1,316 research outputs found
On the formation of hot DQ white dwarfs
We present the first full evolutionary calculations aimed at exploring the
origin of hot DQ white dwarfs. These calculations consistently cover the whole
evolution from the born-again stage to the white dwarf cooling track. Our
calculations provide strong support to the diffusive/convective-mixing picture
for the formation of hot DQs. We find that the hot DQ stage is a short-lived
stage and that the range of effective temperatures where hot DQ stars are found
can be accounted for by different masses of residual helium and/or different
initial stellar masses. In the frame of this scenario, a correlation between
the effective temperature and the surface carbon abundance in DQs should be
expected, with the largest carbon abundances expected in the hottest DQs. From
our calculations, we suggest that most of the hot DQs could be the cooler
descendants of some PG1159 stars characterized by He-rich envelopes markedly
smaller than those predicted by the standard theory of stellar evolution. At
least for one hot DQ, the high-gravity white dwarf SDSS J142625.70+575218.4, an
evolutionary link between this star and the massive PG1159 star H1504+65 is
plausible.Comment: 4 pages, 2 figures. To be published in The Astrophysical Journal
Letter
Lyman-alpha wing absorption in cool white dwarf stars
Kowalski & Saumon (2006) identified the missing absorption mechanism in the
observed spectra of cool white dwarf stars as the Ly-alpha red wing formed by
the collisions between atomic and molecular hydrogen and successfully explained
entire spectra of many cool DA-type white dwarfs. Owing to the important
astrophysical implications of this issue, we present here an independent
assessment of the process. For this purpose, we compute free-free
quasi-molecular absorption in Lyman-alpha due to collisions with H and H2
within the one-perturber, quasi-static approximation. Line cross-sections are
obtained using theoretical molecular potentials to describe the interaction
between the radiating atom and the perturber. The variation of the
electric-dipole transition moment with the interparticle distance is also
considered. Six and two allowed electric dipole transitions due to H-H and H-H2
collisions, respectively, are taken into account. The new theoretical
Lyman-alpha line profiles are then incorporated in our stellar atmosphere
program for the computation of synthetic spectra and colours of DA-type white
dwarfs. Illustrative model atmospheres and spectral energy distributions are
computed, which show that Ly-alpha broadening by atoms and molecules has a
significant effect on the white dwarf atmosphere models. The inclusion of this
collision-induced opacity significantly reddens spectral energy distributions
and affects the broadband colour indices for model atmospheres with Teff<5000
K. These results confirm those previously obtained by Kowalski & Saumon (2006).
Our study points out the need for reliable evaluations of H3 potential energy
surfaces covering a large region of nuclear configurations, in order to obtain
a better description of H-H2 collisions and a more accurate evaluation of their
influence on the spectrum of cool white dwarfs.Comment: 11 pages, 12 figures, 1 table, to be published in MNRA
Outer boundary conditions for evolving cool white dwarfs
White dwarf evolution is essentially a gravothermal cooling process,
which,for cool white dwarfs, sensitively depends on the treatment of the outer
boundary conditions. We provide detailed outer boundary conditions appropriate
for computing the evolution of cool white dwarfs employing detailed non-gray
model atmospheres for pure H composition. We also explore the impact on the
white dwarf cooling times of different assumptions for energy transfer in the
atmosphere of cool white dwarfs. Detailed non-gray model atmospheres are
computed taken into account non-ideal effects in the gas equation of state and
chemical equilibrium, collision-induced absorption from molecules, and the
Lyman alpha quasi-molecular opacity. Our results show that the use of detailed
outer boundary conditions becomes relevant for effective temperatures lower
than 5800 and 6100K for sequences with 0.60 and 0.90 M_sun, respectively.
Detailed model atmospheres predict ages that are up to approx 10% shorter at
log L/L_sun=-4 when compared with the ages derived using Eddington-like
approximations at tau_Ross=2/3. We also analyze the effects of various
assumptions and physical processes of relevance in the calculation of outer
boundary conditions. In particular, we find that the Ly_alpha red wing
absorption does not affect substantially the evolution of white dwarfs. White
dwarf cooling timescales are sensitive to the surface boundary conditions for
T_eff < 6000K. Interestingly enough, non-gray effects have little consequences
on these cooling times at observable luminosities. In fact, collision-induced
absorption processes, which significantly affect the spectra and colors of old
white dwarfs with hydrogen-rich atmospheres, have not noticeable effects in
their cooling rates, except throughout the Rosseland mean opacity.Comment: 6 pages, 9 figures, to be published in Astronomy and Astrophysic
The ages and colours of cool helium-core white dwarf stars
The purpose of this work is to explore the evolution of helium-core white
dwarf stars in a self-consistent way with the predictions of detailed non-gray
model atmospheres and element diffusion. To this end, we consider helium-core
white dwarf models with stellar masses of 0.406, 0.360, 0.327, 0.292, 0.242,
0.196 and 0.169 solar masses and follow their evolution from the end of mass
loss episodes during their pre-white dwarf evolution down to very low surface
luminosities. We find that when the effective temperature decreases below
4000K, the emergent spectrum of these stars becomes bluer within time-scales of
astrophysical interest. In particular, we analyse the evolution of our models
in the colour-colour and colour-magnitude diagrams and we find that helium-core
white dwarfs with masses ranging from approx. 0.18 to 0.3 solar masses can
reach the turn-off in their colours and become blue again within cooling times
much less than 15 Gyr and then remain brighter than M_V approx. 16.5. In view
of these results, many low-mass helium white dwarfs could have had time enough
to evolve to the domain of collision-induced absorption from molecular
hydrogen, showing blue colours.Comment: 11 pages, 9 figures. Accepted for publication in MNRA
An independent constraint on the secular rate of variation of the gravitational constant from pulsating white dwarfs
A secular variation of the gravitational constant modifies the structure and
evolutionary time scales of white dwarfs. Using an state-of-the-art stellar
evolutionary code and an up-to-date pulsational code we compute the effects of
a secularly varying on the pulsational properties of variable white dwarfs.
