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We calculate the thermal-chemical structure of the gaseous atmospheres of the inner disks of T Tauri stars, starting from the density and dust temperature distributions derived by D'Alessio and coworkers in 1999. As a result of processes such as X-ray irradiation or mechanical heating of the surface layers, the gas temperature at the very top of the disk atmosphere in the neighborhood of 1 AU is of the order of 5000 K. Deep down, it drops rapidly into the range of the dust temperature, i.e., several hundred degrees kelvin. In between these upper hot and lower cool layers, there is a transition zone with gas temperatures in the range 500-2000 K. The thickness and location of this warm region depend on the strength of the surface heating. This region also manifests the basic chemical transitions of H to H 2 and C + and C to CO. It is remarkable that even though the H 2 transition begins first (higher up), it does not go to completion until after CO does. Consequently, there is a reasonably thick layer of warm CO that is predominantly atomic H. This thermal-chemical structure is favorable to the excitation of the fundamental and overtone bands of CO because of the large rate coefficients for vibrational excitation in H+CO as opposed to H 2 +CO collisions. This conclusion is supported by the recent observations of the fundamental band transitions in most T Tauri stars. We also argue that layered atmospheres of inner T Tauri disks may play an important role in understanding the observations of H 2 UV fluorescence pumped from excited vibrational levels of that molecule. Possible candidates for surface heating include the interaction of a wind with the upper layers of the disk and dissipation of hydromagnetic waves generated by mechanical disturbances close to the midplane, e.g., by the Balbus-Hawley instability. Detailed modeling of the observations has the potential to reveal the nature of the mechanical surface heating that we model phenomenologically in these calculations and to help explain the nature of the gas in protoplanetary disks.

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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 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 m line of Ne ii has a flux $10 À14 ergs cm À2 s À1, 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.

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Abstract. X-rays from young stellar objects can influence the physical properties of their surroundings. A particularly interesting example is the ionization of the accretion disk by hard X-rays at distances of the order of 1 AU or more. We show that X-rays induce a layered ionization structure in which the outer layer is sufficiently ionized to permit the Balbus-Hawley instability to operate and to allow accretion.