Resolving the Chemistry in the Disk of TW Hydrae. I. Deuterated Species
Abstract
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+ 11,0-10,1 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 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 +/- 0.5) × 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2008
- DOI:
- 10.1086/588516
- arXiv:
- arXiv:0803.2753
- Bibcode:
- 2008ApJ...681.1396Q
- Keywords:
-
- circumstellar matter;
- comets: general;
- ISM: molecules;
- planetary systems: protoplanetary disks;
- stars: individual: TW Hydrae;
- stars: pre-main sequence;
- Astrophysics
- E-Print:
- 12 pages, 12 figures, accepted to ApJ