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In a protoplanetary disk, a combination of thermal and non-thermal desorptionprocesses regulate where volatiles are liberated from icy grain mantles intothe gas phase. Non-thermal desorption should result in volatile-enriched gas indisk-regions where complete freeze-out is otherwise expected. We present ALMAobservations of the disk around the young star IM Lup in 1.4 mm continuum, C18O2-1, H13CO+ 3-2 and DCO+ 3-2 emission at ~0".5 resolution. The images of thesedust and gas tracers are clearly resolved. The DCO+ line exhibits a strikingpair of concentric rings of emission that peak at radii of ~0".6 and 2" (~90and 300 AU, respectively). Based on disk chemistry model comparison, the innerDCO+ ring is associated with the balance of CO freeze-out and thermaldesorption due to a radial decrease in disk temperature. The outer DCO+ ring isexplained by non-thermal desorption of CO ice in the low-column-density outerdisk, repopulating the disk midplane with cold CO gas. The CO gas then reactswith abundant H2D+ to form the observed DCO+ outer ring. These observationsdemonstrate that spatially resolved DCO+ emission can be used to traceotherwise hidden cold gas reservoirs in the outmost disk regions, opening a newwindow onto their chemistry and kinematics.

Double DCO+ rings reveal CO ice desorption in the outer disk around IM Lup