Visualisation of the flow in a convective core.

Visualisation of the flow in the transition region between a convective envelope and the radiative core.

PhD opportunity: Understanding mixing processes in stars with hydrodynamic simulations

Supervisor: Professor Isabelle Baraffe

Stellar evolution models are fundamental to nearly all fields of astrophysics. They are used to derive distances and ages of galactic/extragalactic clusters, to study galactic stellar populations and galaxy evolution. They are also fundamental to determine timescales for star/planet formation or to determine the radius of transiting exoplanets. But many complex stellar physics processes (e.g turbulent convection, mixing, rotation) are still poorly understood and only phenomenologically described. One of the major uncertainties in stellar evolution models, and the main focus of this project, is the treatment of mixing taking place at convective boundaries. Convective motions do not abruptly stop at the classical Schwarzschild boundary, but extend beyond it. The complex dynamics resulting from turbulent convection penetration in stable layers is a major process in stars that drives the transport of chemical species and heat, strongly affecting the structure and the evolution of many types of stars. This process is usually called ”overshooting” or ”convective boundary mixing”. It is one of the oldest unsolved problems of stellar evolution theory and affects all stars that develop a convective envelope or/and core (see figures).

The project is devoted to the study of this mixing process in stars of different masses and stages of evolution, based on multi-dimensional hydrodynamic simulations using the MUSIC code developed by the supervisor and her team. The main objective of this project is to derive new physically-based transport coefficients and parametrizations for one-dimensional stellar evolution models, which can be tested against a wide range of available observations. Our team has developed an original approach to analyse flows that penetrate in overshooting layers, based on statistical approaches to describe extreme events (e.g. extreme value theory) that are commonly used in climate science or in finance. The student will further develop this approach during the PhD project.

During this project, the student will gain a rich expertise in physical processes that take place in stellar interiors, numerical simulations and statistical methods.

For more details on this project please contact Prof Isabelle Baraffe.