2020ApJ...891L..17Z

Recent observations show that the CO gas abundance, relative to H₂, in many 1-10 Myr old protoplanetary disks may be heavily depleted by a factor of 10-100 compared to the canonical interstellar medium (ISM) value of 10^–4. When and how this depletion happens can significantly affect compositions of planetesimals and atmospheres of giant planets. It is therefore important to constrain whether the depletion occurs already at the earliest protostellar disk stage. Here we present spatially resolved observations of C¹⁸O, C¹⁷O, and ¹³C¹⁸O J = 2-1 lines in three protostellar disks. We show that the C¹⁸O line emits from both the disk and the inner envelope, while C¹⁷O and ¹³C¹⁸O lines are consistent with a disk origin. The line ratios indicate that both C¹⁸O and C¹⁷O lines are optically thick in the disk region, and only the ¹³C¹⁸O line is optically thin. The line profiles of the ¹³C¹⁸O emissions are best reproduced by Keplerian gaseous disks at similar sizes as their mm-continuum emissions, suggesting small radial separations between the gas and mm-sized grains in these disks, in contrast to the large separation commonly seen in protoplanetary disks. Assuming a gas-to-dust ratio of 100, we find that the CO gas abundances in these protostellar disks are consistent with the ISM abundance within a factor of 2, nearly one order of magnitude higher than the average value of 1-10 Myr old disks. These results suggest that there is a fast, ∼1 Myr, evolution of the abundance of CO gas from the protostellar disk stage to the protoplanetary disk stage.