A theoretical and observational spectroscopic study of white dwarf stars

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White dwarf stars represent the endpoint of stellar evolution for more than 95% of the stars in our galaxy, including our Sun. Having exhausted their nuclear fuel at their center, white dwarf stars simply cool off quietly over periods of billions of years. While their mass is comparable to that of the Sun, their volume is closer to that of Earth, which makes them extremely compact objects (about a million times the density of the Sun). The study of these dying stars and the determination of their fundamental parameters such as temperature, mass, and chemical composition, provide us with crucial information not only about the nature of these stars, but also on the evolutionary link with their immediate progenitors. The most accurate method used for measuring the fundamental parameters of white dwarf stars is the so-called spectroscopic technique where spectroscopic observations, i.e. the distribution of the stellar flux with wavelength, are compared with the predictions of detailed stellar atmospheres, a method originally developed and continuously being improved at the Université de Montréal. A stellar atmosphere corresponds to this thin superficial layer surrounding the star where all the radiation comes from. The goal of our project is to improve our understanding of white dwarf stars by measuring the fundamental parameters of a large sample of objects with the help of improved calculations of synthetic spectra appropriate for these stars. From an observational point of view, we will secure spectroscopic observations of white dwarf stars with surface abundances composed of hydrogen (DA type) and helium (DB type) using spectrographs located at Steward Observatory in Arizona as well as at Mont Mégantic Observatory in Québec. From a theoretical point of view, we will calculate a new generation of model atmospheres and synthetic spectra, including several improvements in the constitutive physics such as line broadening, opacities, and convective energy transport. The comparison between observed and theoretical spectra will provide us with improved parameters for a large sample of white dwarf stars, which in turn will shed some light on their global properties as well as on their evolution.