Astrophys. J., 681, 1396-1407 (2008/July-2)
Resolving the chemistry in the disk of TW Hydrae. I. Deuterated species.
QI C., WILNER D.J., AIKAWA Yu., BLAKE G.A. and HOGERHEIJDE M.R.
Abstract (from CDS):
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+ 11,0-10,1transitions. 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.
Stars: Circumstellar Matter - Comets: General - ISM: Molecules - Stars: Planetary Systems: Protoplanetary Disks - Stars: Individual: Constellation Name: TW Hydrae - Stars: Pre-Main-Sequence
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