2024ApJ...973..135H

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 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 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.