X-RAY-IRRADIATED PROTOPLANETARY DISK ATMOSPHERES. I. PREDICTED EMISSION-LINE SPECTRUM AND PHOTOEVAPORATION

We present mocassin two-dimensional photoionization and dust radiative transfer models of a prototypical T Tauri disk irradiated by X-rays from the young pre-main-sequence star. The calculations demonstrate a layer of hot gas reaching temperatures of similar to 10(6) K at small radii and similar to 10(4) K at a distance of 1 AU. The gas temperatures decrease sharply with depth, but appear to be completely decoupled from dust temperatures down to a column depth of similar to 5 x 10(21) cm(-2). We predict that several fine-structure and forbidden lines of heavy elements, as well as recombination lines of hydrogen and helium, should be observable with current and future instrumentation, although optical lines may be smothered by the stellar spectrum. Predicted line luminosities are given for the brightest collisionally excited lines (down to similar to 10(-8) L-circle dot) and for recombination transitions from several levels of H I and He I. The mass-loss rate due to X-ray photoevaporation estimated from our models is of the order of 10(-8) M-circle dot yr(-1), implying a dispersal timescale of a few Myr for a disk of mass 0.027 M-circle dot, which is the mass of the disk structure model we employed. We discuss the limitations of our model and highlight the need for further calculations that should include the simultaneous solution of the two-dimensional radiative transfer problem and the one-dimensional hydrostatic equilibrium problem in the polar direction.