Asymptotic giant branch (AGB) stars and formation of white dwarfs
Our research group has developed a numerical code (LPCODE) appropriate for the study of the evolution of degenerate configurations based on an updated and detailed constitutive physics such as a full network for thermonuclear reactions, OPAL radiative opacities, full-spectrum turbulence theories of convection, and detailed equations of state and neutrino emission rates. We have also developed a set of routines that compute the evolution of the chemical abundance distribution caused by gravitational settling, chemical and thermal diffusion of nuclear species. In particular, we have studied the evolution of low-mass, helium-core white dwarfs resulting from the evolution of close binary systems (Althaus et al., 2001, MNRAS, 323, 471). Our results shows that discrepancies between spin-down ages and the predictions of standard white dwarf evolutionary models (van Kerkwijk et al., 2000, ApJ, 530, L37) appear to be the result of ignoring element diffusion in evolutionary calculations. The recent detection of low-mass white dwarfs in compact binaries belonging to globular clusters (see, i.e., Taylor, Grindlay, et al., 2001, ApJ, 553, L169) has also sparked the attention of many researchers. Indeed, the interest in studying low-mass white dwarfs in globular clusters is motivated not only by their importance in the understanding of the formation and evolution of the compact binaries in which these stars are found but also by the possibility they offer of constraining globular cluster dynamics and evolution. It is worth mentioning that our evolutionary results for these stars have received tentative support from the optical detection of the helium white dwarf companion to the millisecond pulsar in 47 Tucanae (Edmonds et al., 2001, ApJ, 557, L57).
On the other hand, it is
well known that white dwarfs are excellent candidates to test the existence
of several weakly interacting
Our group has developed a pulsational code that compute the linear, adiabatic, non-radial stellar pulsations (Corsico, 2003, PhD., University of La Plata). This code is fully coupled to the LPCODE evolutionary code, which has enabled us to study the pulsations of variable white dwarfs. One of our main results concerns the mode trapping properties of white dwarfs. We find that element diffusion strongly smoothes out the chemical profiles, making the mode trapping caused by the outer chemical interfaces notably less important (Corsico et al., 2001, A&A, 380, L17). In collaboration with Michael Montgomery of the University of Texas we started a joint project aimed at exploring the pulsational properties of massive white dwarfs on the basis of new and improved evolutionary models for these stars that take into account time-dependent element diffusion, nuclear burning and the history of the white dwarf progenitor. Our first results suggest that the pulsational properties become very sensitive to the occurrence of core overshooting during the evolutionary stages prior to the white dwarf formation (Althaus L.G., Serenelli A. M., Corsico A. H. & Montgomery M. H., 2003, A&A, 404,593). In this connection, we are currently investigating the effect of a solid core on the pulsational pattern of crystallized white dwarfs.
In the context of pulsating stars, variable white dwarfs with helium-rich envelopes are likewise within our current research interest. With Alfred Gautschy we are studying the non-adiabatic pulsational properties of such stars by employing detailed stellar models which explicitly account for the evolution of chemical distribution due to diffusion processes and modern theories of turbulent convection. Our first results suggest a weaker trapping effect in the periodicities than previously believed (Gautschy & Althaus, 2002, A&A, 382, 141).
An analysis of the secular instability
in intermediate mass stars with core helium burning is likewise within
the scope of our interest. This
In this regard, we are studying
some of the above-mentioned aspects on the basis of new and improved evolutionary
models we are currently developing. We mention the treatment of the
abundance changes which consider nuclear burning, time-dependent
convective mixing and overshooting, semiconvection,
salt finger instability and element diffusion. Our major aim
is the computation of the whole evolution of intermediate-mass
stars from the main sequence stage through the thermally pulsing and mass
loss phases on the AGB to the white dwarf regime. Aspects such as
the study of diffusion-induced hydrogen shell flashes,
the exploration of white dwarf formation
star. Cataclysmic variables and X-ray binaries are commonly associated with white dwarf stars. We are studying the problem of close binary evolution and the formation of low-mass white dwarfs in globular clusters (Serenelli et al. 2002, MNRAS, 337, 1091). These topics are currently of much interest for researchers. In particular, the HST detection of a sequence of low-mass white dwarf candidates in the cluster NGC 6397 (Taylor et al., 2001, ApJ, L169) has prompted us to compute evolutionary models for such white dwarfs with the aim of placing on theoretical grounds some expeculations about the formation and evolution of such white dwarf stars.
In the light of theoretical evidence
suggesting that some of the presumed low-mass helium core white dwarfs
could actually be white dwarfs with oxygen cores we have started a collabortive
effort with Zhanwen Han at Oxford University with the aim of exploring
the formation and evolution of carbon-oxygen white dwarfs
with stellar masses as low as 0.3 solar masses. To this
end we are computing the conservative close binary evolution of a 2.5 solar
mass star in a close binary system from the main sequence to the white
dwarf stage. The stellar mass of the secondary is 1.25 solar masses and
the systems has an initial period of 3 days. Our first results suggest
that both the pulsational and evolutionary properties of oxygen core white
dwarfs differ appreciably from those of their helium-core counterparts.
In particular, our results indicate that future asteroseismology could
be a promising way of distinguishing both