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