On the Formation of Close Binary White Dwarfs

Abstract

Single stars of initial main-sequence mass larger than about 0.8-1.0⊙ and less than approximately 6-8M⊙ evolve into carbon-oxygen white dwarfs of mass in the range 0.55-1.1M⊙ in less than the Hubble time. Single stars do not make helium white dwarfs in a Hubble time. These statements are based on both observational and theoretical considerations. It is not yet established, but there are suspicions that single stars of initial mass in the range 8-10M⊙ may evolve into oxygen-neon white dwarfs of mass in the range 1.1-1.4M⊙. As a single star of the appropriate initial mass develops a hot, electron-degenerate core composed of carbon and oxygen or of oxygen and neon, it becomes large (R > 200R⊙) and luminous (L > 6000L⊙), with the luminosity being generated most of the time by hydrogen burning, interrupted quasiperiodically by a helium shell flash. The star is said to be an asymptotic giant branch (AGB) star. Within 105 - 106yr after arriving on the AGB, an envelope instability, whose nature is still being explored observationally and theoretically, leads to the ejection of most of the hydrogen-rich envelope of the star. The remnant shortly becomes the central star of a planetary nebula composed of the ejected material, and then cools into a white dwarf configuration.