Laboratory rotational spectroscopy leads to the first interstellar detection of singly deuterated methyl mercaptan (CH${}_2$DSH)

Abstract

We report an extensive rotational spectroscopic analysis of singly deuterated methyl mercaptan (CH${}_2$DSH) using both millimeter and far-infrared synchrotron spectra to achieve a global torsional analysis of the three lowest torsional substates (e${}_0$, e${}_1$, and o${}_1$) of this non-rigid species. A fit including 3419 millimeter wave transitions along with 43 infrared torsional subband centers was performed with root mean square deviations of 0.233 MHz and 0.270 cm${}^{-1}$, respectively, resulting in 68 fit parameters. A spectroscopic catalogue built from this analysis for a temperature of 125 K has led to the first interstellar detection of CH${}_2$DSH towards the Solar-like protostar IRAS 16293-2422 B. We report the identification of 46 transitions, including eight relatively unblended lines, resulting in a derived column density of (3.0$\pm$0.3)$\times$10${}^{14}$ cm${}^{-2}$. The column density ratio for HDCS/CH${}_2$DSH compared to HDCO/CH${}_2$DOH suggests a difference in the interstellar chemistry between the sulphur and oxygen complex organics, in particular a different link between H${}_2$CO and CH${}_3$OH and between H${}_2$CS and CH${}_3$SH. This is the first interstellar detection of a deuterated sulphur-bearing COM and therefore an important step into understanding the chemical origin of sulphur-based prebiotics.

Accepted for publication in Astrophysical Journal Letters (ApJL)