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

Abstract

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