X-ray emission from dense plasma in CTTSs: 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 T ∼ 2−3×106 K and ne ∼ 1011−1013 cm−3, unobserved in non-accreting stars. Stationary models suggest that this emission is due to shockheated accreting material, but they do not allow to analyze the stability of such material and its position in the stellar atmosphere. Aims. We investigate the dynamics and the stability of shock-heated accreting material in classical T Tauri stars and the role of the stellar chromosphere in determining the position and the thickness of the shocked region. Methods. We perform 1-D hydrodynamic simulations of the impact of the accretion flow onto 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. Here 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 stays where 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 LX ≈ 7 × 1029 erg s−1, which is comparable to the X-ray luminosity observed in CTTSs of the same mass. Furthermore, the X-ray spectrum synthesized from the simulation matches in detail all the main features of the O viii and O vii lines of the star MP Mus.