First detection of interstellar S2H

TL;DR

This paper reports the first detection of interstellar S2H in the Horsehead nebula, highlighting its formation on icy grain mantles and exploring its desorption mechanisms, which advances understanding of sulfur chemistry in space.

Contribution

It provides the first observational evidence of S2H in space and investigates its formation and desorption processes, combining astronomical detection with laboratory experiments.

Findings

S2H abundance is ~5x10$^{-11}$, close to H2S levels.

Photodesorption yields for H2S and S2H are measured.

Photo-desorption is likely not the main release mechanism for S2H.

Abstract

We present the first detection of gas phase S2H in the Horsehead, a moderately UV-irradiated nebula. This confirms the presence of doubly sulfuretted species in the interstellar medium and opens a new challenge for sulfur chemistry. The observed S2H abundance is ~5x10$^{-11}$, only a factor 4-6 lower than that of the widespread H2S molecule. H2S and S2H are efficiently formed on the UV-irradiated icy grain mantles. We performed ice irradiation experiments to determine the H2S and S2H photodesorption yields. The obtained values are ~1.2x10$^{-3}$ and <1x10$^{-5}$ molecules per incident photon for H2S and S2H, respectively. Our upper limit to the S2H photodesorption yield suggests that photo-desorption is not a competitive mechanism to release the S2H molecules to the gas phase. Other desorption mechanisms such as chemical desorption, cosmic-ray desorption and grain shattering can increase the gaseous S2H abundance to some extent. Alternatively, S2H can be formed via gas phase reactions involving gaseous H2S and the abundant ions S+ and SH+. The detection of S2H in this nebula could be therefore the result of the coexistence of an active grain surface chemistry and gaseous photo-chemistry.