FUV and X-ray irradiated protoplanetary disks: a grid of models I. The disk structure. (arXiv:1208.4959v1 [astro-ph.SR])

FUV and X-ray irradiated protoplanetary disks: a grid of models I. The disk structure. (arXiv:1208.4959v1 [astro-ph.SR]):
Context. Planets are thought to eventually form from the mostly gaseous (~99%
of the mass) disks around young stars. The density structure and chemical
composition of protoplanetary disks are affected by the incident radiation
field at optical, FUV, and X-ray wavelengths, as well as by the dust
properties.

Aims. The effect of FUV and X-rays on the disk structure and the gas chemical
composition are investigated. This work forms the basis of a second paper,
which discusses the impact on diagnostic lines of, e.g., C+, O, H2O, and Ne+
observed with facilities such as Spitzer and Herschel.

Methods. A grid of 240 models is computed in which the X-ray and FUV
luminosity, minimum grain size, dust size distribution, and surface density
distribution are varied in a systematic way. The hydrostatic structure and the
thermo-chemical structure are calculated using ProDiMo.

Results. The abundance structure of neutral oxygen is stable to changes in
the X-ray and FUV luminosity, and the emission lines will thus be useful
tracers of the disk mass and temperature. The C+ abundance distribution is
sensitive to both X-rays and FUV. The radial column density profile shows two
peaks, one at the inner rim and a second one at a radius r=5-10 AU. Ne+ and
other heavy elements have a very strong response to X-rays, and the column
density in the inner disk increases by two orders of magnitude from the lowest
(LX = 1e29 erg/s) to the highest considered X-ray flux (LX = 1e32 erg/s). FUV
confines the Ne+ ionized region to areas closer to the star at low X-ray
luminosities (LX = 1e29 erg/s). H2O abundances are enhanced by X-rays due to
higher temperatures in the inner disk and higher ionization fractions in the
outer disk. The line fluxes and profiles are affected by the effects on these
species, thus providing diagnostic value in the study of FUV and X-ray
irradiated disks around T Tauri stars. (abridged)