Astronomy & Astrophysics manuscript no. aa41297 ©ESO 2021 May 28, 2021 Letter to the Editor The sulphur saga in TMC-1: Discovery of HCSCN and HCSCCH ? J. Cernicharo1, C. Cabezas1, Y. Endo2, M. Agúndez1, B. Tercero3; 4, J. R. Pardo1, N. Marcelino1, P. de Vicente3 1 Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain e-mail: [email protected],[email protected] 2 Department of Applied Chemistry, Science Building II, National Yang Ming Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu 30010, Taiwan 3 Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain 4 Observatorio Astronómico Nacional (IGN), C/Alfonso XII, 3, 28014, Madrid, Spain Received; accepted ABSTRACT We report the detection, for the first time in space, of cyano thioformaldehyde (HCSCN) and propynethial (HCSCCH) towards the starless core TMC-1. Cyano thioformaldehyde presents a series of prominent a- and b-type lines, which are the strongest previously unassigned features in our Q-band line survey of TMC-1. Remarkably, HCSCN is four times more abundant than cyano formaldehyde (HCOCN). On the other hand, HCSCCH is five times less abundant than propynal (HCOCCH). Surprisingly, we find an abundance ratio HCSCCH/HCSCN of ∼ 0.25, in contrast with most other ethynyl-cyanide pairs of molecules for which the CCH-bearing species is more abundant than the CN-bearing one. We discuss the formation of these molecules in terms of neutral-neutral reactions of S atoms with CH2CCH and CH2CN radicals as well as of CCH and CN radicals with H2CS. The calculated abundances for the sulphur- bearing species are, however, significantly below the observed values, which points to an underestimation of the abundance of atomic sulphur in the model or to missing formation reactions, such as ion-neutral reactions. Key words. molecular data – line: identification – ISM: molecules – ISM: individual (TMC-1) – astrochemistry 1. Introduction The chemistry of sulphur-bearing molecules in cold interstel- lar clouds is an active area of research (Vidal et al. 2017; Cer- nicharo et al. 2021a,b). Recently, Cernicharo et al. (2021b) re- ported the detection of five new sulphur-bearing species in TMC- HCSCN HCSCCH 1, a source that presents an interesting carbon-rich chemistry that leads to the formation of long neutral carbon chain radicals and their anions, cyanopolyynes (see Cernicharo et al. 2020a,b Fig. 1. Structure of cyano thioformaldehyde, HCSCN, and ethynyl thio- formaldehyde, HCSCCH, also known as propynethial. and references therein), pure hydrocarbon cycles, and polycyclic aromatic hydrocarbons (Cernicharo et al. 2021c; Burkhardt et al., 2021; McCarthy et al. 2021). A large variety of S-bearing could lead to the formation of these species with the aid of a molecules are also present in this source, which leads to new chemical model. questions regarding the formation routes to these species. It is clear that many ion-neutral and neutral-neutral reactions involv- ing S-bearing species have to be studied to improve the reliabil- 2. Observations ity of chemical models (Petrie 1996; Bulut et al. 2021). Un- 1 veiling new sulphur-bearing species can also help to understand New receivers, built within the Nanocosmos project and in- the chemistry of sulphur by comparing the observed abundances stalled at the Yebes 40m radio telescope, were used for the ob- with those predicted by chemical models. servations of TMC-1. A detailed description of the system is arXiv:2105.12996v1 [astro-ph.GA] 27 May 2021 given by Tercero et al. (2021). The Q-band receiver consists of In this letter we report the discovery of two new sulphur- two high electron mobility transistor cold amplifiers covering bearing molecules: HCSCN (cyano thioformaldehyde) and HC- the 31.0-50.3 GHz band with horizontal and vertical polarisa- SCCH (propynethial). Figure 1 shows the structure of these tions. The backends are 2 × 8 × 2:5 GHz fast Fourier transform molecules. The derived abundances are compared with the oxy- (FT) spectrometers with a spectral resolution of 38.15 kHz that gen analogues of these species, HCOCN (cyano formaldehyde) provide the whole coverage of the Q-band in both polarisations. and HCOCCH (propynal). We discuss plausible reactions that The main beam efficiency varies from 0.6 at 32 GHz to 0.43 at 50 GHz. h m s ? Based on observations carried out with the Yebes 40m telescope The line survey of TMC-1 (αJ2000 = 4 41 41:9 and δJ2000 = (projects 19A003, 20A014, 20D023, and 21A011). The 40m radio tele- +25◦41027:000) in the Q-band was performed in several ses- scope at Yebes Observatory is operated by the Spanish Geographic In- stitute, (IGN, Ministerio de Transportes, Movilidad y Agenda Urbana). 