Constraints on the Gas-phase C/O Ratio of DR Tau's Outer Disk from CS, SO, and C2H Observations

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), C2H N = 3-2, HCN 3-2, HCO+ 3-2, and (HCO+)-C-13 3-2 in the DR Tau protoplanetary disk at a resolution of similar to 04 (80 au). Estimates for the disk-averaged CS/SO ratio range from similar to 0.4 to 0.5, the lowest value reported thus far for a T Tauri disk. At a projected separation of similar to 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 C2H 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 (CO)-C-13 suggest that late infall may contribute to driving down the gas-phase C/O ratio of its disk.