  Using a thermal-chemical model for the generic T-Tauri disk of D'Alessio et
al. (1999), we estimate the strength of the fine-structure emission lines of
NeII and NeIII at 12.81 and 15.55 microns that arise from the warm atmosphere
of the disk exposed to hard stellar X-rays. The Ne ions are produced by the
absorption of keV X-rays from the K shell of neutral Ne, followed by the Auger
ejection of several additional electrons. The recombination cascade of the Ne
ions is slow because of weak charge transfer with atomic hydrogen in the case
of Ne2+ and by essentially no charge transfer for Ne+. For a distance of 140pc,
the 12.81 micron line of Ne II has a flux of 1e-14 erg/cm2s, which should be
observable with the Spitzer Infrared Spectrometer and suitable ground based
instrumentation. The detection of these fine-structure lines would clearly
demonstrate the effects of X-rays on the physical and chemical properties of
the disks of young stellar objects and provide a diagnostic of the warm gas in
protoplanetary disk atmospheres. They would complement the observed H2 and CO
emission by probing vertical heights above the molecular transition layer and
larger radial distances that include the location of terrestrial and giant
planets.
