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.

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.

Massive stars comprise all those with an initial mass exceeding 8 solar masses, and which collapse upon themselves as supernovae at the end of their nuclear "burning" lives, leaving neutron stars or black holes. Since the light produced by a normal star is roughly equivalent to the cube of its mass, a single star of 100 solar masses can emit the equivalent of one million suns. Beyond 20 solar masses, massive stars are distinguished by their strong winds, which can be up to one billion times stronger than that of our Sun, which we already consider quite strong (comets, auroras, etc.). Although they are rare and short-lived, massive stars emit enormous amounts of radiation, most of it in deadly ultraviolet, and matter enriched with heavy elements, into the interstellar environment, ready to form even more generations of stars and planets such as Earth. This process was especially important early in the life of the Universe, when the very first stars were forming, all of them very massive. My research is aimed mainly at exploring: (1) whether the pressure of radiation alone is enough to accelerate the extreme winds of pre-supernova stars, i.e. during the He-burning phase as Wolf-Rayet stars, using the first Canadian spatial telescope on the MOST microsatellite, (2) building a system of microsatellites (BRITE-Constellation) to examine the very low variability of a large sample of luminous stars, (3) how exactly winds accelerate around luminous, hot stars, (4) the role of magnetic fields in accelerating their winds, (5) the mystery of how dust forms and survives in the hostile environment of luminous, hot stars, (6) the upper limit for the most massive stars (100, 150 or 200 solar masses in the current Universe?), (7) the number of WR stars in our entire Galaxy, most of them hidden by interstellar dust, and (8) whether WR stars really do explode into supernovas, leading in some cases to the most energetic (albeit short-lived) phenomenon in the Universe, gamma ray bursts.