Evidence for DCO+ as a probe of ionization in the warm disk surface

TL;DR

This study models DCO$^{+}$ chemistry in protoplanetary disks, revealing it as a tracer of X-ray ionization in warm inner disk surfaces rather than the CO snow line, highlighting its role in active deuterium chemistry.

Contribution

It demonstrates that DCO$^{+}$ traces X-ray ionization in warm inner disk surfaces, challenging previous assumptions about its relation to the CO snow line.

Findings

DCO$^{+}$ abundance peaks at the inner disk surface.

Deuterium fractionation is favored in warmer conditions due to updated reaction exothermicity.

DCO$^{+}$ is not a reliable CO snow line tracer, but indicates X-ray ionization activity.

Abstract

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.