262 research outputs found
The white dwarf population within 40 pc of the Sun
The white dwarf luminosity function is an important tool to understand the
properties of the Solar neighborhood, like its star formation history, and its
age. Here we present a population synthesis study of the white dwarf population
within 40~pc from the Sun, and compare the results of this study with the
properties of the observed sample. We use a state-of-the-art population
synthesis code based on Monte Carlo techniques, that incorporates the most
recent and reliable white dwarf cooling sequences, an accurate description of
the Galactic neighborhood, and a realistic treatment of all the known
observational biases and selection procedures. We find a good agreement between
our theoretical models and the observed data. In particular, our simulations
reproduce a previously unexplained feature of the bright branch of the white
dwarf luminosity function, which we argue is due to a recent episode of star
formation. We also derive the age of the Solar neighborhood employing the
position of the observed cut-off of the white dwarf luminosity function,
obtaining ~8.9+-0.2 Gyr. We conclude that a detailed description of the
ensemble properties of the population of white dwarfs within 40pc of the Sun
allows us to obtain interesting constraints on the history of the Solar
neighborhood.Comment: 8 pages, 7 figures, accepted for publication in A&
A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes
NGC 6791 is a well studied open cluster1 that it is so close to us that can
be imaged down to very faint luminosities. The main sequence turn-off age (~8
Gyr) and the age derived from the termination of the white dwarf cooling
sequence (~6 Gyr) are significantly different. One possible explanation is that
as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the
deep interior of these stars. At lower temperatures, white dwarfs are expected
to crystallise and phase separation of the main constituents of the core of a
typical white dwarf, 12C and 16O, is expected to occur. This sequence of events
is expected to introduce significant delays in the cooling times, but has not
hitherto been proven. Here we report that, as theoretically anticipated,
physical separation processes occur in the cores of white dwarfs, solving the
age discrepancy for NGC 6791.Comment: 3 pages, 2 figures, published in Natur
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
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&
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
Revisiting the luminosity function of single halo white dwarfs
White dwarfs are the fossils left by the evolution of low-and
intermediate-mass stars, and have very long evolutionary timescales. This
allows us to use them to explore the properties of old populations, like the
Galactic halo. We present a population synthesis study of the luminosity
function of halo white dwarfs, aimed at investigating which information can be
derived from the currently available observed data. We employ an up-to-date
population synthesis code based on Monte Carlo techniques, that incorporates
the most recent and reliable cooling sequences for metal poor progenitors as
well as an accurate modeling of the observational biases. We find that because
the observed sample of halo white dwarfs is restricted to the brightest stars
only the hot branch of the white dwarf luminosity function can be used for such
purposes, and that its shape function is almost insensitive to the most
relevant inputs, like the adopted cooling sequences, the initial mass function,
the density profile of the stellar spheroid, or the adopted fraction of
unresolved binaries. Moreover, since the cut-off of the observed luminosity has
not been yet determined only lower limits to the age of the halo population can
be placed. We conclude that the current observed sample of the halo white dwarf
population is still too small to obtain definite conclusions about the
properties of the stellar halo, and the recently computed white dwarf cooling
sequences which incorporate residual hydrogen burning should be assessed using
metal-poor globular clusters.Comment: 9 pages, 9 figures, accepted for publication in A&
Pulsations of massive ZZ Ceti stars with carbon/oxygen and oxygen/neon cores
We explore the adiabatic pulsational properties of massive white dwarf stars
with hydrogen-rich envelopes and oxygen/neon and carbon/oxygen cores. To this
end, we compute the cooling of massive white dwarf models for both core
compositions taking into account the evolutionary history of the progenitor
stars and the chemical evolution caused by time-dependent element diffusion. In
particular, for the oxygen/neon models, we adopt the chemical profile resulting
from repeated carbon-burning shell flashes expected in very massive white dwarf
progenitors. For carbon/oxygen white dwarfs we consider the chemical profiles
resulting from phase separation upon crystallization. For both compositions we
also take into account the effects of crystallization on the oscillation
eigenmodes. We find that the pulsational properties of oxygen/neon white dwarfs
are notably different from those made of carbon/oxygen, thus making
asteroseismological techniques a promising way to distinguish between both
types of stars and, hence, to obtain valuable information about their
progenitors.Comment: 11 pages, including 11 postscript figures. Accepted for publication
in Astronomy and Astrophysic
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