Evolutionary and pulsational properties of white dwarf stars

Abridged. White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. The study of white dwarfs has potential applications to different fields of astrophysics. In particular, they can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, like our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow to use these stars as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. They can be used to constrain fundamental properties of elementary particles such as axions and neutrinos, and to study problems related to the variation of fundamental constants. In this work, we review the essentials of the physics of white dwarf stars. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational instability strips. The existence of these instability strips provides astronomers with an unique opportunity to peer into their internal structure that would otherwise remain hidden from observers. We will show that this allows to measure with unprecedented precision the stellar masses and to infer their envelope thicknesses, to probe the core chemical stratification, and to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white dwarfs and the most recent applications of white dwarf asteroseismology.Comment: 85 pages, 28 figures. To be published in The Astronomy and Astrophysics Revie

Non-Radial Oscillations

The problem of the adiabatic nonradial oscillations of spherical stars is reviewed and results recalled for a variety of models. The anomalous behavior of the eigenfunctions for highly condensed models is related to the apparent mobile singularities depending on the eigenvalues. Tables of Q-values are provided to facilitate possible applications to variable stars. In the case of the gravity modes, the existence of multiple spectra, some stable (g/plus/ modes) and some unstable (g/-/ modes) if superadiabatic and subadiabatic regions alternate, is discussed. As far as vibrational stability is concerned, a general expression is given for the 'damping coefficient'. Attention is drawn to the possibility for g(plus) modes of becoming vibrationally unstable under the effect of various factors and in various models, including the sun where this was advocated as a possibility of relieving the neutrinos difficulty. Finally, the present status of the most obvious candidates among variable stars for nonradial oscillations, the beta Canis Majoris stars and the rapid blue variables (white dwarfs) is briefly reviewed