Simulations of stellar convection and accretion of planetary debris

For a start in 2025

This overarching research project will address the lack of a realistic model of convective overshoot, which is a crucial concern for a wide range of astrophysical topics. So far, most studies of stars have relied upon a 60-year old, 1D prescription of convection, although it has been shown to have a major shortcoming in that it produces an on/off discontinuity at the top and base of the unstable convective layers. It completely neglects convective overshoot; the phenomenon where 3D plumes travel outside the unstable regions. This ambitious goal requires the development of new 3D radiation-hydrodynamics simulations using the CO5BOLD code, such as that shown on the image above. The goals are to 1) aim towards a universal physical description of convective overshoot, following-up on the work of my research group, that can be used to study the evolution of a wide range of stars, 2) improve numerical techniques in 3D stellar simulation codes that can enable further research from a large user base, and 3) measure the composition of rocks accreting onto white dwarfs, including life-forming elements such as water, carbon and silicon, with the same level of accuracy as for the meteorites in our own Solar system.

A large fraction of known white dwarfs, the compact remnants of stars like the Sun, are presently breaking up and absorbing fragments of minor planets. These frequent episodes inform us about the fate of planetary systems and provide the only possible way to characterise the bulk chemical composition of rocky exoplanets. My team has released three press releases on these topics since 2020. This project will rely on these state-of-the-art 3D numerical simulations of the stellar surface to dissect these accretion events to an unprecedented level of accuracy, leading to the first robust characterisation of the diversity in the composition of rocky exoplanets across Galactic age and stellar mass.

Gaia has identified 400,000 stars and white dwarfs within 100 pc of the Sun, and the Warwick group is making a major observational effort by leading spectroscopic follow-up surveys for all these objects starting from 2024 (DESI, SDSS-V, WEAVE, and 4MOST). It is now very timely to improve the stellar models that will be at the forefront of the stellar revolution triggered by Gaia and spectroscopic follow-ups.

• You will start the project by using existing CO5BOLD 3D model atmospheres to provide an analytical model of convective overshoot bulk velocity as a function of density, temperature and depth in the stellar interior.
• You will then use the new MPI parallel computing capabilities of CO5BOLD to calculate new 3D radiation-hydrodynamics simulations for cooler white dwarfs with deeper convection zones, aiming towards an universal model of convective overshoot.
• (Option A - Evolved planetary systems) You will apply these 3D model atmospheres to new metal polluted white dwarf spectra from WEAVE, and 4MOST to extract the most accurate rocky debris composition, such as crust and water fraction.
• (Option B - Stellar evolution) You will continue to lead the improvement of stellar models, using the very precise 100 pc sample as a benchmark for stellar age calibration. You could tackle known issues in white dwarf cooling and crystallisation and their dense atmospheres, or work on 3D convection for planet host M dwarf main-sequence stars.

The successful candidate will join my established team of postdocs and PhD students. The student will receive support from more 20 local experts on white dwarfs, exoplanets and 3D convection, as well as additional support from collaborators in the UK, Germany, Spain, Sweden and more. The PhD project is well suited for a student interested in stellar physics and exoplanets, computing and large data sets. The successful candidate is not expected to have specific prior experience with high-performance computing or white dwarfs. It is possible to tailor the projects to the skills and ambitions of the candidate.

For more information: contact Pier-Emmanuel Tremblay