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

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 $\sim 0."3$ 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 $\sim 0."5$, 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)$\sim 2/3$, 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.

arXiv commentAccepted for publication in The Astrophysical Journal Supplement (27 pages, 13 figures, 5 tables)

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