Sulphur chemistry in the L1544 pre-stellar core

Abstract

The L1544 pre-stellar core has been observed as part of the ASAI IRAM 30m\nLarge Program as well as follow-up programs. These observations have revealed\nthe chemical richness of the earliest phases of low-mass star-forming regions.\nIn this paper we focus on the twenty-one sulphur bearing species (ions,\nisotopomers and deuteration) that have been detected in this spectral-survey\nthrough fifty one transitions: CS, CCS, C3S, SO, SO2, H2CS, OCS, HSCN, NS,\nHCS+, NS+ and H2S. We also report the tentative detection (4 {\\sigma} level)\nfor methyl mercaptan (CH3SH). LTE and non-LTE radiative transfer modelling have\nbeen performed and we used the nautilus chemical code updated with the most\nrecent chemical network for sulphur to explain our observations. From the\nchemical modelling we expect a strong radial variation for the abundances of\nthese species, which mostly are emitted in the external layer where non thermal\ndesorption of other species has previously been observed. We show that the\nchemical study cannot be compared to what has been done for the TMC-1 dark\ncloud, where the abundance is supposed constant along the line of sight, and\nconclude that a strong sulphur depletion is necessary to fully reproduce our\nobservations of the prototypical pre-stellar core L1544.\n