2008ApJ...681.1396Q

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_0,1transitions. 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 <30 to ≥70 AU, 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(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 to HCN is determined to be 1.7±0.5x10^–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.