A quarter of all white dwarf stars are actively swallowing the remains of their own planetary systems, and astronomers are now using these cosmic graveyards to read the chemical composition of worlds that no longer exist. Directly measuring what exoplanets are made of is currently impossible—telescopes can detect their size and orbit but not their elemental makeup. White dwarfs solve this problem: when a Sun-like star dies, its gravity pulls in nearby planetary debris, and the metals from that debris leave clear signatures in the star’s spectrum. But current one-dimensional models badly underestimate how convection stirs these atmospheres, leading to inaccurate chemical readings. This fellowship will build three-dimensional radiation hydrodynamic simulations that capture convective overshoot—turbulent motions that extend beyond their expected boundaries—in ageing white dwarfs older than 450 million years. With roughly 360,000 known white dwarfs and an estimated one-third actively accreting planetary material, better models will unlock a vast dataset on exoplanetary composition. This is fundamental science. It will not change a single product or service tomorrow. But understanding what planets are made of—including, eventually, what will happen to the material of our own Solar System when the Sun dies—is the kind of knowledge that reshapes how we think about our place in the universe.
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Understanding the bulk chemical composition of exoplanets is critical for advancing our knowledge of planetary formation and evolution. However, direct measurement of compositions for planets orbiting distant stars remains impossible. This limitation is addressed through the study of white dwarfs – the remnants of Sun-like stars – which offer a unique window into the composition of exoplanetary material once these stars die. White dwarfs form when a main-sequence star expels a significant portion of its mass, leaving it to collapse into a dense, compact object. The gravitational perturbation caused by this mass loss can redirect nearby planetary bodies to spiral inward and accrete onto the white dwarf's surface. Spectroscopic analyses reveal the presence of metals in over a quarter of white dwarf atmospheres, evidence of accreted debris from planetary disintegration. This process presents a rare opportunity to decipher the elemental make-up of these exoplanetary bodies. To harness this opportunity, accurate atmospheric models of white dwarfs are crucial, particularly in understanding how convection operates within these remnants. Traditional one-dimensional models significantly underestimate the extent and effects of convection, in particular convective overshoot – a phenomenon where turbulent convective motions extend beyond their expected boundaries. During this fellowship, I plan to address this gap by developing sophisticated three-dimensional radiation hydrodynamic (RHD) simulations, which provide a more realistic depiction of convection in white dwarf atmospheres, specifically focusing on convective overshoot and mixing processes. The primary focus will be on ageing white dwarfs, older than 450 Myr, where convective zones deepen and dynamic processes like internal gravity waves become significant. By producing state-of-the-art 3D models, I aim to better capture the behaviour of these evolving atmospheres and improve the accuracy of chemical abundance measurements derived from spectroscopic data. My interdisciplinary approach bridges fluid dynamics and astronomy, employing advanced computational techniques and leveraging novel observational strategies for the Galactic population of white dwarfs. These efforts will be aligned with the latest observational data from next-generation instruments, enhancing our ability to interpret white dwarf spectra and the history of accreted material. Currently, there are about 360,000 known white dwarfs, with an estimated one-third actively consuming remnants of exoplanetary systems. This provides an unparalleled dataset for examining the lifecycle of exoplanetary material and offers crucial insights into the ultimate fate of our own Solar System. My research will significantly advance our understanding of the elemental composition of exoplanets, contributing to the broader field of planetary science.
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