EVIDENCE FOR DCO+ AS A PROBE OF IONIZATION IN THE WARM DISK SURFACE

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.