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The Abundance of Hydrogen Sulfide Ice in Dense Molecular Clouds

Published onJun 01, 2020
The Abundance of Hydrogen Sulfide Ice in Dense Molecular Clouds

Cometary ices may originate directly from the ices in dense molecular clouds or may be the product of physical and chemical re-processing during star formation. A plethora of abundant sulfur-bearing molecular species have been observed in comets, but they appear to be strongly under-abundant in dense molecular clouds. In particular, Hydrogen Sulfide (H2S) is an important sulfur carrier in comets, and is also expected to be so in dense clouds, following astrochemical models. So far H2S ice has not been detected in dense clouds, with an upper limit of <2.0% relative to H2O. We collected observational spectra with the InfraRed Telescope Facility (IRTF) of the target star CK2, located behind the Serpens molecular cloud, in the 3 to 4 μm range. Using published laboratory studies, we compiled a list of the spectral characteristics and band strengths of pure H2S ice, and H2S mixed with H2O and CO. We then analyzed the stellar spectrum and calculated the peak optical, column density and abundance of H2S ice. The spectrum of CK2 shows no absorption due to the stretching mode of H2S ice, so upper limits were determined based on noise values for each mixture. The 3σ upper limit on column density for pure H2S, H2S mixed with H2O, and H2S mixed with CO< are 1.9, 3.5, and 0.8 (×1017 cm-2). This corresponds to <7, <11, and <3% relative to H2O ice, consuming <8, <14, and <3% of the cosmic sulfur budget. We conclude that a significant amount of H2S ice may be hidden in the observed spectrum if it is mixed with H2O.

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