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We present high resolution spectroscopy of mid-infrared molecular emissionfrom two very active T Tauri stars, AS 205 N and DR Tau. In addition tomeasuring high signal-to-noise line profiles of water, we report the firstspectrally resolved mid-infrared line profiles of HCN emission fromprotoplanetary disks. The similar line profiles and temperatures of the HCN andwater emission indicate that they arise in the same volume of the diskatmosphere, within 1-2AU of the star. The results support the earliersuggestion that the observed trend of increasing HCN/water emission with diskmass is a chemical fingerprint of planetesimal formation and core accretion inaction. In addition to directly constraining the emitting radii of themolecules, the high resolution spectra also help to break degeneracies betweentemperature and column density in deriving molecular abundances from lowresolution mid-infrared spectra. As a result, they can improve ourunderstanding 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 orthat infrared pumping of HCN does not dominate at these distances.

Spectrally Resolved Mid-Infrared Molecular Emission from Protoplanetary Disks and the Chemical Fingerprint of Planetesimal Formation