Periodic Reporting for period 2 - COBOM (Convective Boundary Mixing in Stars)
Reporting period: 2020-03-01 to 2021-08-31
Our first results for solar-like models that will be published in several papers are "non conventional", since they are not obtained within the usual "ideal" framework. As a major finding, our results highlight the importance of the impact of penetrative downflows on the thermal background below the convective boundary. They reveal a modification of the thermal background by penetrative down-flows and a local heating in the overshooting layer. This is a new feature that could have significant impacts on stellar structures and is subject to follow-up analyses.
We have also applied the same approach to the study of convective cores for a wide range of stellar masses. 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 first results show that the efficiency of the overshooting process above the convective core increases with stellar mass and stellar luminosity, as suggested by observations. 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.
A new result is the identification of a process of local heating in the overshooting layer of solar-like models. This local modification of the thermal stratification could provide a solution to a long standing problem in stellar physics, known as the "solar modelling problem" which points to a well-known discrepancy between solar models and helioseismology data. We are performing follow-up studies to confirm this result. This discovery was unexpected and, if confirmed, opens a wide range of implications for stellar structures in general.
The next steps involve the analysis of the effect of dimensionality (3D versus 2D), rotation and magnetic field on the efficiency of the overshooting process for convective envelopes and cores in order to provide the most comprehensive analysis of convective boundary mixing in stars ever performed. We are also expecting to demonstrate by the end of the project the success of applying our numerical framework to applications under very different conditions characterising planetary interiors and geophysical problems.