2008ApJ...676..518M

We study atomic-line diagnostics of the inner regions of protoplanetary disks with our model of X-ray-irradiated disk atmospheres, which was previously used to predict observable levels of the Ne II and Ne III fine-structure transitions at 12.81 and 15.55 µm. We extend the X-ray ionization theory to sulfur and calculate the fraction of sulfur in S, S⁺, S²⁺, and sulfur molecules. For the D'Alessio generic T Tauri star disk, we find that the S I fine-structure line at 25.55 µm is below the detection level of the Spitzer Infrared Spectrometer (IRS), in large part due to X-ray ionization of atomic S at the top of the atmosphere and to its incorporation into molecules close to the midplane. We predict that observable fluxes of the S II λ6718/λ6732 forbidden transitions are produced in the upper atmosphere at somewhat shallower depths and smaller radii than the neon fine-structure lines. This and other forbidden-line transitions, such as the O I λ6300/λ6363 and C I λ9826/λ9852 lines, serve as complementary diagnostics of X-ray-irradiated disk atmospheres. We have also analyzed the potential role of the low-excitation fine-structure lines of C I, C II, and O I, which should be observable by SOFIA and Herschel.