Evidence for a CO desorption front in the outer AS 209 disk

TL;DR

This paper presents ALMA observations of the AS 209 disk showing unexpected CO enhancement outside the snowline, suggesting a CO desorption front caused by non-thermal processes or dust migration.

Contribution

It provides the first observational evidence of a CO desorption front in a protoplanetary disk, challenging the assumption of monotonic CO depletion beyond the snowline.

Findings

CO ring observed at 120 AU outside the expected snowline

CO desorption likely caused by non-thermal processes or dust migration

Evidence suggests a complex CO distribution in the outer disk

Abstract

Millimeter observations of CO isotopologues are often used to make inferences about protoplanetary disk gas density and temperature structures. The accuracy of these estimates depends on our understanding of CO freezeout and desorption from dust grains. Most models of these processes indicate that CO column density decreases monotonically with distance from the central star due to a decrease in gas density and freezeout beyond the CO snowline. We present ALMA Cycle 2 observations of $^{12}$CO, $^{13}$CO, and C$^{18}$O $J=2-1$ emission that instead suggest CO enhancement in the outer disk of T Tauri star AS 209. Most notably, the C$^{18}$O emission consists of a central peak and a ring at a radius of $\sim1''$ (120 AU), well outside the expected CO snowline. We propose that the ring arises from the onset of CO desorption near the edge of the millimeter dust disk. CO desorption exterior to a CO snowline may occur via non-thermal processes involving cosmic rays or high-energy photons, or via a radial thermal inversion arising from dust migration.