74 research outputs found
New DA white dwarf evolutionary models and their pulsational properties
In this letter we investigate the pulsational properties of ZZ Ceti stars on
the basis of new white dwarf evolutionary models calculated in a
self-consistent way with the predictions of time dependent element diffusion
and nuclear burning. In addition, full account is taken of the evolutionary
stages prior to the white dwarf formation. Emphasis is placed on the trapping
properties of such models. By means of adiabatic, non-radial pulsation
calculations, we find, as a result of time dependent diffusion, a much weaker
mode trapping effect, particularly for the high-period regime of the pulsation
g-spectrum. This result is valid at least for models with massive hydrogen-rich
envelopes. Thus, mode trapping would not be an effective mechanism to explain
the fact that all the high periods expected from standard models of stratified
white dwarfs are not observed in the ZZ Ceti stars.Comment: 3 pages, 5 figures, accepted for publication in Astronomy &
Astrophysics Letter
The rate of period change in pulsating DB white dwarf stars
In this work, we present the theoretically expected rates of pulsation period
change for V777 Her (DBV) variable stars. To this end we employ new
evolutionary models representative of pulsating DB white dwarf stars computed
in a self-consistent way with the predictions of time-dependent element
diffusion. At the hot edge of the DB instability strip, the envelopes of the
models are characterized by a diffusion-induced double-layered chemical
structure. We compute the numerical values of rates of period change by solving
the equations of linear, adiabatic, nonradial stellar oscillations. We examine
the effects of varying the stellar mass, the mass of the helium envelope and
the neutrino emission on the expected period changes. We present extensive
tabulations of our results which could be useful for comparison with future
detections of the rate of period change in pulsating DB white dwarfs.Comment: 13 pages, including 5 postscript figures and 9 tables. Accepted for
publication in Astronomy and Astrophysic
The double-layered chemical structure in DB white dwarfs
We study the structure and evolution of white dwarf stars with helium-rich
atmospheres (DB) in a self-consistent way with the predictions of
time-dependent element diffusion. Our treatment of diffusion includes
gravitational settling and chemical and thermal diffusion. OPAL radiative
opacities for arbitrary metallicity and carbon-and oxygen-rich compositions are
employed. Emphasis is placed on the evolution of the diffusion-modeled
double-layered chemical structure. This structure, which is characterized by a
pure helium envelope atop an intermediate remnant shell rich in helium, carbon
and oxygen, is expected for pulsating DB white dwarfs, assuming that they are
descendants of hydrogen-deficient PG1159 post-AGB stars. We find that,
depending on the stellar mass, if DB white dwarf progenitors are formed with a
helium content smaller than \approx 10^-3 M_*, a single-layered configuration
is expected to emerge during the DB pulsation instability strip. We also
explore the consequences of diffusively evolving chemical stratifications on
the adiabatic pulsational properties of our DB white dwarf models. In this
context, we find that the evolving shape of the chemical profile translates
into a distinct behaviour of the theoretical period distribution as compared
with the case in which the shape of the profile is assumed to be fixed during
the evolution across the instability strip. Finally, we extend the scope of the
calculations to the domain of the helium-rich carbon-contaminated DQ white
dwarfs. In particular, we speculate that DQ white dwarfs with low detected
carbon abundances would not be descendants of the PG1159 stars.Comment: 10 Pages, including 11 Postscript figures. Accepted for publication
in Astronomy and Astrophysic
The potential of the variable DA white dwarf G117-B15A as a tool for Fundamental Physics
White dwarfs are well studied objects. The relative simplicity of their
physics allows to obtain very detailed models which can be ultimately compared
with their observed properties. Among white dwarfs there is a specific class of
stars, known as ZZ-Ceti objects, which have a hydrogen-rich envelope and show
periodic variations in their light curves. G117-B15A belongs to this particular
set of stars. The luminosity variations have been successfully explained as due
to g-mode pulsations. G117-B15A has been recently claimed to be the most stable
optical clock ever found, being the rate of change of its 215.2 s period very
small: \dot{P}= (2.3 +- 1.4)x10^{-15} s s^-1, with a stability comparable to
that of the most stable millisecond pulsars. The rate of change of the period
is closely related to its cooling timescale, which can be accurately computed.
In this paper we study the pulsational properties of G117-B15A and we use the
observed rate of change of the period to impose constraints on the axion
emissivity and, thus, to obtain a preliminary upper bound to the mass of the
axion. This upper bound turns out to be 4cos^{2}{\beta} meV at the 95%
confidence level. Although there are still several observational and
theoretical uncertainties, we conclude that G117-B15A is a very promising
stellar object to set up constraints on particle physics.Comment: 32 pages, 14 figures, accepted for publication in New Astronom
Time dependent diffusion in pulsating white dwarf stars: Asteroseismology of G117-B15A
We study the structural characteristic of the variable DA white dwarf
G117B-15A by applying the methods of asteroseismology. For such a purpose, we
construct white dwarf evolutionary models considering a detailed and up-to-date
physical description as well as several processes responsible for the
occurrence of element diffusion. We have considered several thickness for the
outermost hydrogen layer, whereas for the inner helium-, carbon- and
oxygen-rich layers we considered realistic profiles predicted by calculations
of the white dwarf progenitor evolution. The evolution of each of the
considered model sequences were followed down to very low effective
temperature; in particular, from 12500K on we computed the dipolar, linear,
adiabatic oscillations with low radial order. We find that asteroseismological
results are not univocal regarding mode identification for the case of
G117B-15A. However, our asteroseismological results are compatible with
spectroscopical data only if the observed periods of 215.2, 271.0 and 304.4 s
are due to dipolar modes with respectively. Our calculations
indicate that the best fit to the observed period pattern of G117B-15A
corresponds to a DA white dwarf structure with a stellar mass of 0.525 \msun,
with a hydrogen mass fraction \lmh-3.83 at an effective temperature
\teff11800K.Comment: 8 pages, 8 figures, to be published in MNRA
New evolutionary models for massive ZZ Ceti stars. II. The effects of crystallization on their pulsational properties
We present in this work new pulsational calculations for improved
carbon-oxygen DA white dwarf models suitable for the study of massive ZZ Ceti
stars. The background models employed in this study, presented in detail in a
recent paper by Althaus et al. (2003), are the result of the complete evolution
of massive white dwarf progenitors from the zero-age main sequence through the
Asymptotic Giant Branch (AGB) and mass loss phases to the white dwarf regime.
