Sulfur Chemistry in Protoplanetary Disks: CS and H2CS

Romane Le Gal,Jamila Pegues,Ryan A Loomis,Karin I Öberg,Jennifer B Bergner

doi:10.3847/1538-4357/ab1416

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.

Highlights

Planets form in dust and gas-rich disks, protoplanetary disks (PPDs), around young stars
In the diffuse interstellar medium (ISM) and in photon-dominated regions (PDRs) the observed sulfur abundance is close to the cosmic value (e.g. Howk et al 2006; Neufeld et al 2015; Goicoechea et al 2006), whereas in dense molecular gas it is found highly depleted: only 0.1% of the sulfur cosmic abundance is observed in the gas phase (Tieftrunk et al 1994), implying a depletion factor of three orders of magnitude (e.g. Wakelam et al 2004; Vastel et al 2018)
A key parameter of the new modeling study we present here, is the use of an up-to-date sulfur chemical network, based on the KInetic Database for Astrochemistry (KIDA), and including recent updates (Vidal et al 2017; Le Gal et al 2017; Fuente et al 2017), that are relevant for the purpose of this work

Summary

Introduction

Planets form in dust and gas-rich disks, protoplanetary disks (PPDs), around young stars. Howk et al 2006; Neufeld et al 2015; Goicoechea et al 2006), whereas in dense molecular gas it is found highly depleted: only 0.1% of the sulfur cosmic abundance is observed in the gas phase (Tieftrunk et al 1994), implying a depletion factor of three orders of magnitude (e.g. Wakelam et al 2004; Vastel et al 2018) This level of depletion suggests that most sulfur is locked into icy mantles coating interstellar dust grains (Millar & Herbst 1990; Ruffle et al 1999; Vidal et al 2017; Laas & Caselli 2019).

Methods

Results

Discussion

Conclusion