First Detection of Interstellar SH.

We present the first detection of gas phase SH 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 SH abundance is ~5×10, only a factor 4-6 lower than that of the widespread HS molecule. HS and SH are efficiently formed on the UV-irradiated icy grain mantles. We performed ice irradiation experiments to determine the HS and SH photodesorption yields. The obtained values are ~1.2×10 and <1×10 molecules per incident photon for HS and SH, respectively. Our upper limit to the SH photodesorption yield suggests that photo-desorption is not a competitive mechanism to release the SH molecules to the gas phase. Other desorption mechanisms such as chemical desorption, cosmic-ray desorption and grain shattering can increase the gaseous SH abundance to some extent. Alternatively, SH can be formed via gas phase reactions involving gaseous HS and the abundant ions S and SH. The detection of SH in this nebula could be therefore the result of the coexistence of an active grain surface chemistry and gaseous photo-chemistry.