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Gas phase Elemental abundances in Molecular cloudS (GEMS) VI. A sulphur journey across star-forming regions: study of thioformaldehyde emission

In the context of the IRAM 30m Large Program GEMS, we present a study of thioformaldehyde in several starless cores located in star-forming filaments of Taurus, Perseus, and Orion. We investigate the influence of the environmental conditions on the abundances of these molecules in the cores, and the effect of time evolution. We have modelled the observed lines of H2CS, HDCS, and D2CS using the radiative transfer code RADEX. We have also used the chemical code Nautilus to model the evolution of these species depending on the characteristics of the starless cores. We derive column densities and abundances for all the cores. We also derive deuterium fractionation ratios, Dfrac, to determine and compare the evolutionary stage between different parts of each star-forming region. Our results indicate that the north region of the B213 filament in Taurus is more evolved than the south, while the north-eastern part of Perseus presents an earlier evolutionary stage than the south-western zone. Model results also show that Dfrac decreases with the cosmic-ray ionisation rate, while it increases with density and with the degree of sulphur depletion. In particular, we only reproduce the observations when the initial sulphur abundance in the starless cores is at least one order of magnitude lower than the solar elemental sulphur abundance. The progressive increase in HDCS/H2CS and D2CS/H2CS with time makes these ratios powerful tools for deriving the chemical evolutionary stage of starless cores. However, they cannot be used to derive the temperature of these regions, since both ratios present a similar evolution at two different temperature ranges (7-11 K and 15-19 K). Regarding chemistry, (deuterated) thioformaldehyde is mainly formed through gas-phase reactions (double-replacement and neutral-neutral displacement reactions), while surface chemistry plays an important role as a destruction mechanism.