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 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 CH${}_2$D${}^{+}$ 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.