Model atmosphere analysis of white dwarfs in the Sloan Digital Sky Survey footprint of Gaia: the definitive work

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White dwarf stars represent the endpoint of more than 97% of the stars in our galaxy, including our own Sun. These Earth-sized objects have degenerate cores (a unique state of matter described by quantum mechanics) in which nuclear fusion no longer occurs. As such, white dwarfs simply cool off slowly over billions of years and they are by far the oldest members of our Galaxy. They can provide insights into Galactic populations and the history of star formation. Hence white dwarfs are fundamentally important for all major areas of astrophysical research, and the determination of their properties (mass, cooling age, atmospheric and internal composition) thus provides a variety of useful information on planetary systems, the stars, the Galaxy and other galaxies as well. Also, being extremely dense objects, white dwarfs provide exquisite physical environments where high-density physics models can be tested and improved. The main scientific objective of this research proposal is to provide a full understanding of the nature and evolution of white dwarf stars using all the currently available spectroscopic and photometric data, but most of all, astrometric data from Gaia space mission (Global Astrometric Interferometer for Astrophysics), which recently released distance measurements of over one billion stars, including around 260,000 white dwarfs. Over the years, our group at University of Montreal has been developing state-of-the-art model atmospheres to measure the fundamental stellar parameters of white dwarfs (temperature, mass, radius, chemical composition, age) using the so-called photometric and spectroscopic techniques. The spectroscopic technique relies on fitting optical spectra of stars with strong enough absorption lines (over 33,000 of which are currently available from the Sloan Digital Sky Survey) with the predictions of model atmospheres, while the photometric method relies on fitting the observed energy distribution with similar models, a technique that requires the knowledge of stellar distances. Until recently, direct distance measurements were only available for 300 white dwarfs or so, severely restricting the applicability of the photometric method to large samples. The Gaia mission has now measured the distance to almost every single spectroscopically confirmed white dwarf, and many white dwarf candidates as well, a true revolution in the white dwarf community. These large data sets will allow us not only to improve our determinations of white dwarf parameters, but most importantly, to study the validity of our model atmosphere calculations by comparing for the first time the parameters obtained from both methods. These results will in turn provide crucial information for studying the initial-to-final mass relation, the stellar formation rate, the initial mass function, and the chemical evolution of the Galaxy, all areas of scientific research that will have a deep impact on other areas of astrophysics.