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TOFU Report Summary

Project ID: 320478
Funded under: FP7-IDEAS-ERC
Country: United Kingdom

Mid-Term Report Summary - TOFU (Toward a new generation of multi-dimensional stellar evolution models: the TOol of the FUture)

The TOFU project is devoted to multi-dimensional stellar and planetary models, based on the development of appropriate numerical tools that allows the multi-dimensional description of a complete star (or planet) interior during timescales relevant to various stellar/planetary evolutionary phases. The major scientific motivation is to improve the description of key stellar/planetary physical processes and to solve long standing problems characterising the life of stars and planets. One innovative aspect of the project is the development of a time implicit approach as it allows the exploration of physical processes on spatial and temporal scales presently out of reach with current numerical codes. In this respect, we have developed and benchmarked within the framework of this project a fully compressible time implicit code MUSIC (Multidimensional Stellar Implicit Code) recently improved with the implementation of a Jacobian-free Newton-Krylov time integration method. The code is particularly well adapted to the description of stars and planets, as it includes all micro-physics (equation of state, radiative opacities) appropriate for the description of these objects interiors and atmospheres.

Based on these developments, we have performed the first hydrodynamics simulations describing the multi-dimensional structure of accreting, convective low mass stars. Accretion is an important process relevant to various fields in astrophysics, from star formation to the study of compact binaries and supernovae type I. Many studies have been devoted to the effect of accretion on the structure and evolution of objects, using either simple semi-analytic approaches or simulations based on one dimensional (1D) stellar evolution codes. But no multi-dimensional numerical study has ever been devoted to the effect of accretion on the structure of accreting objects. Because of this lack of sophisticated modelling, great uncertainties still remain regarding the fraction of accretion luminosity imparted to an accreting object or radiated away, and how the accreted material is redistributed over the stellar surface and the depth it reaches. Early accretion history may impact the properties of stellar objects (luminosity, radius, and effective temperature) in star formation regions and young clusters even after several Myr when the accretion process has ceased. These effects could provide an explanation for several observed features in star-forming regions and young clusters, having important consequences on our general understanding of star formation, an active field of research in Astrophysics.
Our multi-dimensional accretion simulations enable to understand the effect of so-called “hot” accretion, when the accreted material is characterised by a larger entropy than that of the bulk of the stellar material. The multi-D simulations show an accumulation of hot material at the surface that does not sink and instead produces a hot surface layer, which tends to suppress convection in the stellar envelope. These new simulations enable to derive an improved treatment of accretion in 1D stellar evolution models, based on an accretion boundary condition. Such treatment of accretion is more physical than the standard assumption of redistribution of the accreted energy within the stellar interior used in the literature. The impact of this more physical accretion boundary condition on the evolution of young low mass stars and brown dwarfs and on observations in young clusters need to be further explored in follow-up studies. These pioneering simulations show the potential of using time implicit multi-D simulations to improve our understanding of stellar physics et evolution. Our results contribute to the so-called 321D link where multi-dimensional simulations enable to improve 1D stellar evolution calculations, which are still required to produce “observables” and thus for a direct link between models and observations.

Another achievement of this project is linked to a thorough study of turbulent convection in deep stellar/planetary interiors. Convection plays a crucial role on the structure and properties of stars and planets (for e.g energy transport and chemical mixing). We have performed two-dimensional hydrodynamic implicit large-eddy simulations of compressible convection in a young Sun using MUSIC. Simulations in spherical shells, that have different radial extents and under realistic stellar conditions, are performed over long times, permitting the collection of well-converged statistics. We analyze the dynamical pattern of non-uniform stellar convection by examining statistics of the convective turnover time, the radial profiles of velocity, and the overshooting layer width, which characterises the boundary between a convective zone and a stable, radiative zone. These quantities are selected for their relevance to one-dimensional stellar evolution calculations. We find that the inclusion of near-surface layers in the spherical shell can increase the amplitude of convective velocities and can affect the width of the overshooting layer. These results provide support for non-local aspects of convection. They have important impact on studies of stellar convection based on hydrodynamical simulations omitting some portion of a star (like the surface layers or the deeper layers) or based on simple, non physical boundary conditions.

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United Kingdom
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