Molecules with ALMA at Planet-forming Scales (MAPS) XII: Inferring the C/O and S/H ratios in Protoplanetary Disks with Sulfur Molecules

TL;DR

This study uses ALMA observations of sulfur molecules in protoplanetary disks to infer elemental C/O ratios and sulfur chemistry, revealing super-solar C/O and sulfur depletion in disk environments.

Contribution

It provides the first detailed sulfur molecule distributions in multiple disks and demonstrates N(CS)/N(SO) as a new probe for C/O ratios in planet-forming regions.

Findings

CS detected in all disks with diverse spatial profiles

N(H2CS)/N(CS) ratio suggests organic sulfur reservoir

N(CS)/N(SO) ratio indicates super-solar C/O

Abstract

Sulfur-bearing molecules play an important role in prebiotic chemistry and planet habitability. They are also proposed probes of chemical ages, elemental C/O ratio, and grain chemistry processing. Commonly detected in diverse astrophysical objects, including the Solar System, their distribution and chemistry remain, however, largely unknown in planet-forming disks. We present CS ($2-1$) observations at $\sim0."3$ resolution performed within the ALMA-MAPS Large Program toward the five disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. CS is detected in all five disks, displaying a variety of radial intensity profiles and spatial distributions across the sample, including intriguing apparent azimuthal asymmetries. Transitions of C$_2$S and SO were also serendipitously covered but only upper limits are found. For MWC 480, we present complementary ALMA observations at $\sim0."5$, of CS, $^{13}$CS, C$^{34}$S, H$_2$CS, OCS, and SO$_2$. We find a column density ratio N(H$_{2}$CS)/N(CS)$\sim2/3$, suggesting that a substantial part of the sulfur reservoir in disks is in organic form (i.e., C$_x$H$_y$S$_z$). Using astrochemical disk modeling tuned to MWC 480, we demonstrate that $N$(CS)/$N$(SO) is a promising probe for the elemental C/O ratio. The comparison with the observations provides a super-solar C/O. We also find a depleted gas-phase S/H ratio, suggesting either that part of the sulfur reservoir is locked in solid phase or that it remains in an unidentified gas-phase reservoir. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.