The formation and evolution of hydrogen-deficient post-AGB white dwarfs: the emerging chemical profile and the expectations for the PG1159-DB-DQ evolutionary connection

L. G. ALTHAUS; A. M. SERENELLI; J. A. PANEI; A. H. CÓRSICO; E. GARCÍA-BERRO; C. G. SCÓCCOLA

ASTRONOMY AND ASTROPHYSICS

EDP Sciences

Lugar: Les Ulis, France; Año: 2005 vol. 435 p. 631 - 648

0004-6361

This   paper is  designed  to  explore  the  formation  and evolution of  hydrogen-deficient post-AGB white dwarfs.   To this end, we compute  the complete evolution  of an initially 2.7  Mo star from the zero-age main sequence through the thermally pulsing and mass-loss phases  to the white  dwarf stage.  Particular  attention is given to the chemical abundance changes during the whole evolution.  A time-dependent  scheme  for the  simultaneous  treatment of  abundance changes  caused  by nuclear  reactions,  diffusive overshooting,  saltfingers and convection is considered. We employed the double-diffusive mixing-length  theory  of   convection  for  fluids  with  composition gradients  (Grossman  &  Taam  1996).   The study  can  therefore  be considered  as a  test of  its performance  in low-mass  stars.  Also, time-dependent  element diffusion  for multicomponent  gases  is taken into  account  during the  white  dwarf  evolution.  The  evolutionary stages corresponding  to the  last helium thermal  pulse on  the early white-dwarf cooling  branch and  the following born-again  episode arecarefully  explored. Relevant aspects  for PG1159  stars and  DB white dwarf evolution  are studied  in the frame  of these  new evolutionary models  that  take  into  account  the  history  of  the  white  dwarf progenitor.  The scope  of the calculations is extended  to the domain of the  helium-rich, carbon-contaminated DQ white dwarfs  with the aim of   exploring  the  plausibility   of  the   evolutionary  connection PG1159-DB-DQ.  In this regard, the implications for the double-layeredchemical  structure  in pulsating  DB  white  dwarfs is  investigated. Another  aspect of  the  investigation concerned  the consequences  of mass-loss  episodes   during  the   PG1159  stage  for   the  chemical stratification of the outer layer of DB and DQ white dwarfs.