Professor Patrick Dufour’s research work mainly concerns the study of white dwarf atmospheres, from both theoretical (detailed calculations of atmospheric models) and observational (spectroscopic and photometric observations) points of view.
A typical white dwarf consists of a core of carbon and oxygen, representing over 99% of its mass, surrounded by a thin layer of helium. In about 80% of cases, a thin layer of hydrogen completes the envelope. White dwarfs are very compact stars with extremely high surface gravity (over 10,000 times that on our Sun’s surface); as a result, heavy elements initially present on the surface sink beneath the photosphere very rapidly. It is this diffusion phenomenon that leads to the remarkable purity of hydrogen and/or helium observed at the surface of most white dwarfs.
For a large group of white dwarfs, however, observations show the presence of metal lines like 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 recognized that the source of this sudden pollution is the accretion of matter from a disk of debris resulting from the destruction of a rocky object by the white dwarf’s tidal forces. This object is likely a small planet, or an asteroid that survived the pre-white dwarf red giant phase.
Analyzing white dwarfs polluted by heavy elements gives a unique opportunity to study the internal chemical composition of extrasolar planetary systems. It allows to gather information on the formation and evolution of planets in general. Much of Patrick Dufour’s work is devoted to searching for and closely analyzing white dwarfs heavily polluted by planetary debris.