  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.
