| Abstract
| - Context. T Tauri stars have X-ray luminosities in the range LX = 10 28−10 32 erg s -1. These luminosities are similar to their UV luminosities ( LUV ~ 10 30−10 31 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 LX ≥ 10 29 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 H 2 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 +, H 2O +, and H 3O + 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 +, H 2O + and H 3O +, 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 LX > 10 30 erg s -1.
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