Étude de l’évolution spectrale des étoiles naines blanches riches en hélium et le problème de l’origine de l’hydrogène dans les hybrides de type DBA

White dwarf stars represent the end point of stellar evolution for 97% of the stars in the Galaxy. The strong gravitational field present at the surface of these stars leads to an important chemical stratification. This gravitational settling process is responsible, namely, for the extreme purity of their outer layers, and leads to optical spectra generally dominated by a single species, most often hydrogen or helium. Nearly 40 years ago, however, the discovery of significant variations of the observed spectral types as a function of effective temperature revealed the existence of several physical mechanisms that could compete with gravitational settling along the cooling sequence.  The main goal of this thesis is to provide an explanation for the origin of helium-atmosphere white dwarfs below Teff = 30; 000 K. To this end, we study in detail the convective dilution process (for Teff > 15; 000 K) associated with the gradual erosion of a thin radiative hydrogen layer by the underlying helium convection zone. We also examine the convective mixing mechanism (for Teff < 12; 000 K) described by the complete homogenization of the outer layers arising when the superficial hydrogen convection zone reaches the deeper convective helium envelope. Particular attention is given to the problem of the origin of hydrogen detected in some helium-atmosphere white dwarfs.  We first present an analysis of 115 DB (helium-line) white dwarfs and 28 cool, helium-rich DA (hydrogen-line) stars. The atmospheric parameters of each star — effective temperature, surface gravity, and chemical composition—are obtained using the so-called spectroscopic and photometric techniques. Our spectroscopic observations reveal the presence of hydrogen in 63% of our DB sample, thus revealing their DBA hybrid nature. We also confirm the persistence of white dwarfs without any detectable traces of hydrogen at low effective temperature.  Using detailed stellar envelope models, we show that the photospheric hydrogen abundances predicted by the convective dilution scenario are much smaller than those measured in DBA white dwarfs. We also demonstrate that the amount of hydrogen observed in DBA stars, as well as in cool, helium-rich DA white dwarfs, cannot be explained by accretion from external sources, since accretion will build a hydrogen layer that is sufficiently thick to resist the erosion caused by convection. Instead, we show that cool, helium-rich DA white dwarfs most likely represent the end product of the convective mixing of DA white dwarfs with sufficiently thin hydrogen layers.  In order to solve the problem of hydrogen in DBA white dwarfs, we invoke the possibility that significant amounts of hydrogen can be hidden within the deep stellar interior. On the basis of this hypothesis, deduced from realistic models, we construct a physical mechanism where some of this hydrogen located deep in the star is being dredged-up to the surface by the growing convective envelope. The resulting simulations reproduce the H/He abundance ratio observed in the majority of DBA white dwarfs. This convective dredge-up scenario provides for the first time a satisfactory explanation for the origin of hydrogen in these objects, thus potentially solving a long-standing problem in white dwarf spectral evolution.

Keyphrases: Stars: white dwarfs, Stars: abundances, Stars: evolution, Stars: fundamental parameters