X-ray emission from dense plasma in classical T Tauri stars: Hydrodynamic modeling of the accretion shock

Abstract

Context. High spectral resolution X-ray observations of classical T Tauri stars (CTTSs) demonstrate the presence of plasma at temperature T ∼ 2-3 × 106 K and density ne ∼ 1011-10 13 cm-3, which are unobserved in non-accreting stars. Stationary models suggest that this emission is due to shock-heated accreting material, but do not allow us to analyze the stability of the material and its position in the stellar atmosphere.Aims. We investigate the dynamics and stability of shock-heated accreting material in classical T Tauri stars and the role of the stellar chromosphere in determining the position and thickness of the shocked region.Methods. We perform one-dimensional hydrodynamic simulations of the impact of an accretion flow on the chromosphere of a CTTS, including the effects of gravity, radiative losses from optically thin plasma, thermal conduction and a well tested detailed model of the stellar chromosphere. We present the results of a simulation based on the parameters of the CTTS MP Mus.Results. We find that the accretion shock generates an hot slab of material above the chromosphere with a maximum thickness of 1.8 × 109 cm, density ne ∼ 1011-1012 cm-3, temperature T ∼ 3 × 106 K, and uniform pressure equal to the ram pressure of the accretion flow (∼450 dyn cm-2). The base of the shocked region penetrates the chromosphere and remains at a position at which the ram pressure is equal to the thermal pressure. The system evolves with quasi-periodic instabilities of the material in the slab leading to cyclic disappearance and re-formation of the slab. For an accretion rate of ∼10-10 M⊙ yr-1, the shocked region emits a time-averaged X-ray luminosity of LX ≈ 7 × 10 29 erg s-1, which is comparable with the X-ray luminosity observed in CTTSs of identical mass. Furthermore, the X-ray spectrum synthesized from the simulation reproduces in detail all the main features of the O VIII and OVII lines of the star MP Mus.. © 2008 ESO.