First Detection of Interstellar S 2 H

Abstract

We present the first detection of gas-phase S 2 H 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 S 2 H abundance is ∼5 × 10 −11 , only a factor of 4–6 lower than that of the widespread H 2 S molecule. H 2 S and S 2 H are efficiently formed on the UV-irradiated icy grain mantles. We performed ice irradiation experiments to determine the H 2 S and S 2 H photodesorption yields. The obtained values are ∼1.2 × 10 −3 and <1 × 10 −5 molecules per incident photon for H 2 S and S 2 H, respectively. Our upper limit to the S 2 H photodesorption yield suggests that photodesorption is not a competitive mechanism to release the S 2 H molecules to the gas phase. Other desorption mechanisms such as chemical desorption, cosmic-ray desorption, and grain shattering can increase the gaseous S 2 H abundance to some extent. Alternatively, S 2 H can be formed via gas-phase reactions involving gaseous H 2 S and the abundant ions S + and SH + . The detection of S 2 H in this nebula therefore could be the result of the coexistence of an active grain-surface chemistry and gaseous photochemistry.