2017ApJ...851L..49F

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 ∼5 x 10^–11, only a factor of 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 ∼1.2 x 10^–3 and <1 x 10^–5 molecules per incident photon for H₂S and S₂H, respectively. Our upper limit to the S₂H photodesorption yield suggests that photodesorption 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 therefore could be the result of the coexistence of an active grain-surface chemistry and gaseous photochemistry.