Mercapto radical (SH) in translucent interstellar clouds

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 A, assigned as the p P 11 (3/2) and the (q P 21 (3/2) + q Q 11 (3/2)) doublet transitions in the A2 Σ+ –X2 Π (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.