The structure and spectrum of the accretion shock in the atmospheres of young stars

Abstract

The structure and spectrum of the accretion shock have been self-consistently simulated for a wide range of parameters typical for Classical T Tauri Stars (CTTS). Radiative cooling of the shocked gas was calculated, taking into account the self-absorption and non-equilibrium (time-dependent) effects in the level populations. These effects modify the standard cooling curve for an optically thin plasma in coronal equilibrium, however the shape of high-temperature ($T>3\times 10^5$ K) part of the curve remains unchanged. The applied methods allow us to smoothly describe the transition from the cooling flow to the hydrostatic stellar atmosphere. Thanks to this approach, it has been found that the narrow component of HeII lines is formed predominantly in the irradiated stationary atmosphere (hotspot), i.e. at velocities of the settling gas $<2$ km s${}^{-1}$. The structure of the pre-shock region is calculated simultaneously with the heated atmosphere. The simulation shows that the pre-shock gas produces a noticeable emission component in HeII lines and practically does not manifest itself in HeI lines ($\lambda \lambda$ 5876, 10830 Å). The UV spectrum of the hotspot is distorted by the pre-shock gas, namely numerous red-shifted emission and absorption lines overlap each other forming a pseudo-continuum. The spectrum of the accretion region at high pre-shock densities $\sim 10^{14}$ cm${}^{-3}$ is fully formed in the in-falling gas and can be qualitatively described as a spectrum of a star with an effective temperature derived from the Stefan-Boltzmann law via the full energy flux.

12 pages, 9 figures. Accepted for publication in MNRAS

References in corpus (6)

• Magnetospheric accretion on the T Tauri star BP Tauri
• Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph
• X-ray emission from dense plasma in CTTSs: Hydrodynamic modeling of the accretion shock
• Oscillations of MHD shock waves on the surfaces of T Tauri stars
• Impacts of fragmented accretion streams onto Classical T Tauri Stars: UV and X-ray emission lines
• Non-LTE Modelling of the Structure and Spectra of the Hot Accretion Spots on the Surface of Young Stars

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• Orbital parameters and activity of ZZ Tau -- a low mass young binary with circumbinary disc
• Magnetospheric accretion at the late phases of the Pre-Main-Sequence evolution. The case of RZ Psc
• Continuum And Line Emission From Accretion Shocks at T Tauri Stars I. Correlations With Shock Parameters