X-ray emitting MHD accretion shocks in classical T Tauri stars - Case for moderate to high plasma-β values

摘要

Context. Plasma accreting onto classical T Tauristars (CTTS) is believed to impact the stellar surface at free-fallvelocities, generating a shock. Current time-dependent modelsdescribing accretion shocks in CTTSs are one-dimensional, assuming thatthe plasma moves and transports energy only along magnetic field lines(). Aims. We investigate the stability and dynamics ofaccretion shocks in CTTSs, considering the case of in the post-shock region. In these cases the 1D approximation is notvalid and a multi-dimensional MHD approach is necessary. Methods. We model an accretion stream propagatingthrough the atmosphere of a CTTS and impacting onto its chromosphere byperforming 2D axisymmetric MHD simulations. The model takes intoaccount the stellar magnetic field, the gravity, the radiative cooling,and the thermal conduction (including the effects of heat fluxsaturation). Results. The dynamics and stability of the accretionshock strongly depend on the plasma.In the case of shocks with 10$ --> ,violent outflows of shock-heated material (and possiblyMHDwaves) are generated at the base of the accretion columnand intensely perturb the surrounding stellar atmosphere and theaccretion column itself (therefore modifying the dynamics of theshock). In shocks with ,the post-shock region is efficiently confined by the magnetic field.The shock oscillations induced by cooling instability are stronglyinfluenced by:for 10$ --> ,the oscillations may be rapidly dumped by the magnetic field,approaching a quasi-stationary state, or may be chaotic with no obviousperiodicity due to perturbation of the stream induced by the post-shockplasma itself; for the oscillations are quasi-periodic, although their amplitude issmaller and the frequency higher than those predicted by1Dmodels. Key words: accretion, accretion disks -magnetohydrodynamics (MHD) - shock waves - stars: pre-main sequence -X-rays: stars - instabilities