Cold Deuterium Fractionation in the Nearest Planet-forming Disk

Abstract Deuterium fractionation provides a window into the thermal history of volatiles in solar system and protoplanetary disks. While evidence active molecular deuteration has been observed toward handful disks, it remains unclear whether this chemistry affects composition forming planetesimals due to limited observational constraints on radial vertical distribution deuterated molecules. To shed light question, we introduce new Atacama Large Millimeter/submillimeter Array observations DCO + DCN J = 2–1 at an angular resolution 0.″5 (30 au) combine them with archival data higher energy transitions disk around TW Hya. We carry out excitation analysis assuming both LTE non-LTE localize physical conditions traced by emission disk, thus assessing deuterium efficiencies pathways different locations. find similar disk-averaged column densities 1.9 × 10 12 9.8 11 cm −2 for DCN, typical kinetic temperatures molecules 20–30 K, indicating common origin near comet- planet-forming midplane. The /DCN abundance ratio, combined recent modeling results, provide tentative gas-phase C/O enhancement within <40 au. Observations other as well HCN HCO , will be necessary place trends exhibited Hya context, fully constrain main mechanisms