Neon fine-structure line emission by X-ray irradiated protoplanetary disks

Using a thermal-chemical model for the generic T Tauri disk of D'Alessio and colleagues, we estimate the strength of the fine-structure emission lines of Ne II and Ne III at 12.81 and 15.55 mu m 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 of the Ne ions is slow because of weak charge transfer with atomic hydrogen in the case of Ne+2 and by essentially no charge transfer for Ne+. For a distance of 140 pc, the 12.81 mu m line of Ne II has a flux similar to 10(-14) ergs cm(-2) s(-1), 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 H-2 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.