The first step of our project is to perform a systematic numerical analysis of envelope convection and convective boundary mixing in solar like models, including our own Sun.
Our first achievement is the ability to perform fully compressible time implicit simulations with a new numerical tool, namely the MUSIC code developed by our team, for a wide range of stellar interiors and based on realistic geometries and stellar structures. Our numerical approach is novel compared to more conventional works based on ideal setups (cartesian box, ideal physics) and/or on assumptions and simplifications regarding the resolution of the hydrodynamical equations. We also combine the power of such numerical simulations with a novel approach to analyse the results, namely using rare event statistics to analyse convective, penetrative plumes responsible for mixing. This statistical method is usually applied in finance or climate science. As a major finding, our results highlight the importance of the impact of penetrative downflows on the thermal background below the convective boundary of a solar-like model. They reveal a modification of the thermal background by penetrative down-flows and a local heating in the overshooting layer.
We have also applied the same approach to the study of convective cores for a wide range of stellar masses. As a important finding, we show that we can successfully apply the same statistical analysis based on rare events to convective cores. As found for penetrative down-flows for convective envelopes, we can confirm the existence of extreme and rare events of penetrating up-flows that penetrate much further than the average penetrating flow. Our results show that the efficiency of the overshooting process above the convective core increases with stellar mass and stellar luminosity, which is a property suggested by observations of many stars. We provide a numerically calibrated relationship describing the overshooting width as a function of the stellar luminosity. Such a relationship can be implemented in stellar evolution codes and is very much needed by the stellar community.
We have also developed a sophisticated tool to analyse the properties of internal waves generated by convection. An important result concerns the analysis of waves generated at the boundary of a convective core in a massive star model. We show that in contradiction to recent claims, these waves will most likely be damped in the stellar interior before reaching the surface and cannot explain the photometric variability observed in many such stars. We performed the first numerical simulations which show this strong damping of waves, which was predicted by the theory more than forty years ago.
All our results were published in peer review journals and presented at about 40 international conferences and workshops.
