X-ray impact on the protoplanetary disks around T Tauri stars

¹ Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
e-mail: giambattista.aresu@gmail.com
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
³ UK Astronomy Technology Center, Royal Observatory, Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ Laboratoire d’Astrophysique de Grenoble, CNRS/Université Joseph Fourier (UMR 5571), BP 53, 38041 Grenoble Cedex 9, France
⁶ SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory Edinburgh, UK

Received: 22 July 2010
Accepted: 24 November 2010

Abstract

Context. T Tauri stars have X-ray luminosities in the range L_X = 10²⁸−10³²   erg   s^-1. These luminosities are similar to their UV luminosities (L_UV ~ 10³⁰−10³¹   erg   s^-1) and therefore X-rays are expected to affect the physics and chemistry of the upper layers of their surrounding protoplanetary disks.

Aims. The effects and importance of X-rays on the chemical and hydrostatic structure of protoplanetary disks are investigated, species tracing X-ray irradiation (for L_X ≥ 10²⁹   erg   s^-1) are identified and predictions for [O i], [C ii], and [N ii] fine structure line fluxes are provided.

Methods. We implemented X-ray physics and chemistry into the chemo-physical disk code ProDiMo. We include Coulomb heating and H₂ ionization as heating processes and both primary and secondary ionization due to X-rays in the chemistry.

Results. X-rays heat the gas causing it to expand in the optically thin surface layers. Neutral molecular species are not significantly affected in terms of their abundance and spatial distribution, but charged species such as N⁺, OH⁺, H₂O⁺, and H₃O⁺ display enhanced abundances in the disk surface.

Conclusions. Coulomb heating by X-rays changes the vertical structure of the disk, yielding temperatures of  ~8000 K out to distances of 50 AU. The chemical structure is altered by the high electron abundance of the gas at the disk surface, causing an efficient ion-molecule chemistry. The products of this, OH⁺, H₂O⁺ and H₃O⁺, are of great interest for observations of low-mass young stellar objects with the Herschel Space Observatory. Both [O i] (at 63 and 145 μm) and [C ii] (at 158   μm) fine structure emission are affected only for L_X > 10³⁰ erg s^-1.