A Computational and Observational Study of Interstellar Thioformaldehyde Masers

Interstellar spectroscopy of thioformaldehyde (H$_{2}$CS) holds substantial promise because of the close relationship between the H$_{2}$CS molecule and the well-studied formaldehyde (H$_{2}$CO) molecule. We present here a summary of our computational investigation of H$_{2}$CS level populations and their relationship to known H$_{2}$CO 6-cm masers, as well as the details of an observational campaign of four H$_{2}$CS isotopologues. The maser pump model developed by Boland and de Jong (1981) for a known 4.8-GHz H$_{2}$CO maser in the Galactic star-forming region NGC 7538 has been extended to the analogous ground state transitions of four thioformaldehyde isotopologues: H$_{2}^{12}$C$^{32}$S, H$_{2}^{13}$C$^{32}$S, H$_{2}^{12}$C$^{34}$S, and H$_{2}^{12}$C$^{33}$S. Preliminary results from this model provide strong evidence for non-thermal maser emission from any of these isotopologues. Of considerable interest is the fact that this J=1 transition of H$_{2}$CS has never been detected astronomically. Higher-J transitions of H$_{2}$CS have been detected in various Galactic sources through thermal absorption but interpretations of these observations are ambiguous. A detection of the J=1 transition of H$_{2}$CS would alleviate many of these ambiguities. We describe our forthcoming experiment to search in NGC 7538 for both thermal and non-thermal emission and absorption from the considered H$_{2}$CS isotopologues. Both parts of this research effort will provide valuable and novel constraints on H$_{2}$CS and H$_{2}$CO.