Issue |
A&A
Volume 371, Number 3, June I 2001
|
|
---|---|---|
Page(s) | 1107 - 1117 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361:20010416 | |
Published online | 15 June 2001 |
Two-dimensional distributions and column densities of gaseous molecules in protoplanetary disks
II. Deuterated species and UV shielding by ambient clouds
1
Department of Earth and Planetary Sciences, Kobe University, Kobe 657-8501, Japan
2
Departments of Physics and Astronomy, The Ohio State University, Columbus, OH 43210, USA e-mail: herbst@mps.ohio-state.edu
Corresponding author: Y. Aikawa, aikawa@jet.planet.sci.kobe-u.ac.jp
Received:
22
November
2000
Accepted:
19
March
2001
We have investigated the two-dimensional () distribution of
deuterated molecular species in circumstellar disks around young
stellar objects. The abundance ratios between singly deuterated and
normal molecules ("D/H ratios") in disks evolve in a similar way as
in molecular clouds. Fractionation is caused by rapid exchange
reactions that are exothermic because of energy differences between
deuterated and normal species. In the midplane region, where
molecules are heavily depleted onto grain surfaces, the D/H ratios of
gaseous molecules are higher than at larger heights.
The D/H ratios for the vertical column
densities of NH3, H2O, and HCO+ are sensitive to the
temperature, and decrease significantly with decreasing radial
distance for
AU. The analogous D/H ratios for CH4
and H2CO, on the other hand, are not very sensitive to the
temperature in the range (
K) we are concerned with, and do
not decrease with decreasing R at
AU. The D/H column-density
ratios also depend on disk mass. In a disk with a larger mass, the
ratios of deuterated species to normal species are higher, because of
heavier depletion of molecules onto grains.
In the second part of the paper, we report molecular column densities
for disks embedded in ambient cloud gas. Our results suggest that CN
and HCO+ can be tracers of gaseous disks, especially if the central
object is a strong X-ray source.
Our results also suggest that the radial distributions of CN,
C2H, HCN, and H2CO may vary among disks depending on the X-ray
luminosity of the central star.
Key words: ISM: molecules / stars: T Tauri / circumstellar matter / protoplanetary disk
© ESO, 2001
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