Double DCO+ Rings Reveal CO Ice Desorption in the Outer Disk Around IM Lup

In a protoplanetary disk, a combination of thermal and non-thermal desorption processes regulate where volatiles are liberated from icy grain mantles into the gas phase. Non-thermal desorption should result in volatile-enriched gas in disk-regions where complete freeze-out is otherwise expected. We present Atacama Large Millimeter/Submillimeter Array observations of the disk around the young star IM Lup in 1.4 mm continuum, C¹⁸O 2-1, H¹³CO⁺ 3-2 and DCO⁺ 3-2 emission at ∼0.″5 resolution. The images of these dust and gas tracers are clearly resolved. The DCO⁺ line exhibits a striking pair of concentric rings of emission that peak at radii of ∼0.″6 and 2″ (∼90 and 300 AU, respectively). Based on disk chemistry model comparison, the inner DCO⁺ ring is associated with the balance of CO freeze-out and thermal desorption due to a radial decrease in disk temperature. The outer DCO⁺ ring is explained by non-thermal desorption of CO ice in the low-column-density outer disk, repopulating the disk midplane with cold CO gas. The CO gas then reacts with abundant H₂D⁺ to form the observed DCO⁺ outer ring. These observations demonstrate that spatially resolved DCO⁺ emission can be used to trace otherwise hidden cold gas reservoirs in the outmost disk regions, opening a new window onto their chemistry and kinematics.