Journal
ASTROPHYSICAL JOURNAL LETTERS
Volume 851, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/2041-8213/aaa01b
Keywords
astrochemistry; ISM: abundances; ISM: individual objects (Horsehead); ISM: molecules; methods: laboratory: solid state; photon-dominated region (PDR)
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Funding
- ERC Starting Grant (3DICE) [336474]
- Spanish MINECO [AYA2016-75066-C2-1/2-P, AYA2012-32032]
- Spanish MINECO from ERC [ERC-2013-SyG, G. A. 610256 NANOCOSMOS]
- Programme National Physique et Chimie du Milieu Interstellaire (PCMI) of CNRS/INSU
- INC/INP - CEA
- CNES
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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 similar to 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 similar to 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.
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