H2S in the L1157-B1 bow shock

Abstract

Sulphur-bearing molecules are highly reactive in the gas phase of the interstellar medium. However, the form in which most of the sulphur is locked on to interstellar dust grains is unknown. By taking advantage of the short time-scales of shocks in young molecular outflows, one could track back the main form of sulphur in the ices. In this paper, six transitions of H<SUB>2</SUB>S and its isotopologues in the L1157-B1 bow shock have been detected using data from the Herschel-Chemical HErschel Surveys of Star forming regions survey and the IRAM-30m Astrochemical Surveys At IRAM large programme. These detections are used to calculate the properties of H<SUB>2</SUB>S gas in L1157-B1 through use of a rotation diagram and to explore the possible carriers of sulphur on the grains. The isotopologue detections allow the first calculation of the H<SUB>2</SUB>S deuteration fraction in an outflow from a low-mass protostar. The fractional abundance of H<SUB>2</SUB>S in the region is found to be 6.0 × 10<SUP>-7</SUP> and the deuteration fraction is 2 × 10<SUP>-2</SUP>. In order to investigate the form of sulphur on the grains, a chemical model is run with four different networks, each with different branching ratios for the freeze out of sulphur-bearing species into molecules such as OCS and H<SUB>2</SUB>S. It is found that the model best fits the data when at least half of each sulphur-bearing species hydrogenates when freezing. We therefore conclude that a significant fraction of sulphur in L1157-B1 is likely to be locked in H<SUB>2</SUB>S on the grains.