First Detection of Interstellar S2H

• Published in: Astrophysical journal. Letters Vol. 851; no. 2
• Main Authors: Fuente, Asunción, Goicoechea, Javier R., Pety, Jérôme, Le Gal, Romane, Martín-Doménech, Rafael, Gratier, Pierre, Guzmán, Viviana, Roueff, Evelyne, Loison, Jean Christophe, Muñoz Caro, Guillermo M., Wakelam, Valentine, Gerin, Maryvonne, Riviere-Marichalar, Pablo, Vidal, Thomas
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 20.12.2017; IOP Publishing; Bristol : IOP Publishing
• Subjects: Abundance; astrochemistry; Astrophysics; Atmospheric chemistry; Cosmic rays; Desorption; Galactic Astrophysics; Hydrogen sulfide; Interstellar chemistry; Interstellar gas; Interstellar matter; Interstellar medium; Irradiation; ISM: abundances; ISM: individual objects (Horsehead); ISM: molecules; methods: laboratory: solid state; Nebulae; Organic chemistry; Photochemistry; photon-dominated region (PDR); Sciences of the Universe; Sulfur; Surface chemistry; Vapor phases; Horsehead; photondominated region (PDR); Astrochemistry; ISM: molecules; methods: laboratory: solid state; ISM: abundances
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/aaa01b

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 × 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 × 10−3 and <1 × 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.