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

TL;DR

This study uses ALMA observations to reveal double-ring structures of DCO+ in the IM Lup disk, demonstrating that non-thermal desorption processes can liberate CO ice in outer disk regions, unveiling hidden cold gas reservoirs.

Contribution

First spatially resolved detection of double DCO+ rings in a protoplanetary disk, linking them to thermal and non-thermal CO desorption processes.

Findings

Inner DCO+ ring linked to CO freeze-out and thermal desorption.

Outer DCO+ ring explained by non-thermal CO desorption.

DCO+ rings trace cold gas reservoirs in outer disk regions.

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 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.