H 2 CS deuteration maps towards the pre-stellar core L1544

Context. Deuteration is a crucial tool for understanding the complexity of interstellar chemical processes, especially when they involve the interplay of gas-phase and grain-surface chemistry. In the case of multiple deuteration, comparing observation with the results of chemical modelling is particularly effective to study how molecules are inherited in the different stages within the process of star and planet formation.Aims. We aim to study the D/H ratio in H2CS across the prototypical pre-stellar core L1544. This study allows us to test current gas-dust chemical models involving sulfur in dense cores.Methods. Here, we present single-dish observations of H2CS, HDCS and D2CS with the IRAM 30 m telescope. We analysed their column densities and distributions and compared these observations with gas-grain chemical models. The deuteration maps of H2CS in L1544 were compared with the deuteration maps of methanol, H2CO, N2H+, and HCO+ towards the same source. Furthermore, we compared the single and double deuteration of H2CS towards the dust peak of L1544 with H2CO and c-C3H2. The difference between the deuteration of these molecules in L1544 is discussed and compared with the prediction of chemical models.Results. The maximum deuterium fractionation for the first deuteration of H2CS is N(HDCS)/N(H2CS) similar to 30% and is located towards the north-east at a distance of about 10 000 AU from the dust peak. While for c-C3H2 the first and second deuteration have a similar efficiency, for H2CS and H2CO the second deuteration is more efficient, leading to D2CX/HDCX similar to 100% (with X = O or S).Conclusions. Our results imply that the large deuteration of H2CO and H2CS observed in protostellar cores as well as in comets is likely inherited from the pre-stellar phase. However, comparison with state-of-the-art chemical models suggests that the reaction network for the formation of the doubly deuterated H2CS and H2CO it is not complete yet.