457 research outputs found
White dwarf cooling sequences and cosmochronology
The evolution of white dwarfs is a simple gravothermal process. This means
that their luminosity function, i.e. the number of white dwarfs per unit
bolometric magnitude and unit volume as a function of bolometric magnitude, is
a monotonically increasing function that decreases abruptly as a consequence of
the finite age of the Galaxy. The precision and the accuracy of the white dwarf
luminosity functions obtained with the recent large surveys together with the
improved quality of the theoretical models of evolution of white dwarfs allow
to feed the hope that in a near future it will be possible to reconstruct the
history of the different Galactic populations.Comment: Proceedings of the 40th Liege International Astrophysical Colloquium:
Aging low mass stars: from red giants to white dwarf
A population synthesis study of the luminosity function of hot white dwarfs
We present a coherent and detailed Monte Carlo simulation of the population
of hot white dwarfs. We assess the statistical significance of the hot end of
the white dwarf luminosity function and the role played by the bolometric
corrections of hydrogen-rich white dwarfs at high effective temperatures. We
use the most up-to-date stellar evolutionary models and implement a full
description of the observational selection biases to obtain realistic
simulations of the observed white dwarf population. Our theoretical results are
compared with the luminosity function of hot white dwarfs obtained from the
Sloan Digital Sky Survey (SDSS), for both DA and non-DA white dwarfs. We find
that the theoretical results are in excellent agreement with the observational
data for the population of white dwarfs with hydrogen deficient atmospheres
(non-DA white dwarfs). For the population of white dwarfs with hydrogen-rich
atmospheres (white dwarfs of the DA class), our simulations show some
discrepancies with the observations for the brightest luminosity bins. These
discrepancies can be attributed to the way in which the masses of the white
dwarfs contributing to this luminosity bin have been computed, as most of them
have masses smaller than the theoretical lower limit for carbon-oxygen white
dwarfs. We conclude that the way in which the observational luminosity function
of hot white dwarfs is obtained is very sensitive to the particular
implementation of the method used to derive the masses of the sample. We also
provide a revised luminosity function for hot white dwarfs with hydrogen-rich
atmospheres.Comment: 6 pages, 5 figures, accepted for publication in A&
Monte Carlo simulations of the halo white dwarf population
The interpretation of microlensing results towards the Large Magellanic Cloud
(LMC) still remains controversial. Whereas white dwarfs have been proposed to
explain these results and, hence, to contribute significantly to the mass
budget of our Galaxy, there are as well several constraints on the role played
by white dwarfs. In this paper we analyze self-consistently and simultaneously
four different results, namely, the local halo white dwarf luminosity function,
the microlensing results reported by the MACHO team towards the LMC, the
results of Hubble Deep Field (HDF) and the results of the EROS experiment, for
several initial mass functions and halo ages. We find that the proposed
log-normal initial mass functions do not contribute to solve the problem posed
by the observed microlensing events and, moreover, they overproduce white
dwarfs when compared to the results of the HDF and of the EROS survey. We also
find that the contribution of hydrogen-rich white dwarfs to the dynamical mass
of the halo of the Galaxy cannot be more than .Comment: 17 pages, 10 figures; accepted for publication in Astronomy and
Astrophysic
The ages of very cool hydrogen-rich white dwarfs
The evolution of white dwarfs is essentially a cooling process that depends
primarily on the energy stored in their degenerate cores and on the
transparency of their envelopes. In this paper we compute accurate cooling
sequences for carbon-oxygen white dwarfs with hydrogen dominated atmospheres
for the full range of masses of interest. For this purpose we use the most
accurate available physical inputs for both the equation of state and opacities
of the envelope and for the thermodynamic quantities of the degenerate core. We
also investigate the role of the latent heat in the computed cooling sequences.
