H₂CS deuteration maps towards the pre-stellar core L1544^⋆

¹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
e-mail: spezzano@mpe.mpg.de
² University of Bologna, Dipartimento di Chimica “Giacomo Ciamician”, Bologna, Italy
³ Observatorio Astronómico Nacional (OAN), Alfonso XII, 3, 28014 Madrid, Spain

Received: 10 January 2022
Accepted: 6 March 2022

Abstract

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) ~ 30% and is located towards the north-east at a distance of about 10000 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 ~ 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.