Atomic diagnostics of X-ray-irradiated protoplanetary disks

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 mu m. 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 mu m 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 lambda 6718/lambda 6732 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 lambda 6300/lambda 6363 and C I lambda 9826/lambda 9852 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.