Far-ultraviolet and X-ray irradiated protoplanetary disks: a grid of models

I. The disk structure^⋆

¹ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: meijerink@astro.rug.nl
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, Netherlands
³ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR5274, 38041 Grenoble, France
⁴ SUPA, School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, UK

Received: 22 June 2012
Accepted: 1 August 2012

Abstract

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 (L_X = 10²⁹ erg s^-1) to the highest considered X-ray flux (L_X = 10³² erg s^-1). FUV confines the Ne⁺ ionized region to areas closer to the star at low X-ray luminosities (L_X = 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 L_FUV ≥ 10³¹ erg s^-1), whereas the column density already drops below this value at radii r > 20 AU at L_FUV = 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.