Gas Dust Stratification in Young Disks

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, ¹³C), 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 form the CO absorption lines are much higher than expected for the measured extinction for each source and suggest the gas to dust ratio is measuring the dust settling and grain coagulation in these extended disks. We provide a model of turbulence, dust settling and grain growth to explain the results.