Welcome

Accretion is the most efficient energy release mechanisms in the cosmos and it is often associated with powerful hydrodynamic phenomena, which in the case of young stars can take the form of parsec scale jets. Accretion and ejection phenomena dominate stellar formation. Althought the mass ejection rate in young stars can be reasonably constrained by multi-wavelength observations, the accretion rate rests on physics of accretion flows and shocks which is less certain, both observationally and theoretically.

Accretion on young stars, like classical T-Tauri stars, is supposed to occur by chanelling, through magnetospheric columns, the disk matter at high velocities of about 500 km/s to the stellar photosphere. This induces a strong shock with a high temperature of about 105K with associated spectral signatures. The indirect signatures are for instance the veiling, line dilution, spectral evidence of velocity fields and x-ray emission. They are used to derive the accretion rate which is typically 10-8 solar mass per year.

In spite of the theoretical and observational activity, a number of fundamental questions remain pending: the geometry and the stability of the accretion columns, the location of the accretion shock, its structure and stability, the importance of the magnetic field on the shock and of phenomena like the reconnection on the stellar surface. Understanding these and other issues is hampered by the limitation in angular resolution that can be obtained on the current facilities.

The aim of the ANR STARSHOCK (2009-2013) is to understand the dynamics and the radiative properties of accretion columns near the photosphere and to calculate the emerging spectra. This supposes an accurate modelling of the accretion shock, based on 3D radiative hydrodynamical studies and also the modelling of the radiative signatures ( photometry and spectroscopy ) of the strong radiative shock.

These studies will be constrained by experiments on the radiative shocks performed on the large-scale experimental plasma facilities present in Europe. Under similar conditions to the accretion shocks, the experiments will serve to study the small scale structure of radiative shocks, which is not directly observable, and to test the numerical codes : in particular the coupling of the radiation transport with the hydrodynamics and the capacity of the radiative transfer code to reproduce the spectral profiles obtained experimentally.

C. STEHLÉ, LERMA
ANR Scientific Coordinator