Resolving the Chemistry in the Disk of TW Hydrae. I. Deuterated Species

We present Submillimeter Array observations of several deuterated species in the disk around the classical T Tauri star TW Hydrae at arcsecond scales, including detections of the DCN J = 3–2 and DCO+ J = 3–2 lines and upper limits to the HDO 31,2–22,1, ortho-H2D+ 11,0–11,1, and para-D2H+ 11,0–10,1 transitions. We also present observations of the HCN J = 3–2, HCO+ J = 3–2, and H13CO+ J = 4–3 lines for comparison with their deuterated isotopologues. We constrain the radial and vertical distributions of various species in the disk by fitting the data using a model where the molecular emission from an irradiated accretion disk is sampled with a two-dimensional Monte Carlo radiative transfer code. We find that the distribution of DCO+ differs markedly from that of HCO+. The D/H ratios inferred change by at least 1 order of magnitude (0.01-0.1) for radii