f). (b) −1 (c) ∗ tainty 10 kHz. Integrated line intensity in mK km s . Line width The intensity scale, antenna temperature (TA), was calibrated at half intensity derived by fitting a Gaussian function to the observed using two absorbers at different temperatures and the atmo- line profile (in km s−1). spheric transmission model ATM (Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties have been adopted to be 10 % Table 2. Observationally derived and theoretical spectroscopic parame- based on the observed repeatability of the line intensities be- ters (in MHz) for CH2DC3N. tween different observing runs. All data have been analyzed us- 2 ing the GILDAS package . Constant Spacea Ab initiob A 120120.70c 120120.70 B 1993.493107(593) 1993.53 3. Results C 1985.363112(657) 1985.74 −5 ∆J 9.107(230) 10 - −2 The identification of most of the features from our TMC-1 Q- ∆JK 1.4767(249)10 - band line survey was done using the MADEX code (Cernicharo rmsd 13.6 - 2012) and the CDMS and JPL catalogues (Müller et al. 2005; Jmin=Jmax 8/12 - Kmin=Kmax 0/1 - Pickett et al. 1998). Nevertheless many lines remain unidentified. Ne 15 - Among these U-lines we found a series of five lines with a har- (a) (b) monic relation 8:9:10:11:12 between them. This series of lines Notes. Fit to the lines of CH2DC3N observed in TMC-1. CCSD/cc- could be fitted using a Hamiltonian for a linear molecule obtain- pVTZ level of theory.
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