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

Abstract

Context. High resolution X-ray observations of classical T Tauri stars (CTTSs) show a soft X-ray excess due to high density plasma (n e = 10 11 −10 13 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 postshock 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 × 10 3 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.

Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars

“…As discussed below, the evolution of the accretion shock and of the postshock plasma is analogous to that described in Paper I when the magnetic field is weak (β > ∼ 1 in the postshock plasma; run B50-D11-Unif) and to that described by Sacco et al (2010) in the limit of strong magnetic field (β 1; run B500-D11-Unif). Figure 2 shows maps of temperature, density, and β for runs B50-D11-Unif and B500-D11-Unif at the labeled times.…”

“…In the present study, we restrict our analysis to accretion stream impacts that are able to produce detectable X-ray emission. Sacco et al (2010) have shown that X-ray observations preferentially reveal emission from the impact of low density (n str0 < ∼ 10 12 cm −3 ) and high velocity (|u str0 | > ∼ 300 km s −1 ) accretion streams due to the large absorption of dense postshock plasma. X-ray observations show that the above requirements are fitted, for instance, in the well-studied young accreting star MP Mus (Argiroffi et al 2007) which is an ideal test case for our analysis.…”

“…On the other hand, Sacco et al (2010) have shown that the denser the stream, the smaller the thickness of the shocked slab due to the larger efficiency of the radiative losses. In the case of heavy streams, therefore, we expect that the postshock plasma is located very close to the chromosphere (possibly even buried in it) and with a very small stand-off height.…”

Multiwavelength diagnostics of accretion in an X-ray selected sample of CTTSs

“…Recent hydrodynamical simulations (Sacco et al 2008;2010) have demonstrated that, even if the post-shock zone is not stationary (and characterized by quasi-periodic shock oscillations), the time-averaged properties of the post-shock zone are, in general, well-described by the stationary model. Sacco et al (2010) have also shown that the higher the post-shock temperature the larger the discrepancies between the stationary and the hydrodynamical models. This is mainly due to the fact that the thermal conduction (which is not taken into account in the stationary model and that is more efficient for higher post-shock temperatures) drains energy from the shock-heated plasma to the chromosphere through a thin transition region, and acts as an additional cooling mechanism (see also Orlando et al 2010).…”

Variable X-ray emission from the accretion shock in the classical T Tauri star V2129 Ophiuchi

“…From a theoretical point of view, it has been debated whether X-rays emitted by shock-heated plasma in CTTS are able to escape from the shock region, and hence be detected (Drake 2005;Sacco et al 2010). Their escape probability depends on the height of the shock surface in the stellar atmosphere and on the direction of emission.…”