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White
dwarf evolution
White dwarf stars are the electron-degenerate
stellar remnants of the majority of all stars. These stars constitute a
natural laboratory to confront our current theory of stellar evolution
with the physical properties of dense matter. In addition to their
intrinsic interest to the stellar evolution, they have recently
received particular attention as they may be linked to other astrophysical
fields. They are strong candidates for the dark matter of the Galaxy,
(e.g. Alcock et al., 1998, ApJ, 491, L11;
1999 ApJ 518, 44). They also provide estimations of ages
for different stellar systems such as the local Galactic disk (Winget
et al. 1987 ApJ 315, L77; Wood 1992, ApJ, 386, 539) and
globular and open clusters (cf. von Hippel & Gilmore 2000, AJ,
120, 1384). In this sense, white dwarfs can be used to trace
the evolutionary history of our Galaxy.
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
massive particles such as
axions. In this connection, we started a collaborative effort with
Drs. Enrique Garcia-Berro and Jordi Isern of the University of Barcelona.
In particular, we have used our evolutionary models to perform a
comprehensive study of the pulsational characteristics of the
variable hydrogen-rich white dwarf G117-B15A. This has allowed us to place
tight constraints on the mass of the axion (Corsico et al.
2001, New Astronomy, 6, 197), improving previous efforts.
In collaboration with the above mentioned authors we are currently
working on a project aimed at understanding the evolutionary and pulsational
properties of massive white dwarf stars with oxygen and neon cores.
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Stellar
pulsations
White dwarf stars are pulsationally
unstable in three temperature regimes, with typical periods in the range
100-1000 seconds. Over the last past decade, the study of pulsational pattern
of variable white dwarfs through asteroseismological techniques has become
a very powerful tool for probing the internal structure and evolution of
these stars. In particular, asteroseismology of massive white dwarfs has
recently drawn the attention of researchers in view of the possibility
it offers to place constraints on the crystallization process in the
interior of white dwarfs. This has been motivated by the discovery
of pulsation in the star BPM 37093, a massive white dwarf which
should be largely crystallized.
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
aspect is currently underwent in
a joint project with Alfred Gautschy.
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Asymptotic
giant branch (AGB) stars and formation of white dwarfs
There are a number of relevant and
open astrophysical problems about the advanced evolutionary phases
of low-and intermediate-mass stars. Amongst these are a quantification
according to first principles of the various mechanisms that lead
nuclearly processed matter to the surface; the occurrence driving
mass loss and the relation between initial stellar mass and final white
dwarf mass; the value of the minimum mass for intermediately degenerate
carbon ignition; and the origin of neon- and magnesium-rich
white dwarfs. Solution to these problems is required as a whole, and as
an input for Galactic chemical evolution, population synthesis, interpretation
of colors of distant galaxies and so on. All these problems deal
with evolutionary phases following central helium exhaustion, from
the base of the asymptotic giant branch (AGB) to the final
ejection of planetary nebula, after which the blueward excursion
leading to white dwarfs begins.
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
the study of pulsation of
hot white dwarfs, AGB and post-AGB stars, carbon
stars and the PG 1159--DB--DQ evolutionary connection
are within the scope of our immediate objectives. |
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Binary
evolution
White dwarfs are often found
in binary systems containing usually another white dwarf, a millisecond
pulsar or a main sequence
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
types of stars if low-mass white
dwarfs were in fact found to pulsate. |