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In the context of the IRAM 30m Large Program GEMS, we present a study ofthioformaldehyde in several starless cores located in star-forming filaments ofTaurus, Perseus, and Orion. We investigate the influence of the environmentalconditions on the abundances of these molecules in the cores, and the effect oftime evolution. We have modelled the observed lines of H2CS, HDCS, and D2CSusing the radiative transfer code RADEX. We have also used the chemical codeNautilus to model the evolution of these species depending on thecharacteristics of the starless cores. We derive column densities andabundances for all the cores. We also derive deuterium fractionation ratios,Dfrac, to determine and compare the evolutionary stage between different partsof each star-forming region. Our results indicate that the north region of theB213 filament in Taurus is more evolved than the south, while the north-easternpart of Perseus presents an earlier evolutionary stage than the south-westernzone. Model results also show that Dfrac decreases with the cosmic-rayionisation rate, while it increases with density and with the degree of sulphurdepletion. In particular, we only reproduce the observations when the initialsulphur abundance in the starless cores is at least one order of magnitudelower than the solar elemental sulphur abundance. The progressive increase inHDCS/H2CS and D2CS/H2CS with time makes these ratios powerful tools forderiving the chemical evolutionary stage of starless cores. However, theycannot be used to derive the temperature of these regions, since both ratiospresent a similar evolution at two different temperature ranges (7-11 K and15-19 K). Regarding chemistry, (deuterated) thioformaldehyde is mainly formedthrough gas-phase reactions (double-replacement and neutral-neutraldisplacement reactions), while surface chemistry plays an important role as adestruction mechanism.

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