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Experts in: Faint blue stars, White dwarfs, degenerate stars, nuclei of planetary nebulae

Bergeron, Pierre

BERGERON, Pierre

Professeur titulaire

I am interested in the study of white dwarf stars and, in particular, the calculation of model atmospheres. White dwarf stars represent the final evolutionary stage of more than 97% of stars in our galaxy, including our Sun. Having exhausted the nuclear power sources in their centre, white dwarfs cool slowly over several billion years. They have a mass comparable to that of the Sun but in a volume equal to that of the Earth, thus making them extremely compact objects whose density is a million times that of the Sun. The study of these stellar remnants and the determination of their fundamental parameters such as surface temperature, mass and chemical composition tell us not only about the nature of these stars, but also about the evolutionary link with the stars that produced them. The most accurate method for measuring the basic parameters of white dwarf stars is to compare in detail the spectroscopic data, i.e. the flux distribution as a function of wavelength, with theoretical predictions obtained from model atmospheres we have been constantly refining here at the Université de Montréal. The stellar atmosphere corresponds to the thin surface layer where the stellar radiation originates. I am also interested in the study of pulsating white dwarfs, called ZZ Ceti stars, and in particular the determination of the empirical boundaries of their instability strips. All of these theoretical projects rely on photometric and spectroscopic data obtained at different observatories at Kitt Peak in Arizona (2.3 m Steward, 2.1 m and 4 m Kitt Peak) and the Mont Mégantic Observatory.

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Dufour, Patrick

DUFOUR, Patrick

Professeur agrégé

My research is oriented mainly toward the study of white dwarf atmospheres, from both the theoretical (detailed model atmosphere calculations) and observational (spectroscopic and photometric observations) viewpoints. White dwarfs are the remnants of low-mass stars that have used up their reserves of nuclear fuel. A typical white dwarf consists of a nucleus of carbon and oxygen representing over 99% of its mass, surrounded by a thin layer of helium that is itself surrounded, in about 80% of cases, by another thin layer of hydrogen. These layers, although thin, are optically opaque and regulate the rate at which the star loses energy (i.e. its cooling rate). To properly understand the evolution of white dwarfs, it is essential to understand the physical properties of these surface layers. The spectroscopic analysis of light from white dwarfs' atmospheres is the main technique used to gather information on the external parts of white dwarfs. My work is focussed on analyzing stars with traces of heavy elements (DZ and DQ spectral types) and stars with a carbon atmosphere.

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Fontaine, Gilles

FONTAINE, Gilles

Professeur titulaire

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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