Evidence for Multiple Pathways to Deuterium Enhancements in Protoplanetary Disks

TL;DR

This study uses spatially resolved observations of deuterated molecules in the TW Hya protoplanetary disk to reveal multiple formation pathways for deuterium enhancements, highlighting temperature-dependent chemistry.

Contribution

It provides observational evidence for distinct formation pathways of DCN and DCO+ in a protoplanetary disk, emphasizing the importance of temperature in deuterium chemistry.

Findings

DCN is centrally peaked, unlike DCO+ which increases with radius.

DCN forms at higher temperatures than DCO+.

Deuterium fractionation occurs efficiently across a range of temperatures.

Abstract

The distributions of deuterated molecules in protoplanetary disks are expected to depend on the molecular formation pathways. We use observations of spatially resolved DCN emission from the disk around TW Hya, acquired during ALMA Science verification with a ~3" synthesized beam, together with comparable DCO+ observations from the Submillimeter Array, to investigate differences in the radial distributions of these species and hence differences in their formation chemistry. In contrast to DCO+, which shows an increasing column density with radius, DCN is better fit by a model that is centrally peaked. We infer that DCN forms at a smaller radii and thus at higher temperatures than DCO+. This is consistent with chemical network model predictions of DCO+ formation from H2D+ at T<30 K and DCN formation from additional pathways involving CH2D+ at higher temperatures. We estimate a DCN/HCN abundance ratio of ~0.017, similar to the DCO+/HCO+ abundance ratio. Deuterium fractionation appears to be efficient at a range of temperatures in this protoplanetary disk. These results suggest caution in interpreting the range of deuterium fractions observed in Solar System bodies, as multiple formation pathways should be taken into account.