Midplane Deuterium Fractionation in the Nearest Planet-Forming Disk

Deuterium fractionation offers 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 present new 0.5" ALMA observations of DCO⁺ and DCN J=2-1, combined with archival data of higher energy transitions, towards the protoplanetary disk around TW Hydrae (TW Hya). We carry out a detailed radial excitation analysis in both LTE and non-LTE to localize the physical conditions and chemical origins of DCO⁺ and DCN in the disk, thus assessing deuterium fractionation efficiencies and pathways at different disk locations. We find similar disk-averaged column densities of 1.9×10¹² and 9.8×10¹¹ cm^-2 for DCO⁺ and DCN, with typical kinetic temperatures for both molecules in the range of 20-30K throughout the disk, indicating a common origin near the comet- and planet-forming midplane of TW Hya.