Gas phase Elemental abundances in Molecular cloudS (GEMS). II. On the quest for the sulphur reservoir in molecular clouds: the H2S case

Context. Sulphur is one of the most abundant elements in the Universe. Surprisingly, sulphuretted molecules are not as abundant as expected in the interstellar medium and the identity of the main sulphur reservoir is still an open question.
Aims: Our goal is to investigate the H₂S chemistry in dark clouds, as this stable molecule is a potential sulphur reservoir.
Methods: Using millimeter observations of CS, SO, H₂S, and their isotopologues, we determine the physical conditions and H₂S abundances along the cores TMC 1-C, TMC 1-CP, and Barnard 1b. The gas-grain model NAUTILUS is used to model the sulphur chemistry and explore the impact of photo-desorption and chemical desorption on the H₂S abundance.
Results: Our modeling shows that chemical desorption is the main source of gas-phase H₂S in dark cores. The measured H₂S abundance can only be fitted if we assume that the chemical desorption rate decreases by more than a factor of 10 when n_H > 2 × 10⁴. This change in the desorption rate is consistent with the formation of thick H₂O and CO ice mantles on grain surfaces. The observed SO and H₂S abundances are in good agreement with our predictions adopting an undepleted value of the sulphur abundance. However, the CS abundance is overestimated by a factor of 5-10. Along the three cores, atomic S is predicted to be the main sulphur reservoir.
Conclusions: The gaseous H₂S abundance is well reproduced, assuming undepleted sulphur abundance and chemical desorption as the main source of H₂S. The behavior of the observed H₂S abundance suggests a changing desorption efficiency, which would probe the snowline in these cold cores. Our model, however, highly overestimates the observed gas-phase CS abundance. Given the uncertainty in the sulphur chemistry, we can only conclude that our data are consistent with a cosmic elemental S abundance with an uncertainty of a factor of 10.