Constraints on the Gas-phase C/O Ratio of DR Tau's Outer Disk from CS, SO, and C₂H 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 ∼0.″4 (80 au). Estimates for the disk-averaged CS/SO ratio range from ∼0.4 to 0.5, the lowest value reported thus far for a T Tauri disk. At a projected separation of ∼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.