Pier-Emmanuel Tremblay’s project will develop a robust theoretical framework that can underpin the observations of white dwarfs polluted by exoplanet debris
Snapshot of convective velocities in a vertical slice from a 3D simulation of a pure-hydrogen atmosphere white dwarf at a temperature of 12,000 degrees (Kelvin) and a surface gravity 100,000 times greater than that on the Earth. The horizontal solid line represents the classical 1D definition (Schwarzschild boundary) of the convective layers. This demonstrates that 3D convective overshoot contributes significant gas flows outside of the 1D convection zone. Credit: University of Warwick/Tim Cunningham.
Open questions such as “Are there other habitable worlds?” and “Is there life elsewhere in the Universe?” depend on the internal chemical composition of planets outside of our solar system. For the foreseeable future, this is impossible to observe directly. However, many of the known exoplanets will outlive their host stars becoming compact white dwarfs, the cooling embers of most former nuclear-burning main-sequence stars in the Milky Way. In our solar system, Mars, Jupiter, Saturn, Uranus and Neptune are expected to survive the Sun. Gravitational interactions between these planets will then scatter asteroids, comets and possibly some of the moons and planets themselves onto the central white dwarf, where they will be accreted.
My project will rely on state-of-the-art three-dimensional (3D) numerical simulations of the white dwarf surface to dissect these frequent planetesimal accretion events to an unprecedented level of accuracy. As many as 95% of these stellar remnants have a convective surface made of pristine hydrogen/helium gas, in which metallic debris plunge and become mixed by bulk fluid motions in a matter of hours. So far, almost all studies 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. I will address the lack of a realistic model of convective overshoot, which is a crucial concern not only for studies of white dwarfs but also for a wide range of astrophysical topics, including how stars process the products of nuclear burning, grow old and pulsate.
My research area will soon progress rapidly thanks to the spacecraft Gaia of the European Space Agency which has provided, for the first time, a near complete census of all stellar systems within 300 light-years of the Sun, including thousands of white dwarfs polluted by asteroid debris. Using these data and spectroscopic follow-ups, I will lead a robust characterisation of the diversity in the composition of rocky exoplanets across Galactic age and stellar mass. This includes chemical abundances for life-forming elements such as water, carbon and silicon, which can then be compared to the properties of meteorites in our own solar system and planet formation models.
