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Resolving the chemistry in the disk of TW Hydrae I. Deuterated species | | C. Qi
; D.J. Wilner
; Y. Aikawa
; G.A. Blake
; M.R. Hogerheijde
; | | Date: |
19 Mar 2008 | | Abstract: | We present Submillimeter Array (SMA) 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 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 2D 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 one order of
magnitude (0.01 to 0.1) for radii <30 AU 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 1.7pm0.5 imes 10^-2; however, this ratio is very sensitive to the
poorly constrained vertical distribution of HCN. The resolved radial
distribution of DCO+ indicates that {it in situ} deuterium fractionation
remains active within the TW Hydrae disk and must be considered in the
molecular evolution of circumstellar accretion disks. | | Source: | arXiv, 0803.2753 | | Services: | Forum | Review | PDF | Favorites | |
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