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 ALMA
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.
