2006ApJ...646..342R

We present high-resolution infrared spectra of four YSOs (T Tau N, T Tau S, RNO 91, and HL Tau). The spectra exhibit narrow absorption lines of ¹²CO, ¹³CO, and C¹⁸O, as well as broad emission lines of gas-phase ¹²CO. The narrow absorption lines of CO are shown to originate from the colder circumstellar gas. We find that the line-of-sight gas column densities resulting from the CO absorption lines are much higher than expected for the measured extinction for each source and suggest the gas/dust ratio is measuring the dust settling and/or grain coagulation in these extended disks. We provide a model of turbulence, dust settling, and grain growth to explain the results.