Stellar seismology: Models and numerical simulations of waves in rotating stars

Supervisors: Professor Isabelle Baraffe and Dr Thomas Guillet

Stellar seismology (also called Asteroseismology) is a powerful approach which allows to probe the deep interior of stars. Many observational efforts are devoted to the detection of pulsation modes (i.e waves) at the surface of various types of stars. The detection of waves propagating from the deep interior to the surface (see Fig.1) provides extremely valuable information about the internal structure of a star. Acoustic waves, driven by pressure variations, and gravity waves, driven by buoyancy forces, can be excited by convection in a convective core where nuclear burning proceeds (for example in massive stars) or in a convective envelope (like in our Sun), and detected at the stellar surface. The Coriolis force can also become a dominant restoring force for gravity modes in rotating stars. One defines the gravito-inertial modes for which both Coriolis force and buoyancy are important. These waves generated in the deep interior of massive stars can provide key information about e.g. the size of the convective core and thus test our understanding of stellar structure and evolution.

The study of the processes by which these waves are excited in convective regions and how they propagate to the surface is an active field of research in Astrophysics, boosted by the current and coming space missions (Kepler, TESS, PLATO). Various theories exist describing wave excitation mechanisms and predicting different power spectrum of the waves which can now be confronted to advanced numerical simulations and to observations (see Fig.2).

The general goal of the project is to advance on the physical understanding of the excitation mechanisms of waves driven by convective processes, using both analytical and numerical approaches. We are performing in the Exeter group multi-dimensional hydrodynamic simulations of stellar convection in rotating and non rotating stars. One goal of the PhD project is to improve existing analytical theories describing wave excitation mechanisms using current observations and the results of multi-dimensional hydrodynamic simulations of stellar convection. Analysing the impact of rotation on the excitation mechanisms and on the properties of waves is another important aim of this project.

This PhD project will allow the candidate to develop a good understanding of stellar evolution and oscillations and to gain insight into the theory of wave excitation by convective processes in stars. The candidate will also get familiar with our hydrodynamical code MUSIC (MUlti-dimensional Stellar Implicit Code) and with the python tools developed in the team to extract from the output of hydrodynamical simulations of convection the properties of waves (frequency, power spectrum, etc.).