Spectrally Resolved Mid-Infrared Molecular Emission from Protoplanetary
  Disks and the Chemical Fingerprint of Planetesimal Formation
  We present high resolution spectroscopy of mid-infrared molecular emission
from two very active T Tauri stars, AS 205 N and DR Tau. In addition to
measuring high signal-to-noise line profiles of water, we report the first
spectrally resolved mid-infrared line profiles of HCN emission from
protoplanetary disks. The similar line profiles and temperatures of the HCN and
water emission indicate that they arise in the same volume of the disk
atmosphere, within 1-2AU of the star. The results support the earlier
suggestion that the observed trend of increasing HCN/water emission with disk
mass is a chemical fingerprint of planetesimal formation and core accretion in
action. In addition to directly constraining the emitting radii of the
molecules, the high resolution spectra also help to break degeneracies between
temperature and column density in deriving molecular abundances from low
resolution mid-infrared spectra. As a result, they can improve our
understanding of the extent to which inner disks are chemically active.
Contrary to predictions from HCN excitation studies carried out for AS 205 N,
the mid-infrared and near-infrared line profiles of HCN are remarkably similar.
The discrepancy may indicate that HCN is not abundant beyond a couple of AU or
that infrared pumping of HCN does not dominate at these distances.
