Letter to the Editor

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

¹ Consorzio COMETA, via S. Sofia 64, 95123 Catania, Italy e-mail: sacco@astropa.inaf.it
² INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ DSFA - Università degli Studi di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

Received: 5 August 2008
Accepted: 1 October 2008

Abstract

Context. High spectral resolution X-ray observations of classical T Tauri stars (CTTSs) demonstrate the presence of plasma at temperature  K and density  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  cm, density  cm^-3, temperature  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 ~ yr^-1, the shocked region emits a time-averaged X-ray luminosity of  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 and  lines of the star MP Mus.