Millimeter- and submillimeter-wave spectroscopy of thioformamide and interstellar search toward Sgr B2(N)

R A Motiyenko,R T Garrod,K M Menten,H S P Müller,J.-C Guillemin,A Belloche,L Margulès

doi:10.1051/0004-6361/202038723

Abstract

Context. Thioformamide NH2CHS is a sulfur-bearing analog of formamide NH2CHO. The latter was detected in the interstellar medium back in the 1970s. Most of the sulfur-containing molecules detected in the interstellar medium are analogs of corresponding oxygen-containing compounds. Therefore, thioformamide is an interesting candidate for a search in the interstellar medium. Aims. A previous study of the rotational spectrum of thioformamide was limited to frequencies below 70 GHz and to transitions with J ≤ 3. The aim of this study is to provide accurate spectroscopic parameters and rotational transition frequencies for thioformamide to enable astronomical searches for this molecule using radio telescope arrays at millimeter wavelengths. Methods. The rotational spectrum of thioformamide was measured and analyzed in the frequency range 150−660 GHz using the Lille spectrometer. We searched for thioformamide toward the high-mass star-forming region Sagittarius (Sgr) B2(N) using the ReMoCA spectral line survey carried out with the Atacama Large Millimeter/submillimeter Array. Results. Accurate rigid rotor and centrifugal distortion constants were obtained from the analysis of the ground state of parent, 34S, 13C, and 15N singly substituted isotopic species of thioformamide. In addition, for the parent isotopolog, the lowest two excited vibrational states, v12 = 1 and v9 = 1, were analyzed using a model that takes Coriolis coupling into account. Thioformamide was not detected toward the hot cores Sgr B2(N1S) and Sgr B2(N2). The sensitive upper limits indicate that thioformamide is nearly three orders of magnitude at least less abundant than formamide. This is markedly different from methanethiol, which is only about two orders of magnitude less abundant than methanol in both sources. Conclusions. The different behavior shown by methanethiol versus thioformamide may be caused by the preferential formation of the latter (on grains) at late times and low temperatures, when CS abundances are depressed. This reduces the thioformamide-to-formamide ratio, because the HCS radical is not as readily available under these conditions.

Highlights

Several small sulfur compounds have recently been observed in the interstellar medium (ISM): HS2 (Fuente et al 2017), NS+ (Cernicharo et al 2018), HCS, and HSC (Agúndez et al 2018)
We have presented a comprehensive study of the rotational spectrum of thioformamide that includes characterization of the
On the basis of the results of this study, accurate frequency predictions of the ground-state rotational spectra of thioformamide were obtained for the transitions involving levels with J ≤ 90 and Ka ≤ 35 and in the frequency range at least up to 1 THz

Summary

Introduction

Several small sulfur compounds have recently been observed in the interstellar medium (ISM): HS2 (Fuente et al 2017), NS+ (Cernicharo et al 2018), HCS, and HSC (Agúndez et al 2018). While the first four elements are represented in molecules ranging from 2 to 13 atoms with the exception of fullerenes (McGuire 2018), sulfur is only present in small molecules with 2−4 or 6 and possibly 9 atoms with the tentatively detected ethanethiol (Kolesniková et al 2014). For all of the detected sulfur compounds, the corresponding oxygen derivatives are observed in. The current question is to know if the larger sulfur derivatives (Herbst & van Dishoeck 2009) are difficult to form in the ISM, or have a too short lifetime in interstellar clouds to have sufficiently high abundances allowing their detection.

Objectives

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Discussion

Conclusion