2008A&A...491L..17S

High spectral resolution X-ray observations of classical T Tauri stars (CTTSs) demonstrate the presence of plasma at temperature T∼2-3x10⁶K and density n_e∼10¹¹-10¹³cm^–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. 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. 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. We find that the accretion shock generates an hot slab of material above the chromosphere with a maximum thickness of 1.8x10⁹cm, density n_e∼10¹¹-10¹²cm^–3, temperature T∼3x10⁶K, and uniform pressure equal to the ram pressure of the accretion flow (∼450dyn/cm²). 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^–10M_☉/yr, the shocked region emits a time-averaged X-ray luminosity of L_X≃7x10²⁹erg/s, 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.