Sulphur-bearing species in molecular clouds

Abstract

We study several molecules that could help in the solution of the missing sulphur problem in dense clouds and circumstellar regions, as well as in the clarification of the sulphur chemistry in comets. These sulphur molecules are the trimer (CH₂S)₃ and the tetramer (CH₂S)₄ of thioformaldehyde, pentathian S₅CH₂, hexathiepan S₆CH₂, thiirane C₂H₄S, trisulfane HSSSH, and thioacetone (CH₃)₂CS. Infrared spectra of these species are calculated using the density functional theory methods. The majority of calculated bands belong to the mid-infrared, with some of them occurring in the near- and far-infrared region. We suggest that some of unidentified spectral features measured by Infrared Space Observatory in several active galactic nuclei and starburst galaxies could be caused by 1,3,5-trithiane ((CH₂S)₃), 1,3,5,7-tetrathiocane ((CH₂S)₄), and thiirane (C₂H₄S). The objects whose unidentified infrared features we compare with calculated bands are NGC 253, M82, NGC 1068, Circinus, Arp 220, 30 Doradus, Orion KL, and Sgr B2.