Far-ultraviolet and X-ray irradiated protoplanetary disks : a grid of models I. The disk structure

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, far-ultraviolet (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 Protoplanetary Disk Model (ProDiMo), with the recent addition of X-rays. Results: The abundance structure of neutral oxygen is very 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. The minimum is caused by shadowing from the inner rim. The fluctuations in value of the column density as a function of radius are smoothed out when FUV and X-ray luminosities increase. Ne+ and other heavy elements with an ionization potential higher than IP > 13.6 eV 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 = 1029 erg s-1) to the highest considered X-ray flux (LX = 1032 erg s-1). FUV confines the Ne+ ionized region to areas closer to the star at low X-ray luminosities (LX = 1029 erg s-1). This is indirectly caused by changes in the disk structure. The radial column densities of Ne+ are higher than 1012 cm-1 out to radii r > 100 AU (at LFUV ≥ 1031 erg s-1), whereas the column density already drops below this value at radii r > 20 AU at LFUV = 1032 erg s-1. H2O abundances are enhanced by X-rays due to higher temperatures in the inner disk than in the FUV only case, thus leading to a more efficient neutral-neutral formation channel. Also, the higher ionization fraction provides an ion-molecule route 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. Appendix B is available in electronic form at http://www.aanda.org