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 7₆-6₅ and 5₆-4₅, C₂H $N=3-2$, HCN $3-2$, HCO⁺ $3-2$, and H¹³CO⁺ $3-2$ in the DR Tau protoplanetary disk at a resolution of ~0.4'' (80 au). Estimates for the disk-averaged CS/SO ratio range from ~0.4-0.5, 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 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 ¹³CO suggest that late infall may contribute to driving down the gas-phase C/O ratio of its disk.