Sulfur chemistry in protoplanetary disks: CS and H2CS

Abstract

The nature and abundance of sulfur chemistry in protoplanetary disks (PPDs) may impact the sulfur inventory on young planets and therefore their habitability. PPDs also present an interesting test bed for sulfur chemistry models, since each disk present a diverse set of environments. In this context, we present new sulfur molecule observations in PPDs, and new S-disk chemistry models. With ALMA we observed the CS 5-4 rotational transition toward five PPDs (DM Tau, DO Tau, CI Tau, LkCa 15, MWC 480), and the CS 6-5 transition toward three PPDs (LkCa 15, MWC 480 and V4046 Sgr). Across this sample, CS displays a range of radial distributions, from centrally peaked, to gaps and rings. We also present the first detection in PPDs of $^{13}$CS 6-5 (LkCa 15 and MWC 480), C$^{34}$S 6-5 (LkCa 15), and H$_2$CS $8_{17}-7_{16}$, $9_{19}-8_{18}$ and $9_{18}-8_{17}$ (MWC 480) transitions. Using LTE models to constrain column densities and excitation temperatures, we find that either $^{13}$C and $^{34}$S are enhanced in CS, or CS is optically thick despite its relatively low brightness temperature. Additional lines and higher spatial resolution observations are needed to distinguish between these scenarios. Assuming CS is optically thin, CS column density model predictions reproduce the observations within a factor of a few for both MWC 480 and LkCa 15. However, the model underpredicts H$_2$CS by 1-2 orders of magnitude. Finally, comparing the H$_2$CS/CS ratio observed toward the MWC~480 disk and toward different ISM sources, we find the closest match with prestellar cores.