On the Temperature Distribution and CO Depletion in the Embedded Disk around L1489 IRS

ABSTRACT :
Molecular gas distributions in young protoplanetary disks may severally affect chemical evolution and subsequent planet formation. Previous ALMA observations of the Class I protostar L1489 IRS have revealed CO dips at radii of ~140–250 AU in its protostellar disk extending up to a radius of 600 AU. This suggests that gas phase CO abundance decreases around the dip radii due to CO freeze-out, but interestingly it again increases at further larger radii. In this talk, we will present our findings on the temperature distribution in the gas disk around L1489 IRS and discuss the physical origin of the non-monotonic distribution of CO gas in the disk. We analyzed ALMA archival data of C18O (J = 2-1) and (J = 3-2) lines and probed radial profiles of the temperature and column density in the disk through LTE analysis. It was confirmed that both lines were mostly optically thin across the entire disk. The CO column density dropped by a factor of ~1.5 at the CO dip radii on both northeast (redshifted) and southwest (blueshifted) sides, indicating CO depletion there. We found that the disk temperature is almost constant around ~25 K but increases to ~37 K just outside the CO depletion radius on northeast side. On the other hand, towards southwest side, it slightly decreases from ~40 K to ~30 K with increasing radius within the CO depletion radius and increases back to ~40 K just outside the depletion radius. These results suggest that thermal desorption of CO in the outer disk may cause the non-monotonic distribution of CO gas in the disk. We will discuss the possible origin of the temperature increase in the outer disk.