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 3(1,2)-2(2,1), ortho-H2D+ 1(1,0)-1(1,1), and para-D2H+ 1(1,0)-1(0,1) transitions. We also present observations of the HCN J = 3-2, HCO+ J = 3-2, and (HCO+)-C-13 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 < 30 to >= 70AU, and there is a rapid falloff of the abundance of DCO+ at radii larger than 90 AU. Using a simple analytical chemical model, we constrain the degree of ionization, x(e-) = n(e-)/n(H-2), to be similar to 10(-7) in the disk layer(s) where these molecules are present. Provided the distribution of DCN follows that of HCN, the ratio of DCN to HCN is determined to be (1.7 +/- 0.5) x 10(-2); however, this ratio is very sensitive to the poorly constrained vertical distribution of HCN. The resolved radial distribution of DCO+ indicates that in situ deuterium fractionation remains active within the TW Hydrae disk and must be considered in the molecular evolution of circumstellar accretion disks.