1 https://nanocosmos.iff.csic.es/ Article number, page 1 of 9 A&A proofs: manuscript no. aa41297 Fig. 2. Selected a-type transitions of HCSCN. The abscissa corresponds to the rest frequency of the lines assuming a local standard of rest velocity of 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 millikelvin. The quantum numbers for each transition are indicated in the upper left corner of 12 −2 the corresponding panel. The red lines show the computed synthetic spectrum for this species for Tr=5 K and a column density of 1.3×10 cm . Blanked channels correspond to negative features produced in the folding of the frequency switching data. sions. The first results (Cernicharo et al. 2020a,b) were based on One surprising result in our data is the presence of a few two observing runs performed in November 2019 and February strong unknown features with intensities around 10 mK. Three 2020. Two different frequency coverages were achieved, 31.08- of them are in an excellent harmonic relation of 6:7:8. A fit to 49.52 GHz and 31.98-50.42 GHz, in order to check that no spuri- the standard Hamiltonian of a linear molecule provides a ro- ous spectral ghosts are produced in the down-conversion chain. tational constant, B, of 3082.8334±0.0029 MHz and a distor- Additional data were taken in October 2020, December 2020, tion constant, D, of 40.890±0.027 kHz. Such a large distortion and January-April 2021. The observing procedure was frequency constant points towards an asymmetric rotor. Hence, these three switching with a frequency throw of 10 MHz for the two first lines could correspond to the transitions Ka=0 of Jup=6, 7, and runs and of 8 MHz for all the others. Receiver temperatures in 8 of a molecule with (B + C)=2=3082.8 MHz. This suggests the runs achieved in 2020 vary from 22 K at 32 GHz to 42 that the molecule could have a total of either four or five N, C, K at 50 GHz. Some power adaptation in the down-conversion and/or O atoms. We explored around these features, looking for chains reduced the receiver temperatures in 2021 to 16 K at 32 the Ka=1 lines. After some iterative work, we found six of these GHz and 25 K at 50 GHz. The sensitivity of the survey varies lines that can be fitted with a reduced number of rotational and along the Q-band between 0.25 and 1 mK, which is a consider- distortion constants for a total of nine transitions. These lines are able improvement over previous line surveys in the 31-50 GHz shown in Fig. 2. The assigned quantum numbers correspond to frequency range (Kaifu et al. 2004). The acronym we have as- a-type transitions. The carrier is definitely an asymmetric rotor. signed to this line survey is QUIJOTE (Q-band Ultrasensitive Hence, we could expect to have b-type transitions if the molecule Inspection Journey to the Obscure Tmc-1 Environment). has a large µb. From the rotational and distortion constants de- The intensity scale used in this work, the antenna tempera- rived from the a-type transitions, we predicted the spectrum for ∗ ture (TA), was calibrated using two absorbers at different tem- b-type transitions and found nine of them, as shown in Fig. 3. peratures and the atmospheric transmission model ATM (Cer- Moreover, these b-type transitions show a series of lines around nicharo 1985; Pardo et al. 2001). Calibration uncertainties of the predicted frequencies, which is reminiscent of the hyperfine 10 % were adopted. All data were analysed using the GILDAS structure of an N-nucleus. Assuming this is the case, we fitted package2. all these lines and obtained the rotational constants provided in Table 1. The derived values for χaa and χbb definitively corre- spond to a CN group. The hyperfine structure of the a-type tran- 3. Assignment of unidentified lines to HCSCN sitions is not resolved with our spectral resolution. Hence, the Our millikelvin-sensitive line survey of TMC-1 has revealed three strongest components appear as a single feature. No c-type hundreds of unknown features whose identification challenges transitions were found at the level of sensitivity of our survey. our state-of-the-art understanding of the chemistry of this object (Cernicharo et al. 2021b,c). Line identification in this work was done using the catalogues MADEX (Cernicharo 2012), CDMS (Müller et al. 2005), and JPL (Pickett et al. 1998). 2 http://www.iram.fr/IRAMFR/GILDAS Article number, page 2 of 9 Cernicharo et al.: The sulphur saga in TMC-1 Fig. 3. Same as Fig. 2 but for the b-type transitions of HCSCN. The red lines show the computed synthetic spectrum for this species for Tr=5 K and a column density of 1.3×1012 cm−2.