Mercapto radical (SH) in translucent interstellar clouds^⋆

¹ Sackler Laboratory for Astrophysics, Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: zhao@strw.leidenuniv.nl
² Instituto de Astronomia, Universidad Catolica del Norte, Av. Angamos 0610, Antofagasta, Chile
³ Pulkovo Observatory, Pulkovskoe Shosse 65, 196140 Saint-Petersburg, Russia
⁴ Center for Astronomy, Nicholas Copernicus University, Gagarina 11, 87-100 Toruń, Poland

Received: 7 May 2015
Accepted: 24 May 2015

Abstract

We present the first detection of rotationally resolved electronic transitions of the mercapto radical (SH) in the near-ultraviolet in translucent interstellar clouds. Observational data acquired by the Ultraviolet and Visual Echelle Spectrograph of the Very Large Telescope were used to search for the near-ultraviolet absorption spectra of SH. Two weak absorption features at ~3242.40 and 3240.66 Å, assigned as the ^pP₁₁(3/2) and the (^qP₂₁(3/2) + ^qQ₁₁(3/2)) doublet transitions in the A²Σ⁺–X²Π (0, 0) band of SH, respectively, were identified in the averaged spectrum of four selected lines of sight: HD 73882, HD 78344, HD 80077, and HD 154368. These features are weak, but can be identified within a 3σ detection limit, and are found in other published data sets as well. We compiled a set of line positions and oscillator strengths of SH to facilitate the data analysis. We derived an averaged value of the SH column density, ~1.5 ± 0.3 × 10¹³ cm^-2, that is most likely representative for the magnitude of SH column densities in individual SH-rich translucent clouds. This value is also consistent with predictions from models for C-type shocks and turbulent dissipation regions with typical translucent cloud conditions.