Abstract
Context.Material accreted onto classical T Tauri stars is expected to form a hot quasi-periodic plasma structure that radiates in X-rays. Simulations of this phenomenon only partly match observations. They all rely on a static model for the chromosphere and on the assumption that radiation and matter are decoupled.Aims.We explore the effects of a shock-heated chromosphere and of the coupling between radiation and hydrodynamics on the structure and dynamics of the accretion flow.Methods.We simulated accretion columns that fall onto a stellar chromosphere using the 1D ALE code AstroLabE. This code solves the hydrodynamics equations along with the first two moment equations for radiation transfer, with the help of a dedicated opacity table for the coupling between matter and radiation. We derive the total electron and ion densities from collisional-radiative model.Results.The chromospheric acoustic heating affects the duration of the cycle and the structure of the heated slab. In addition, the coupling between radiation and hydrodynamics leads to a heating of the accretion flow and of the chromosphere: the whole column is pushed up by the inflating chromosphere over several times the steady chromosphere thickness. These last two conclusions are in agreement with the computed monochromatic intensity. Acoustic heating and radiation coupling affect the amplitude and temporal variations of the net X-ray luminosity, which varies between 30 and 94% of the incoming mechanical energy flux, depending on which model is considered.
Highlights
Classical T Tauri stars (CTTSs) are solar-type pre-mainsequence stars that are surrounded by a thick disc composed of gas and dust
We only considered hydrogen (H I, H II) and helium (He I, He II, He III); the chemical composition is completed by a “catch-all” metal “M” with a number abundance of 0.12% and a mass of 17 u
The time-averaged surface luminosity (Eq (9)) is here equal to 4.0 × 109 erg cm−2 s−1, namely 94% of the mechanical energy flux FM. These results show that compared to the reference case, the dynamical heating of the chromosphere affects the duration of the quasi-periodic oscillations (QPOs) period and its observability
Summary
Classical T Tauri stars (CTTSs) are solar-type pre-mainsequence stars that are surrounded by a thick disc composed of gas and dust (see e.g. Feigelson & Montmerle 1999). 2002 and Stelzer & Schmitt 2004 for TW Hya, Schmitt et al 2005 for BP Tau, Günther et al 2006 for V4046 Sgr, Argiroffi et al 2007, 2009 for MP Mus, Robrade & Schmitt 2007 for RU Lup, and Huenemoerder et al 2007 for Hen 3-600) Another signature is the UV and optical veiling, which is attributed to the post-shock medium, the heated atmosphere, and the pre-shock medium (Calvet & Gullbring 1998). The energy balance between radiation and gas in the lower stellar atmosphere is replaced by a nonphysical tuning (heating function, off threshold, etc.). Such an assumption may affect the sinking of the post-shock structure as well as the accretion structure itself. We detail in Sect. 2.2.3 the two extreme radiative regimes we encountered in this context and describe
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