We present separately cooling sequences in which the effects of phase
separation of the carbon-oxygen binary mixture upon crystallization have been
neglected, and the delay introduced in the cooling times when this mechanism is
properly taken into account, in order to compare our results with other
published cooling sequences which do not include a treatment of this
phenomenon. We find that the cooling ages of very cool white dwarfs with pure
hydrogen atmospheres have been systematically underestimated by roughly 1.5 Gyr
at log(L/Lo)=-4.5 for an otherwise typical 0.6 Mo white dwarf, when phase
separation is neglected. If phase separation of the binary mixture is included
then the cooling ages are further increased by roughly 10%. Cooling tracks and
cooling isochrones in several color-magnitude diagrams are presented as well.Comment: 8 Pages; ApJ, accepted for publicatio
New phase diagrams for dense carbon-oxygen mixtures and white dwarf evolution
Cool white dwarfs are reliable and independent stellar chronometers. The most
common white dwarfs have carbon-oxygen dense cores. Consequently, the cooling
ages of very cool white dwarfs sensitively depend on the adopted phase diagram
of the carbon-oxygen binary mixture. A new phase diagram of dense carbon-oxygen
mixtures appropriate for white dwarf interiors has been recently obtained using
direct molecular dynamics simulations. In this paper, we explore the
consequences of this phase diagram in the evolution of cool white dwarfs. To do
this we employ a detailed stellar evolutionary code and accurate initial white
dwarf configurations, derived from the full evolution of progenitor stars. We
use two different phase diagrams, that of Horowitz et al. (2010), which
presents an azeotrope, and the phase diagram of Segretain & Chabrier (1993),
which is of the spindle form. We computed the evolution of 0.593 and 0.878M_sun
white dwarf models during the crystallization phase, and we found that the
energy released by carbon-oxygen phase separation is smaller when the new phase
diagram of Horowitz et al. (2010) is used. This translates into time delays
that are on average a factor about 2 smaller than those obtained when the phase
diagram of Segretain & Chabrier (1993) is employed. Our results have important
implications for white dwarf cosmochronology, because the cooling ages of very
old white dwarfs are different for the two phase diagrams. This may have a
noticeable impact on the age determinations of very old globular clusters, for
which the white dwarf color-magnitude diagram provides an independent way of
estimating their age.Comment: 7 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
The white dwarf cooling sequence of NGC 6791: a unique tool for stellar evolution
NGC 6791 is a well-studied, metal-rich open cluster that is so close to us
that can be imaged down to luminosities fainter than that of the termination of
its white dwarf cooling sequence, thus allowing for an in-depth study of its
white dwarf population. We use a Monte Carlo simulator that employs up-to-date
evolutionary cooling sequences for white dwarfs with hydrogen-rich and
hydrogen-deficient atmospheres, with carbon-oxygen and helium cores. The
cooling sequences for carbon-oxygen cores account for the delays introduced by
both Ne^22 sedimentation in the liquid phase and by carbon-oxygen phase
separation upon crystallization. We do not find evidence for a substantial
fraction of helium-core white dwarfs, and hence our results support the
suggestion that the origin of the bright peak of the white dwarf luminosity
function can only be attributed to a population of unresolved binary white
dwarfs. Moreover, our results indicate that the number distribution of
secondary masses of the population of unresolved binaries has to increase with
increasing mass ratio between the secondary and primary components of the
progenitor system. We also find that the observed cooling sequence appears to
be able to constrain the presence of progenitor sub-populations with different
chemical compositions and the fraction of non-DA white dwarfs. Our simulations
place interesting constraints on important characteristics of the stellar
populations of NGC 6791. In particular, we find that the fraction of single
helium-core white dwarfs must be smaller than 5%, that a sub-population of
stars with zero metallicity must be <12%, while if the adopted metallicity of
the sub-population is solar the upper limit is ~8%. Finally, we also find that
the fraction of non-DA white dwarfs in this particular cluster is surprinsingly
small <6%.Comment: 9 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
On the White Dwarf distances to Galactic Globular Clusters
We analyze in detail various possible sources of systematic errors on the
distances of globular clusters derived by fitting a local template DA white
dwarf sequence to the cluster counterpart (the so-called WD-fitting technique).
We find that the unknown thickness of the hydrogen layer of white dwarfs in
clusters plays a non negligible role. For reasonable assumptions - supported by
the few sparse available observational constraints - about the unknown mass and
thickness of the hydrogen layer for the cluster white dwarfs, a realistic
estimate of the systematic error on the distance is within +-0.10 mag. However,
particular combinations of white dwarf masses and envelope thicknesses - which
at present cannot be excluded a priori - could produce larger errors.
Contamination of the cluster DA sequence by non-DA white dwarfs introduces a
very small systematic error of about -0.03 mag in the Mv/(V-I) plane, but in
the Mv/(B-V) plane the systematic error amounts to ~ +0.20 mag. Contamination
by white dwarfs with helium cores should not influence appreciably the
WD-fitting distances. Finally, we obtain a derivative D((m-M)v)/D(E(B-V))~ -5.5
for the WD-fitting distances, which is very similar to the dependence found
when using the Main Sequence fitting technique.Comment: 12 pages, 11 figures A&A, accepted for publicatio
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