Comparing the the theoretical results obtained taking into account the effects
of a running with the observed periods and measured rates of change of the
periods of two well studied pulsating white dwarfs, G117--B15A and R548, we
place constraints on the rate of variation of Newton's constant. We derive an
upper bound yr using the variable
white dwarf G117--B15A, and yr using
R548. Although these upper limits are currently less restrictive than those
obtained using other techniques, they can be improved in a future measuring the
rate of change of the period of massive white dwarfs.Comment: 13 pages, 4 tables, 3 figures. To be published in the Journal of
Cosmology and Astroparticle Physic
The seismic properties of low-mass He-core white dwarf stars
We present here a detailed pulsational study applied to low-mass He-core
white dwarfs, based on full evolutionary models representative of these
objects. The background stellar models on which our pulsational analysis was
carried out were derived by taking into account the complete evolutionary
history of the progenitor stars, with special emphasis on the diffusion
processes acting during the white dwarf cooling phase. We computed nonradial
-modes to assess the dependence of the pulsational properties of these
objects with stellar parameters such as the stellar mass and the effective
temperature, and also with element diffusion processes. We also performed a g-
and p-mode pulsational stability analysis on our models and found well-defined
blue edges of the instability domain, where these stars should start to exhibit
pulsations. We found substantial differences in the seismic properties of white
dwarfs with and the extremely low-mass (ELM) white
dwarfs (). Specifically, -mode pulsation modes
in ELM white dwarfs mainly probe the core regions and are not dramatically
affected by mode-trapping effects by the He/H interface, whereas the opposite
is true for more massive He-core white dwarfs. We found that element diffusion
processes substantially affects the shape of the He/H chemical transition
region, leading to non-negligible changes in the period spectrum of low-mass
white dwarfs. Our stability analysis successfully predicts the pulsations of
the only known variable low-mass white dwarf (SDSS J184037.78+642312.3), and
also predicts both - and -mode pulsational instabilities in a significant
number of known low-mass and ELM white dwarfs.Comment: 14 pages, 15 figures, 2 tables. To be published in Astronomy &
Astrophysic
Probing the internal rotation of pre-white dwarf stars with asteroseismology: the case of PG 122+200
We put asteroseismological constraints on the internal rotation profile of
the GW Vir (PG1159-type) star PG 0122+200. To this end we employ a
state-of-the-art asteroseismological model for this star and we assess the
expected frequency splittings induced by rotation adopting a forward approach
in which we compare the theoretical frequency separations with the observed
ones assuming different types of plausible internal rotation profiles. We also
employ two asteroseismological inversion methods for the inversion of the
rotation profile of PG 0122+200. We find evidence for differential rotation in
this star. We demonstrate that the frequency splittings of the rotational
multiplets exhibited by PG 0122+200 are compatible with a rotation profile in
which the central regions are spinning about 2.4 times faster than the stellar
surface.Comment: 8 pages, 6 figures, 2 tables. To be published in MNRA
Asteroseismological study of massive ZZ Ceti stars with fully evolutionary models
We present the first asteroseismological study for 42 massive ZZ Ceti stars
based on a large set of fully evolutionary carbonoxygen core DA white dwarf
models characterized by a detailed and consistent chemical inner profile for
the core and the envelope. Our sample comprise all the ZZ Ceti stars with
spectroscopic stellar masses between 0.72 and known to date.
The asteroseismological analysis of a set of 42 stars gives the possibility to
study the ensemble properties of the massive pulsating white dwarf stars with
carbonoxygen cores, in particular the thickness of the hydrogen envelope and
the stellar mass. A significant fraction of stars in our sample have stellar
mass high enough as to crystallize at the effective temperatures of the ZZ Ceti
instability strip, which enables us to study the effects of crystallization on
the pulsation properties of these stars. Our results show that the phase
diagram presented in Horowitz et al. (2010) seems to be a good representation
of the crystallization process inside white dwarf stars, in agreement with the
results from white dwarf luminosity function in globular clusters.Comment: 58 pages, 11 figures, accepted in Ap
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