The TW Hya Rosetta Stone Project I: Radial and vertical distributions of DCN and DCO+

TL;DR

This study uses ALMA observations to map the distributions of DCO+ and DCN in the TW Hya disk, revealing active deuterium chemistry in certain regions and providing insights into the chemical history of Solar System volatiles.

Contribution

It presents high-resolution spatial distributions of DCO+ and DCN in TW Hya, highlighting the regions of active deuterium fractionation and their implications for Solar Nebula chemistry.

Findings

DCO+ is found in a broad ring extending beyond the pebble disk.

DCN is concentrated in a narrow ring around 30 au.

Active deuterium chemistry occurs outside the cold midplane and inner disk regions.

Abstract

Molecular D/H ratios are frequently used to probe the chemical past of Solar System volatiles. Yet it is unclear which parts of the Solar Nebula hosted an active deuterium fractionation chemistry. To address this question, we present 0".2-0".4 ALMA observations of DCO+ and DCN 2-1, 3-2 and 4-3 towards the nearby protoplanetary disk around TW Hya, taken as part of the TW Hya Rosetta Stone project, augmented with archival data. DCO+ is characterized by an excitation temperature of ~40 K across the 70 au radius pebble disk, indicative of emission from a warm, elevated molecular layer. Tentatively, DCN is present at even higher temperatures. Both DCO+ and DCN present substantial emission cavities in the inner disk, while in the outer disk the DCO+ and DCN morphologies diverge: most DCN emission originates from a narrow ring peaking around 30~au, with some additional diffuse DCN emission present at larger radii, while DCO+ is present in a broad structured ring that extends past the pebble disk. Based on parametric disk abundance models, these emission patterns can be explained by a near-constant DCN abundance exterior to the cavity, and an increasing DCO+ abundance with radius. There appears to be an active deuterium fractionation chemistry in multiple disk regions around TW Hya, but not in the cold planetesimal-forming midplane and in the inner disk. More observations are needed to explore whether deuterium fractionation is actually absent in these latter regions, and if its absence is a common feature, or something peculiar to the old TW Hya disk.