Professor Gilles Fontaine’s research in astroseismology and stellar evolution recently led him to look at the fascinating challenges involved in characterizing exoplanets, one of the most important issues in contemporary astrophysics. Most of the time, an exoplanet’s properties cannot be determined unless the fundamental characteristics of its host star are known.
The astroseismological method is an excellent tool for this purpose and, if properly used, can determine the structural parameters of a variable star with great precision, along with its internal stratification and its age. Gilles Fontaine has begun to explore this research approach, which has already proven its merits on several occasions when planets have been discovered orbiting variable stars.
An impressive and growing number of white dwarfs seem to bear signs of planetary debris, opening the possibility of determining the bulk composition of these debris, a unique tool in planetology. The potential in this case is huge and very promising. White dwarfs play the role of a “substrate,” in a way, on which heavy elements constituting planetary debris are deposited, but a permeable substrate that allows the elements to pass through at different rates.
The first step is to determine the current abundance of these heavy elements in a given white dwarf. This calls for sophisticated atmospheric models, a specialty of Gilles Fontaine young colleague Patrick Dufour. Second, these levels of abundance (which vary over time) have to be interpreted with the possible accretion rates, the effects of differential diffusion among the different elements, the presence of convection zones, thermohaline convection and other mechanisms that can partially mix external layers and ultimately extend back to the primordial composition. This is a considerable challenge, but Professor Fontaine has already begun addressing it with a major review of calculations of diffusion coefficients that he did in 1986 in co-operation with Georges Michaud at the Université de Montréal. He is counting on the collaboration of expert numerical analyst Pierre Brassard. He hopes to be soon able to effectively simulate accretion-diffusion episodes of planetary debris on white dwarfs.
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