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