Double DCO$^+$ Rings Reveal CO Ice Desorption in the Outer Disk Around IM Lup
Öberg, Karin I., Furuya, Kenji, Loomis, Ryan, Aikawa, Yuri, Andrews, Sean M., Qi, Chunhua, van Dishoeck, Ewine F., & Wilner, David J.
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, C$^18$O 2-1, H$^13$CO$^+$ 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 H$_2$D$^+$ 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.