Multiple Carbon monoxide Snow-lines in Disks Sculpted by Radial Drift

TL;DR

This paper demonstrates that radial drift in protoplanetary disks can create multiple CO snow-lines by altering the temperature structure, affecting the phase state of CO and the disk's chemical composition.

Contribution

It introduces a simple parametric model showing how radial drift-induced density structures lead to multiple CO snow-lines in disks.

Findings

Radial drift increases outer disk temperature by 10-30%.

Multiple CO snow-lines are formed at different radii.

Outer disk CO phase transitions are influenced by thermal and UV desorption.

Abstract

Observations of protoplanetary disks suggest that the gas and dust follow significantly different radial distributions. This finding can be theoretically explained by a combination of radial drift and gas drag of intermediate-sized dust grains. Using a simple parametric model to approximate the different distributions of the gas and dust components, we calculate and examine the impact of radial drift on the global dust temperature structure. We find that the removal of large grains beyond the "truncation radius" allows this region to become significantly warmer from reprocessed stellar radiation shining down from the disk upper layers, increasing the outer disk temperature by $\sim10-30\%$. This change is sufficient to raise the local temperature to a value exceeding the CO desorption temperature. These findings imply that the disk density structures induced by radial drift are able to create multiple CO snow-lines. The inner disk CO is in the gas phase, freezing out near the classical snow-line at $R\sim20-40$ AU. Moving outward, the CO sublimates once again beyond the truncation radius (80 AU in our models) and subsequently re-freezes out at sufficiently large stellar distances, beyond $R\gtrsim130-200$ AU. We find that thermal desorption of CO in the outer disk becomes competitive with external UV photodesorption and that this additional transition from solid state CO to the gas-phase has significant implications for the C/O ratio in the outer disk.