Accretion shocks in Young Stellar Objects: From laboratory experiments to the astrophysical context – STARSHOCK

Accretion is the one of 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 shows indirect signatures such as veiling, line dilution, spectral evidence of velocity fields and x-ray emission. 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 need for multidimensional numerical simulations is therefore inevitable. However it is necessary in that respect to use numerical tools capable of coupling the radiation transport with hydrodynamics and magneto-hydrodynamics. We propose to develop the first global interdisciplinary study of the accretion shocks in YSOs, which combines laboratory experiments and multidimensional numerical simulations; the aim is to understand the dynamics and the radiative properties of accretion columns near the photosphere and to calculate the emerging spectra. More precisely, the projects aims at describing the role of magnetic fields and of radiation on the structure of the flows and shocks, their dynamics and stability. With the aid of our current multidimensional radiative- magneto-hydrodynamic codes we will be able to go beyond the current approximations, and to verify the assumption made. Coupled with the a post-processing 3D radiative transfer code developed within the project, we will compare the spectral properties, at low or high resolution, predicted by the simulations to actual observations in the visible, UV and X-rays. Those studies will be constrained by experiments on the radiative shocks which we will perform 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. Experiment may also point to new physics not included in the model, as it was the case in earlier work. A limited number of published observations will provide a guide to our studies. We shall be able to produce multi-wavelength synthetic spectra of accretion shocks. We will put particular stress on the x-ray part of the spectrum, which traces hottest plasma in the accretion shock. Those studies, together with improved numerical tools and experimental verification, will lead to a better physical understanding and possibly improved mass accretion rates. These studies, performed with the best numerical simulation tools and experimentally settled, will allow to revisit the determination of the accretion rate. The hydrodynamical self consistent structures and the associated spectral data will be freely accessible to astrophysical community. These studies will also deliver an open source 3D radiative transfer code which will valorize the recent french effords in the field of multidimensional hydrodynamics and will be a precious tool for the quantitative studies which are required to garantee the scientific return of present and future observations.