# Deuterated methyl mercaptan (CH$_3$SD): Laboratory rotational spectroscopy and search toward IRAS 16293-2422 B

## TL;DR

This study provides precise spectroscopic data for deuterated methyl mercaptan to aid astronomical detection efforts, but the molecule was not observed in the targeted protostar, setting an upper limit on its abundance.

## Contribution

The paper reports new laboratory rotational spectra and spectroscopic parameters for CH3SD, enhancing tools for astronomical searches of deuterated sulfur-containing molecules.

## Key findings

- Laboratory spectra of CH3SD measured in 150-510 GHz range.
- Spectroscopic parameters modeled with RAM36 program.
- CH3SD not detected in ALMA data, upper limit on deuteration ~18%.

## Abstract

Methyl mercaptan (also known as methanethiol), CH3SH, has been found in the warm and dense parts of high -- as well as low -- mass star-forming regions. The aim of the present study is to obtain accurate spectroscopic parameters of the S-deuterated methyl mercaptan CH$_3$SD to facilitate astronomical observations by radio telescope arrays at (sub)millimeter wavelengths. We have measured the rotational spectrum associated with the large-amplitude internal rotation of the methyl group of methyl mercaptan using an isotopically enriched sample in the 150-510 GHz frequency range using the K\"oln millimeter wave spectrometer. The analysis of the spectra has been performed up to the second excited torsional state. We present modeling results of these data with the RAM36 program. CH$_3$SD was searched for, but not detected, in data from the Atacama Large Millimeter/submillimeter Array (ALMA) Protostellar Interferometric Line Survey (PILS) of the deeply embedded protostar IRAS 16293-2422. The derived upper limit corresponds to a degree of deuteration of at most ~18%.