Understanding the Spectral Evolution of White Dwarf Stars

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White dwarf stars represent the endpoint of more than 97% of the stars in the Galaxy, including the Sun, and the determination of their global properties thus provides a variety of useful information on the star formation history in our galaxy, and on stellar evolution in general. Having exhausted the nuclear fuel at their center, white dwarfs gradually cool off along an evolutionary sequence over several billions of years. Because of the intense gravitational field present at the surface of these stars (10,000 times that of the Sun), light elements such as hydrogen and helium tend to float to the surface, while heavier elements sink out of sight. Consequently, about 80% of white dwarfs have atmospheres (the outer layers of a star) dominated by hydrogen, the so-called DA stars, while the remaining 20% have helium-dominated atmospheres. The latter include the DB stars, whose spectra are dominated by neutral helium lines (or ionized helium for the DO stars), the DQ stars with strong molecular carbon bands, the DZ stars with absorption features of heavier elements such as calcium, iron, sodium, etc., and the DC stars, whose spectra are completely featureless. However, these various types of white dwarfs are found only in some specific range of temperatures, indicating that there exists a variety of physical mechanisms that compete with the gravitational field to alter the chemical composition of the outer layers of white dwarfs as they evolve along their cooling sequence. Such physical mechanisms include convective mixing, convective dredge-up from the core, accretion from the interstellar medium or circumstellar material, radiative acceleration, and stellar winds. The objective of this program is to provide a better understanding of the Spectral Evolution of White Dwarf Stars by analyzing the 25,000 or so white dwarfs discovered in the Sloan Digital Sky Survey with state-of-the-art model atmosphere techniques and evolutionary models being developed at the Université de Montréal, in order to identify the various evolutionary channels that produce white dwarfs, thus providing a better understanding of the connection with the earlier phases of stellar evolution.****************