On the observability of T Tauri accretion shocks in the X-ray band

Context. High resolution X-ray observations of classical T Tauri stars (CTTSs) show a soft X-ray excess due to high density plasma (ne = 1011-113 cm-3). This emission has been attributed to shock-heated accreting material impacting onto the stellar surface. Aims: We investigate the observability of the shock-heated accreting material in the X-ray band as a function of the accretion stream properties (velocity, density, and metal abundance) in the case of plasma-β ≪ 1 (thermal pressure ≪ magnetic pressure) in the post-shock zone. Methods: We use a 1-D hydrodynamic model describing the impact of an accretion stream onto the chromosphere of a CTTS, including the effects of radiative cooling, gravity stratification and thermal conduction. We explore the space of relevant parameters and synthesize from the model results the X-ray emission in the [0.5-8.0] keV band and in the resonance lines of O vii (21.60 Å) and Ne ix (13.45 Å), taking into account the absorption from the chromosphere. Results: The accretion stream properties largely influence the temperature and the stand-off height of the shocked slab and its sinking in the chromosphere, determining the observability of the shocked plasma affected by chromospheric absorption. Our model predicts that X-ray observations preferentially detect emission from low density and high velocity shocked accretion streams due to the large absorption of dense post-shock plasma. In all the cases examined, the post-shock zone exhibits quasi-periodic oscillations due to thermal instabilities with periods ranging from 3×10-2 to 4×103 s. In the case of inhomogeneous streams and β ≪ 1, the shock oscillations are hardly detectable. Conclusions: We suggest that, if accretion streams are inhomogeneous, the selection effect introduced by the absorption on observable plasma components may easily explain the discrepancy between the accretion rate measured by optical and X-ray data as well as the different densities measured using different He-like triplets in the X-ray band.