Discovery of interstellar mercapto radicals (SH) with the GREAT instrument on SOFIA

¹ The Johns Hopkins University, 3400 North Charles St. Baltimore, MD 21218, USA
e-mail: neufeld@pha.jhu.edu
² LERMA, CNRS UMR 8112, École Normale Supérieure & Observatoire de Paris, Paris, France
³ Departamento de Astrofísica, Centro de Astrobiología, CSIC-INTA, Torrejón de Ardoz, Madrid, Spain
⁴ Department of Chemistry, University of Virginia, McCormick Road, PO Box 400319, Charlottesville, VA 22904, USA
⁵ Institut d’Astrophysique Spatiale, CNRS UMR 8617, Université Paris-Sud, Orsay, France
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ I. Physikalisches Institut der Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany

Received: 23 January 2012
Accepted: 14 February 2012

Abstract

We report the discovery of interstellar mercapto radicals (SH) along the sight-line to the submillimeter continuum source W49N. We have used the GREAT instrument on SOFIA to observe the 1383 GHz ²Π_3/2 J = 5/2 ← 3/2 lambda doublet in the upper sideband of the L1 receiver. The resulting spectrum reveals SH absorption in material local to W49N, as well as in foreground gas, unassociated with W49N, that is located along the sight-line. For the foreground material at velocities in the range 37–44 km s^-1 with respect to the local standard of rest, we infer a total SH column density ~4.6 × 10¹² cm^-2, corresponding to an abundance of ~7 × 10^-9 relative to H₂, and yielding an SH/H₂S abundance ratio ~0.13. The observed SH/H₂S abundance ratio is much smaller than that predicted by standard models for the production of SH and H₂S in turbulent dissipation regions and shocks, and suggests that the endothermic neutral-neutral reaction SH + H₂ → H₂S + H must be enhanced along with the ion-neutral reactions believed to produce CH⁺ and SH⁺ in diffuse molecular clouds.