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Millimeter observations of CO isotopologues are often used to make inferencesabout protoplanetary disk gas density and temperature structures. The accuracyof these estimates depends on our understanding of CO freezeout and desorptionfrom dust grains. Most models of these processes indicate that CO columndensity decreases monotonically with distance from the central star due to adecrease in gas density and freezeout beyond the CO snowline. We present ALMACycle 2 observations of $^{12}$CO, $^{13}$CO, and C$^{18}$O $J=2-1$ emissionthat 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 aradius of $\sim1''$ (120 AU), well outside the expected CO snowline. We proposethat the ring arises from the onset of CO desorption near the edge of themillimeter dust disk. CO desorption exterior to a CO snowline may occur vianon-thermal processes involving cosmic rays or high-energy photons, or via aradial thermal inversion arising from dust migration.

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