2015ApJ...802L..23F

In this Letter, we model the chemistry of DCO⁺ in protoplanetary disks. We find that the overall distribution of the DCO⁺ abundance is qualitatively similar to that of CO but is dominated by a thin layer located at the inner disk surface. To understand its distribution, we investigate the different key gas-phase deuteration pathways that can lead to the formation of DCO⁺. Our analysis shows that the recent update in the exothermicity of the reaction involving CH₂D⁺ as a parent molecule of DCO⁺ favors deuterium fractionation in warmer conditions. As a result, the formation of DCO⁺ is enhanced in the inner warm surface layers of the disk where X-ray ionization occurs. Our analysis points out that DCO⁺ is not a reliable tracer of the CO snow line as previously suggested. We thus predict that DCO⁺ is a tracer of active deuterium and, in particular, X-ray ionization of the inner disk.