Cold Deuterium Fractionation in the Nearest Planet-Forming Disk

TL;DR

This study uses ALMA observations to analyze deuterium fractionation in the TW Hya protoplanetary disk, revealing insights into chemical processes and conditions relevant to planet formation.

Contribution

It provides new high-resolution observations of DCO$^+$ and DCN in a protoplanetary disk, combining LTE and non-LTE analysis to localize physical conditions and assess deuteration pathways.

Findings

Similar column densities for DCO$^+$ and DCN around 20-30K temperatures.

Tentative evidence of gas phase C/O enhancement within 40 au.

Deuteration processes are linked to disk midplane conditions.

Abstract

Deuterium fractionation provides a window to the thermal history of volatiles in the solar system and protoplanetary disks. While evidence of active molecular deuteration has been observed towards a handful of disks, it remains unclear whether this chemistry affects the composition of forming planetesimals due to limited observational constraints on the radial and vertical distribution of deuterated molecules. To shed light on this question, we introduce new ALMA observations of DCO$^+$ and DCN $J=2-1$ at an angular resolution of $0.5"$ (30 au) and combine them with archival data of higher energy transitions towards the protoplanetary disk around TW Hya. We carry out a radial excitation analysis assuming both LTE and non-LTE to localize the physical conditions traced by DCO$^+$ and DCN emission in the disk, thus assessing deuterium fractionation efficiencies and pathways at different disk locations. We find similar disk-averaged column densities of $1.9\times10^{12}$ and $9.8\times10^{11}$ cm$^{-2}$ for DCO$^{+}$ and DCN, with typical kinetic temperatures for both molecules of 20-30K, indicating a common origin near the comet- and planet-forming midplane. The observed DCO$^+$/DCN abundance ratio, combined with recent modeling results, provide tentative evidence of a gas phase C/O enhancement within $<40$ au. Observations of DCO$^+$ and DCN in other disks, as well as HCN and HCO$^+$, will be necessary to place the trends exhibited by TW Hya in context, and fully constrain the main deuteration mechanisms in disks.