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 1⁄4 3Y2 and DCO J 1⁄4 3Y2 lines and upper limits to the HDO 31;2Y22;1, ortho-H2D 11;0Y11;1, and para-D2H 11;0Y10;1 transitions. We also present observations of the HCN J 1⁄4 3Y2, HCO J 1⁄4 3Y2, and HCO J 1⁄4 4Y3 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 (0.01Y0.1) 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 ) 1⁄4 n(e )/n(H2), to be 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 toHCN is determined to be 1:7 0:5 ð Þ ; 10 ; 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. Subject headingg s: circumstellar matter — comets: general — ISM: molecules — planetary systems: protoplanetary disks — stars: individual (TW Hydrae) — stars: preYmain-sequence Online material: color figures