Professor Patrick Dufour’s research at the Université de Montréal focuses on the atmospheres of white dwarf stars, both theoretically (detailed atmospheric model calculations) and observationally (spectroscopic and photometric observations).
A typical white dwarf consists of a core of carbon and oxygen, making up over 99% of its mass, surrounded by a thin layer of helium. In around 80% of cases, a thin additional layer of hydrogen completes the envelope. White dwarfs are very compact stars with extremely high surface gravities (more than 10,000 times that of the Sun’s surface); the heavy elements initially present on the surface of these stars sink very rapidly beneath the photosphere. It is this diffusion phenomenon that is responsible for the remarkable purity of hydrogen and/or helium observed at the surface of most white dwarfs.
For a large group of white dwarfs, however, we observe the presence of metallic lines such as calcium, iron or magnesium. The presence of heavy elements in the atmosphere of a white dwarf necessarily implies that an external source has recently polluted the star’s surface. It is now accepted that the source of this sudden pollution is the accretion of matter from a debris disk resulting from the destruction of a rocky object by the tidal forces of the white dwarf. The object in question is probably a small planet, or an asteroid that survived the pre-white dwarf red giant phase.
The analysis of white dwarfs polluted with heavy elements offers us a unique opportunity to study the internal chemical composition of extrasolar planetary systems. It also provides us with information on the formation and evolution of planets in general. A large part of my work is therefore devoted to the search for and detailed analysis of white dwarf stars heavily polluted by planetary debris.