Formación y evolución de estrellas enanas blancas deficientes en hidrógeno

PANEI J. A.,; ALTHAUS L. G.; CÓRSICO A. H.

La Plata, Bs. As., Argentina

Congreso; XLVI Reunión Anual de la Asociación Argentina de Astronomía; 2003

Asociación Argentina de Astronomía

In the present work, we compute the formation and evolution of hydrogen - deficient white dwarfs, classifaied as spectral type DB. They constitute about $20 %$ of all observed white dwarfs. White dwarfs are the most abundant stars and they are important for the study of globular clusters and galaxies. It´s well known that these stars are ideal laboratories that allow us to test the stellar evolution theory. It is currently believed that DB white dwarfs are the results of post-AGB stars experiencing a last helium thermal pulse on the white dwarf cooling branch, where it burns most of his hydrogen content. During this last flash, the star expand to red giant dimensions, suffering the so - called born - again scenario. We calculate the formation and evolution of DB white dwarfs progenitor from the ZAMS, through the thermally pulsing phase, stage of mass loss, planetary nebulae, the formation the PG~1159 stars and born - again scenario. To this end we employed a full evolutionary code with takes into account a time - dependent scheme for treating no-instantaneous mixing and diffusive overshooting. We present preliminary results, particulary regarding the chemistry variations along the whole evolution. We find that after the born-again scenario surface abundances are in good agreement with observational expectations for PG~1159 stars.