X-ray optical depth diagnostics of T Tauri accretion shocks

et al. 2013

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Context. According to the magnetospheric accretion model, hot spots form on the surface of classical T Tauri stars (CTTSs) in regions where accreting disk material impacts the stellar surface at supersonic velocity, generating a shock. Aims. We investigate the dynamics and stability of postshock plasma that streams along nonuniform stellar magnetic fields at the impact region of accretion columns. We study how the magnetic field configuration and strength determine the structure, geometry, and location of the shock-heated plasma. Methods. We model the impact of an accretion stream onto the chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our model considers the gravity, the radiative cooling, and the magnetic-field-oriented thermal conduction (including the effects of heat flux saturation). We explore different configurations and strengths of the magnetic field. Results. The structure, stability, and location of the shocked plasma strongly depend on the configuration and strength of the magnetic field. In the case of weak magnetic fields (plasma β > ∼ 1 in the postshock region), a large component of B may develop perpendicular to the stream at the base of the accretion column, which limits the sinking of the shocked plasma into the chromosphere and perturbs the overstable shock oscillations induced by radiative cooling. An envelope of dense and cold chromospheric material may also develop around the shocked column. For strong magnetic fields (β < 1 in the postshock region close to the chromosphere), the field configuration determines the position of the shock and its stand-off height. If the field is strongly tapered close to the chromosphere, an oblique shock may form well above the stellar surface at the height where the plasma β ≈ 1. In general, we find that a nonuniform magnetic field makes the distribution of emission measure vs. temperature of the postshock plasma at T > 10 6 K lower than when there is uniform magnetic field. Conclusions. The initial magnetic field strength and configuration in the region of impact of the stream are expected to influence the chromospheric absorption and, therefore, the observability of the shock-heated plasma in the X-ray band. In addition, the field strength and configuration also influence the energy balance of the shocked plasma with its emission measure at T > 10 6 K, which is lower than expected for a uniform field. The above effects contribute to underestimating the mass accretion rates derived in the X-ray band.

Section: Distributions Of Emission Measure Vs Temperaturesupporting

confidence: 84%

“…As shown, for instance, by Argiroffi et al (2009) these distributions allow straightforwardly to compare the model results with the observations of accretion shocks in CTTSs. Figure 9 shows the EM(T ) distributions averaged over 3 ks (i.e.…”

Section: Distributions Of Emission Measure Vs Temperaturementioning

confidence: 95%

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

Orlando

Bonito

et al. 2013

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“…On the other hand, the mass accretion rate of MP Mus, as deduced from optical observations, isṀ opt ≈ 3 × 10 −9 M yr −1 (Argiroffi et al 2009), which exceeds by more than one order of magnitude the value obtained from X-rays. Similar Fig.…”

Section: Mass Accretion Ratesmentioning

confidence: 62%

“…This idea has been challenged recently by Argiroffi et al (2009), who compared the distributions of emission measure EM(T ) of two CTTSs with evidence of X-ray emitting dense plasma (MP Mus and TW Hya) with that of a star (TWA 5) with no evidence of accretion (i.e. only the coronal component is present) and with the EM(T ) derived from a 1D hydrodynamic model of accretion shocks (i.e.…”

Section: Distribution Of Emission Measure Vs Temperature Of the Shocmentioning

confidence: 99%

X-ray emitting MHD accretion shocks in classical T Tauri stars

Orlando¹,

Sacco

et al. 2010

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103

Context. Plasma accreting onto classical T Tauri stars (CTTS) is believed to impact the stellar surface at free-fall velocities, generating a shock. Current time-dependent models describing accretion shocks in CTTSs are one-dimensional, assuming that the plasma moves and transports energy only along magnetic field lines (β 1). Aims. We investigate the stability and dynamics of accretion shocks in CTTSs, considering the case of β > ∼ 1 in the post-shock region. In these cases the 1D approximation is not valid and a multi-dimensional MHD approach is necessary. Methods. We model an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere by performing 2D axisymmetric MHD simulations. The model takes into account the stellar magnetic field, the gravity, the radiative cooling, and the thermal conduction (including the effects of heat flux saturation). Results. The dynamics and stability of the accretion shock strongly depend on the plasma β. In the case of shocks with β > 10, violent outflows of shock-heated material (and possibly MHD waves) are generated at the base of the accretion column and intensely perturb the surrounding stellar atmosphere and the accretion column itself (therefore modifying the dynamics of the shock). In shocks with β ≈ 1, the post-shock region is efficiently confined by the magnetic field. The shock oscillations induced by cooling instability are strongly influenced by β: for β > 10, the oscillations may be rapidly dumped by the magnetic field, approaching a quasi-stationary state, or may be chaotic with no obvious periodicity due to perturbation of the stream induced by the post-shock plasma itself; for β ≈ 1 the oscillations are quasi-periodic, although their amplitude is smaller and the frequency higher than those predicted by 1D models.

“…This component could be produced by the shock-heated plasma at the base of the accretion column, the plasma density being much higher than that (n e ≤ 10 10 cm −3 ) measured for coronae of active stars (Testa et al 2004). The discovery of optical depths effects on the X-ray spectrum of MP Mus further supports this hypothesis (Argiroffi et al 2009). …”

Section: Introductionmentioning

confidence: 63%

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

Sacco

Orlando

et al. 2010

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114

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.