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 H 2 CS 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 H 2 CS, HDCS and D 2 CS 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 H 2 CS in L1544 were compared with the deuteration maps of methanol, H 2 CO, N 2 H + , and HCO + towards the same source. Furthermore, we compared the single and double deuteration of H 2 CS towards the dust peak of L1544 with H 2 CO and c -C 3 H 2 . 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 H 2 CS is N(HDCS)/N(H 2 CS) ~ 30% and is located towards the north-east at a distance of about 10000 AU from the dust peak. While for c -C 3 H 2 the first and second deuteration have a similar efficiency, for H 2 CS and H 2 CO the second deuteration is more efficient, leading to D 2 CX/HDCX ~ 100% (with X = O or S). Conclusions. Our results imply that the large deuteration of H 2 CO and H 2 CS 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 H 2 CS and H 2 CO it is not complete yet.