Metal polluted white dwarfs
After more than twenty years after the discovery of the first planet outside our Solar System little is still known about the detailed composition of rocky, Earth-like exoplanets. Current detection techniques enable estimates of exoplanet radius and mass, using mainly the transit and radial velocity methods, but density alone does not yield a unique solution for the bulk composition of these planets.
In this context, white dwarfs offer a unique laboratory to study exoplanetary compositions. Asteroid polluted white dwarfs enable to derive accurate information on the chemical composition of rocky minor planets or asteroids, which are the building blocks of terrestrial exoplanets. Many years of research in this field have demonstrated that the metala observed at a fraction of about 30% of cool white dwarfs can only be explained the accretion of small planetesimals or large asteroids, which were likely part of larger rocky planetary bodies.
Due to high surface gravity and negligible radiative forces, heavy elements sink on short timescales within the atmospheres of relatively cool white dwarfs and hence the presence of metals in their atmospheres must be sign of external accretion. The source of this accreting material was initially attributed to the interstellar medium or comets, but both theories had problems in explaining the high accretion rates found at hydrogen dominated white dwarf. Today accretion from circumstellar dust, resulting from the tidal disruption of large asteroids or minor planets, is the only and most compelling explanation to the metal pollution seen at a large fraction of white dwarfs.
Nowadays, metal enriched white dwarfs have become a unique and powerful tool to indirectly analyse the composition of exoterrestrial matter, enabling to obtain detailed information on the chemistry of terrestrial exoplanets.
- M. Rocchetto, J. Farihi, B. T. Gaensicke, C. Bergfors (2014). The frequency and infrared brightness of circumstellar discs at white dwarfs arXiv:1408.0229 [arXiv]
- C. Bergfors, J. Farihi, P. Dufour, M. Rocchetto (2014). Signs of a faint disc population at polluted white dwarfs MNRAS, 444, 2147 [arXiv, MNRAS]