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tokens: the surface, which naturally underestimates the extinction. The curves are shown in Figure 3 (top) for both the truncated and full disks. At a given location, the optical depth of the outer disk is generally lower for the truncated disk with radial drift, as we would expect. Note that it does not move all the way inward to the truncation radius since there are still small grains out to the gas disk radius. Figure 3. Top row: effective opacity () to external FUV radiation for the different small/large dust mass distributions over steradians. The left and right columns show the full model and drift-dominated model, respectively. The dashed and solid lines indicate and . The line color corresponds to the different small grain dust fractions, gray (10%) and black (1%). Bottom row: for the case, the ratio of desorbed molecule flux from photodesorption vs. thermal desorption (). The solid line indicates where the processes are equal. We compute the local flux, , and adopt a value for the yield of photodesorbed CO of molecules photon −1 , which is appropriate for the FUV photodesorption experiments of Fayolle et al. ( 2011 ). Figure 3 (bottom) compares the relative desorption fluxes from the ISRF to thermal processes () for the case. Large values correspond to efficiency in each process, i.e., regions of the disk where is large implies photodesorption dominates thermal desorption and vice versa. In all models, thermal desorption dominates the inner disk and transitions to photodesorption in the outer disk. We find that the transition is substantially further out for the drift-dominated model and that the transition occurs rapidly. In the case with drift, thermal desorption dominates ISRF photodesorption inside of , and for the full disk the transition occurs at ∼80 AU. These results are non-obvious, as radial drift increases outer disk heating but also decreases the opacity to external FUV. We find similar results for 1% small grain mass case, with values of 400 AU (drift) and 70 AU (full), respectively. 4. DISCUSSION Based on these results, we expect radial drift to alter the disk temperature profile and enhance thermal desorption of CO in the outer disk. To approximately estimate the enhancement in CO column density, we take the temperature profile and assign a CO abundance of 10 −4 per H 2 for dust temperatures between 17 and 200 K. We calculate the vertically integrated column density of CO versus radius and then compare the models with and without drift and find the maximum enhancement in CO column is ∼2–2.5 times higher outside of the large grain radius. Recently, observations of IM Lup have shown multiple rings of DCO + , a direct product of CO-driven chemistry (Öberg et al. 2015 ). Furthermore, Tang et al. ( 2015 ) report multiple CO ring structures in both 12 CO and 13 CO in GG Tau. Radial temperature variations increasing CO sublimation in the outer disk may be contributing to these structures, though they should be modeled in detail to