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

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Abstract

  • 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.

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  • 2015

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