Astronomy and Astrophysics, volume 491, L17-20 (2008/11-4)
X-ray emission from dense plasma in classical T Tauri stars: hydrodynamic modeling of the accretion shock.
SACCO G.G., ARGIROFFI C., ORLANDO S., MAGGIO A., PERES G. and REALE F.
Abstract (from CDS):
High spectral resolution X-ray observations of classical T Tauri stars (CTTSs) demonstrate the presence of plasma at temperature T∼2-3x106K and density ne∼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 ne∼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 LX≃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.
X-rays: stars - stars: formation - accretion, accretion disks - hydrodynamics - shock waves - methods: numerical
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