Sulphur storage in cold molecular clouds: the case of the NH4+SH- salt on interstellar dust grains

Abstract

ABSTRACT In comets and in the cold phase of the interstellar medium (ISM), ammonium salts are key molecular species due to their role in the retention of volatile compounds on cold surfaces. In the case of sulphur, the H$_2$S/OCS ratio observed in protostars could be explained by the presence of ammonium hydrosulphide (NH$_4$SH) salts. However, laboratory data on the properties of NH4SH in ISM cold relevant conditions are rather scarce, as they usually focus on the atmosphere of Jupiter. We propose to consolidate the laboratory data regarding NH$_4$SH on grains, by performing temperature programmed desorption experiments and Fourier transform infrared reflection spectroscopy. The salt was also exposed to H atoms to mimic the ISM conditions. NH$_4$SH was found to form in situ at 10 K, from a mixture of ammonia (NH$_3$) and hydrogen sulphide (H$_2$S). The NH$_4^+$ infrared feature (1485 cm$^{-1}$) is the most prominent one at 80 K. As pure species, H$_2$S and NH$_3$ desorb at 76 and 90 K, respectively, whereas they are released into the gas phase at 153 K when adsorbed in the form of salt. The presence of water delays the desorption of the salt until the very end of the water desorption, but does not affect the desorption kinetics. During H-exposure, the salt is dissociated and no new product was detected. As a comparative study, salts have been included in the Nautilus gas–grain model. The results show a good correlation with the observations of IRAS 16293−2422B, as opposed to when NH$_4$SH is not included in the model.