667 research outputs found
Gravitational settling of 22Ne and white dwarf evolution
We study the effects of the sedimentation of the trace element 22Ne in the
cooling of white dwarfs. In contrast with previous studies, which adopted a
simplified treatment of the effects of 22Ne sedimentation, this is done
self-consistently for the first time, using an up-to-date stellar evolutionary
code in which the diffusion equation is coupled with the full set of equations
of stellar evolution. Due the large neutron excess of 22Ne, this isotope
rapidly sediments in the interior of the white dwarf. Although we explore a
wide range of parameters, we find that using the most reasonable assumptions
concerning the diffusion coefficient and the physical state of the white dwarf
interior the delay introduced by the ensuing chemical differentation is minor
for a typical 0.6 Msun white dwarf. For more massive white dwarfs, say M_Wd
about 1.0 Msun, the delay turns out to be considerably larger. These results
are in qualitatively good accord with those obtained in previous studies, but
we find that the magnitude of the delay introduced by 22Ne sedimentation was
underestimated by a factor of about 2. We also perform a preliminary study of
the impact of 22Ne sedimentation on the white dwarf luminosity function.
Finally, we hypothesize as well on the possibility of detecting the
sedimentation of 22Ne using pulsating white dwarfs in the appropriate effective
temperature range with accurately determined rates of change of the observed
periods.Comment: To apper in The Astrophysical Journa
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&
Evolution of white dwarf stars with high-metallicity progenitors: the role of 22Ne diffusion
Motivated by the strong discrepancy between the main sequence turn-off age
and the white dwarf cooling age in the metal-rich open cluster NGC 6791, we
compute a grid of white dwarf evolutionary sequences that incorporates for the
first time the energy released by the processes of 22Ne sedimentation and of
carbon/oxygen phase separation upon crystallization. The grid covers the mass
range from 0.52 to 1.0 Msun, and it is appropriate for the study of white
dwarfs in metal-rich clusters. The evolutionary calculations are based on a
detailed and self-consistent treatment of the energy released from these two
processes, as well as on the employment of realistic carbon/oxygen profiles, of
relevance for an accurate evaluation of the energy released by carbon/oxygen
phase separation. We find that 22Ne sedimentation strongly delays the cooling
rate of white dwarfs stemming from progenitors with high metallicities at
moderate luminosities, whilst carbon/oxygen phase separation adds considerable
delays at low luminosities. Cooling times are sensitive to possible
uncertainties in the actual value of the diffusion coefficient of 22Ne.
Changing the diffusion coefficient by a factor of 2, leads to maximum age
differences of approx. 8-20% depending on the stellar mass. We find that the
magnitude of the delays resulting from chemical changes in the core is
consistent with the slow down in the white dwarf cooling rate that is required
to solve the age discrepancy in NGC 6791.Comment: 10 pages, 6 figures, to be published in The Astrophysical Journa
The Possible White Dwarf-Neutron Star Connection
The current status of the problem of whether neutron stars can form, in close
binary systems, by accretion-induced collapse (AIC) of white dwarfs is
examined. We find that, in principle, both initially cold C+O white dwarfs in
the high-mass tail of their mass distribution in binaries and O+Ne+Mg white
dwarfs can produce neutron stars. Which fractions of neutron stars in different
types of binaries (or descendants from binaries) might originate from this
process remains uncertain.Comment: 6 pages. To appear in "White Dwarfs", ed. J. Isern, M. Hernanz, and
E. Garcia-Berro (Dordrecht: Kluwer
Crystallization of classical multi-component plasmas
We develop a method for calculating the equilibrium properties of the
liquid-solid phase transition in a classical, ideal, multi-component plasma.
Our method is a semi-analytic calculation that relies on extending the accurate
fitting formulae available for the one-, two-, and three-component plasmas to
the case of a plasma with an arbitrary number of components. We compare our
results to those of Horowitz, Berry, & Brown (Phys. Rev. E, 75, 066101, 2007),
who use a molecular dynamics simulation to study the chemical properties of a
17-species mixture relevant to the ocean-crust boundary of an accreting neutron
star, at the point where half the mixture has solidified. Given the same
initial composition as Horowitz et al., we are able to reproduce to good
accuracy both the liquid and solid compositions at the half-freezing point; we
find abundances for most species within 10% of the simulation values. Our
method allows the phase diagram of complex mixtures to be explored more
thoroughly than possible with numerical simulations. We briefly discuss the
implications for the nature of the liquid-solid boundary in accreting neutron
stars.Comment: 14 pages, 5 figures, submitted to Phys. Rev.
Gravitational Settling of ^{22}Ne in Liquid White Dwarf Interiors--Cooling and Seismological Effects
We assess the impact of the trace element ^{22}Ne on the cooling and
seismology of a liquid C/O white dwarf (WD). Due to this elements' neutron
excess, it sinks towards the interior as the liquid WD cools. The subsequent
gravitational energy released slows the cooling of the WD by 0.25--1.6 Gyrs by
the time it has completely crystallized, depending on the WD mass and the
adopted sedimentation rate. The effects will make massive WDs or those in metal
rich clusters (such as NGC 6791) appear younger than their true age. Our
diffusion calculations show that the ^{22}Ne mass fraction in the crystallized
core actually increases outwards. The stability of this configuration has not
yet been determined. In the liquid state, the settled ^{22}Ne enhances the
internal buoyancy of the interior and changes the periods of the high radial
order g-modes by approximately 1%. Though a small adjustment, this level of
change far exceeds the accuracy of the period measurements. A full assessment
and comparison of mode frequencies for specific WDs should help constrain the
still uncertain ^{22}Ne diffusion coefficient for the liquid interior.Comment: 26 pages (11 text pages with 15 figures); to appear in The
Astrophysical Journa
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
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