Paper: astro-ph/0506254
Date: Sat, 11 Jun 2005 09:01:42 GMT (150kb)

Title: Deuterated H$_3^+$ in proto-planetary disks

Authors: Cecilia Ceccarelli and Carsten Dominik
Comments: accepted for publication in Astronomy & Astrophysics
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Probing the gas and dust in proto-planetary disks is central for
understanding the process of planet formation. In disks surrounding solar type
protostars, the bulk of the disk mass resides in the outer midplane, which is
cold ($\leq$20 K), dense ($\geq 10^7$ cm$^{-3}$) and depleted of CO. Observing
the disk midplane has proved, therefore, to be a formidable challenge.
Ceccarelli et al. (2004) detected H$_2$D$^+$ emission in a proto-planetary disk
and claimed that it probes the midplane gas. Indeed, since all heavy-elements
bearing molecules condense out onto the grain mantles, the most abundant ions
in the disk midplane are predicted to be H$_3^+$ and its isotopomers. In this
article, we carry out a theoretical study of the chemical structure of the
outer midplane of proto-planetary disks. Using a self-consistent physical model
for the flaring disk structure, we compute the abundances of H$_3^+$ and its
deuterated forms across the disk midplane. We also provide the average column
densities across the disk of H$_3^+$, H$_2$D$^+$, HD$_2^+$ and D$_3^+$, and
line intensities of the ground transitions of the ortho and para forms of
H$_2$D$^+$ and HD$_2^+$ respectively. We discuss how the results depend on the
cosmic ray ionization rate, dust-to-gas ratio and average grain radius, and
general stellar/disk parameters. An important factor is the poorly understood
freeze-out of N$_2$ molecules onto grains, which we investigate in depth. We
finally summarize the diagnostic values of observations of the H$_3^+$
isotopomers.

\\ ( http://arXiv.org/abs/astro-ph/0506254 , 150kb)