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Molecular D/H ratios are frequently used to probe the chemical past of SolarSystem volatiles. Yet it is unclear which parts of the Solar Nebula hosted anactive deuterium fractionation chemistry. To address this question, we present0".2-0".4 ALMA observations of DCO+ and DCN 2-1, 3-2 and 4-3 towards the nearbyprotoplanetary disk around TW Hya, taken as part of the TW Hya Rosetta Stoneproject, augmented with archival data. DCO+ is characterized by an excitationtemperature of ~40 K across the 70 au radius pebble disk, indicative ofemission from a warm, elevated molecular layer. Tentatively, DCN is present ateven higher temperatures. Both DCO+ and DCN present substantial emissioncavities in the inner disk, while in the outer disk the DCO+ and DCNmorphologies diverge: most DCN emission originates from a narrow ring peakingaround 30~au, with some additional diffuse DCN emission present at largerradii, while DCO+ is present in a broad structured ring that extends past thepebble disk. Based on parametric disk abundance models, these emission patternscan be explained by a near-constant DCN abundance exterior to the cavity, andan increasing DCO+ abundance with radius. There appears to be an activedeuterium fractionation chemistry in multiple disk regions around TW Hya, butnot in the cold planetesimal-forming midplane and in the inner disk. Moreobservations are needed to explore whether deuterium fractionation is actuallyabsent in these latter regions, and if its absence is a common feature, orsomething peculiar to the old TW Hya disk.

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