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

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?1011-1013?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? 1029?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 Oviii and Ovii?lines of the star MP?Mus.