The deuterated forms of H3+ : a unique probe of the outer disk midplane
          Carsten Dominik, University of Amsterdam
          Cecilia Ceccarelli, Laboratoire d'Astrophysique de l'Observatoire de Grenoble

          Young proto-planetary disks are gas rich, as probed by several observations of molecular transitions. Yet, molecules are abundant only in an intermediate zone of the disk, shielded from UV photons from the star are shielded but heated enough to prevent freeze out of molecules onto dust grains.  In the low-temperature midplane of the outer disk, heavy-elements bearing molecules are indeed frozen onto the grain mantles.  However, molecules containing only H and D remain in the gas phase.  Recently, Ceccarelli et al. (2004) have shown that observations of the ground transition of the o-H2D+ can be used to probe the outer disk midplane, potentially providing information about the degree of ionization and the dust-to gas ratios, two key parameters.

In this contribution, we present model calculations for the abundances and emission line intensities of singly, doubly and triply deuterated H3+ in a proto-planetary disk.  Using a self-consistent disk model and a small set of chemical reactions, we compute the CO depletion and abundances of H2D+, D2H+ and D3+ as a function of time and location in the disk.  We show that these three different molecules are important charge carriers throughout most of the outer disk.  Which molecule dominates depends upon the proprieties of the disk (mass, radius...), chemical age of the disk and mixing time scales.  We translate the abundances into line intensities and predict absorption and emission line strengths for all three molecules.