Atomic Diagnostics of X-Ray-Irradiated Protoplanetary Disks

• Published in: The Astrophysical journal 2008-03, Vol.676 (1), p.518-531
• Main Authors: Meijerink, R, Glassgold, A. E, Najita, J. R
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/527411

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[image], S[image], 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.