Constraints on the gas-phase C/O ratio of DR Tau's outer disk from CS, SO, and C${}_2$H observations

Abstract

Millimeter wavelength observations of Class II protoplanetary disks often display strong emission from hydrocarbons and high CS/SO values, providing evidence that the gas-phase C/O ratio commonly exceeds 1 in their outer regions. We present new NOEMA observations of CS $5-4$, SO $7_6-6_5$ and $5_6-4_5$, C${}_2$H $N=3-2$, HCN $3-2$, HCO${}^{+}$ $3-2$, and H${}^{13}$CO${}^{+}$ $3-2$ in the DR Tau protoplanetary disk at a resolution of $\sim 0.4^{\prime \prime }$ (80 au). Estimates for the disk-averaged CS/SO ratio range from $\sim 0.4-0.5$, the lowest value reported thus far for a T Tauri disk. At a projected separation of $\sim 180$ au northeast of the star, the SO moment maps exhibit a clump that has no counterpart in the other lines, and the CS/SO value decreases to $<0.2$ at its location. Thermochemical models calculated with DALI indicate that DR Tau's low CS/SO ratio and faint C${}_2$H emission can be explained by a gas-phase C/O ratio that is $<1$ at the disk radii traced by NOEMA. Comparisons of DR Tau's SO emission to maps of extended structures traced by ${}^{13}$CO suggest that late infall may contribute to driving down the gas-phase C/O ratio of its disk.

Published in ApJ

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