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Stellar cannibalism: a planet engulfed by a white dwarf star

A team of canadian and american astrophysicists led Patrick Dufour, postdoctoral researcher at the Physics Department of the Université de Montréal and member of the Centre de Recherche en Astrophysique du Québec (CRAQ), discovered, by examining thousands of spectra from the Sloan Digital Sky Survey, a white dwarf with the most metal-rich surface composition ever observed. Their analysis, based on observations from the Gemini North telescope (Hawaii) and the Multi-Mirror Telescope (MMT, Arizona) demonstrated clearly that a small planet-like object, with a mass equivalent to that of Ceres in our Solar System, sank inside a white dwarf star.

White dwarfs are "dead" stars in which the core does not produce thermonuclear reactions anymore. Therefore, they are slowly cooling down and loosing their thermal energy. These astronomical objects are extremely compact and have a surface gravitational field about 100000 times that of the Earth. Consequently, the heaviest chemical elements (like calcium, magnesium, iron, etc…) have a strong tendency to sink toward the centre of the star while lighter elements (mainly hydrogen and helium) remain at the surface. This explains that the great majority of white dwarfs have an atmosphere composed of pure hydrogen or pure helium (the two lightest elements). When heavier elements are detected on the surface of those stars, it is natural to attribute their presence to an external source, either by accretion from the interstellar medium, or more probably due to an asteroid or a minor planet that would have survived the different evolution phases of the white dwarf.

The team led by Patrick Dufour, which includes the Professors Gilles Fontaine (UdeM, CRAQ) and Pierre Bergeron (UdeM, CRAQ), demonstrated that the white dwarf SDSS J073842.56+183509.6 possesses the most "heavy-elements polluted" atmosphere, breaking the previous record by a factor of 10. Using their theoretical models of white dwarf atmospheres, the team of scientists managed to calculated the total mass of heavy elements present in the star. This mass is equivalent to one half of that of the dwarf planet Ceres in our Solar System. The luminosity of the white dwarf, obtained with the infrared imager NIRI at the Gemini North telescope, also revealed an excess of infrared light, which demonstrates evidence of the presence of an accretion disk around the star. This disk of debris, which probably originates from the destruction of a dwarf planet or an asteroid by the gravitational forces of the white dwarf, is the source of the heavy elements observed at the surface of the star. By estimating the total mass of heavy elements present in the white dwarf and in the surrounding disk, the authors concluded that this disk is most probably the residue of a dwarf planet of the size of Ceres.

A spectroscopic analysis of SDSS J073842.56+183509.6 performed at the MMT revealed that the amount of heavy elements found is remarkably similar to the composition of the terrestrial crust. This analysis demonstrates that telluric extrasolar planets have a composition similar to that of the rocky planets in our Solar System. This discovery, together with the optical development of future giant telescopes, proves that it will be possible to study very precisely the chemical composition of extrasolar objects.

© Gemini Observatory and CRAQ: Illustration of Jon Lomberg and Olivier Hernandez

Figure caption: Artistic view of an accretion disk around a white dwarf. The disk diameter is roughly equal to the Earth-Moon distance. The mass of the disk is equivalent to that of the dwarf planet Ceres.

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