Abundance changes are accounted for by means of a full coupling between nuclear
evolution and time-dependent mixing due to convection, salt fingers, and
diffusive overshoot. In addition, time-dependent element diffusion for
multicomponent gases has been considered during the white dwarf evolution.
Crystallization and chemical rehomogenization due to phase separation upon
crystallization in the core of our models have been fully considered. The
effects of crystallization on the period spectrum of these massive white dwarf
models are assessed by means of a detailed pulsational analysis. We find that
the theoretical pulsation spectrum is strongly modified when crystallization is
considered, in particular concerning the mode trapping properties of the
equilibrium models. We also discuss at some length the implications of our
study for BPM 37093, the most massive ZZ Ceti star presently known. We find
that if BPM 37093 has a stellar mass of \msun its observed
spectrum could bear the signature of overshoot episodes during the helium core
burning.Comment: 15 Pages, including 17 Postscript figures. Accepted for publication
in Astronomy and Astrophysic
New evolutionary models for massive ZZ Ceti stars, I : first results for their pulsational properties
Chaotic orbits suffer significant changes as a result of small perturbations. One can thus wonder whether the dynamical friction suffered by a satellite on a regular orbit, and interacting with the stars of a galaxy, will be different if the bulk of the stars of the galaxy are in regular or chaotic orbits. In order to check that idea, we investigated the orbital decay (caused by dynamical friction) of a rigid satellite moving within a larger stellar system (a galaxy) whose potential is nonintegrable. We performed numerical experiments using two kinds of triaxial galaxy models: (1) the triaxial generalization of Dehnen's spherical mass model (Dehnen; Merritt & Fridman); (2) a modified Satoh model (Satoh; Carpintero, Muzzio, & Wachlin). The percentages of chaotic orbits present in these models were increased by perturbing them. In the first case, a central compact object (black hole) was introduced; in the second case, the perturbation was produced by allowing the galaxy to move on a circular orbit in a logarithmic potential. The equations of motion were integrated with a non-self-consistent code. Our results show that the presence of chaotic orbits does not affect significantly the orbital decay of the satellite.Fil: Althaus, Leandro Gabriel. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Serenelli, A. M.. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Corsico, Alejandro Hugo. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Montgomery, M. H.. Institute of Astronomy; Reino Unid
Seismology Of A Massive Pulsating Hydrogen Atmosphere White Dwarf
We report our observations of the new pulsating hydrogen atmosphere white dwarf SDSS J132350.28+010304.22. We discovered periodic photometric variations in frequency and amplitude that are commensurate with nonradial g-mode pulsations in ZZ Ceti stars. This, along with estimates for the star's temperature and gravity, establishes it as a massive ZZ Ceti star. We used time-series photometric observations with the 4.1 m SOAR Telescope, complemented by contemporary McDonald Observatory 2.1 m data, to discover the photometric variability. The light curve of SDSS J132350.28+010304.22 shows at least nine detectable frequencies. We used these frequencies to make an asteroseismic determination of the total mass and effective temperature of the star: M-star = 0.88 +/- 0.02 M-circle dot and T-eff = 12,100 +/- 140 K. These values are consistent with those derived from the the optical spectra and photometric colors.CNPqFAPERGS/PronexUS National Science Foundation AST-0909107Norman Hackerman Advanced Research Program 003658-0252-2009MICINN grant AYA08-1839/ESPESF EUROCORES Program EuroGENESIS (MICINN grant) EUI2009-04170Generalitat de Catalunya 2009SGR315EU-FEDER fundsAGENCIA through the Programa de Modernizacion Tecnologica BID 1728/OC-ARCONICET PIP 112-200801-00940Astronom
Measuring The Evolutionary Rate Of Cooling Of ZZ Ceti
We have finally measured the evolutionary rate of cooling of the pulsating hydrogen atmosphere (DA) white dwarf ZZ Ceti (Ross 548), as reflected by the drift rate of the 213.13260694 s period. Using 41 yr of time-series photometry from 1970 November to 2012 January, we determine the rate of change of this period with time to be dP/dt = (5.2 +/- 1.4) x 10(-15) s s(-1) employing the O - C method and (5.45 +/- 0.79) x 10(-15) s s(-1) using a direct nonlinear least squares fit to the entire lightcurve. We adopt the dP/dt obtained from the nonlinear least squares program as our final determination, but augment the corresponding uncertainty to a more realistic value, ultimately arriving at the measurement of dP/dt = (5.5 +/- 1.0) x 10(-15) s s(-1). After correcting for proper motion, the evolutionary rate of cooling of ZZ Ceti is computed to be (3.3 +/- 1.1) x 10(-15) s s(-1). This value is consistent within uncertainties with the measurement of (4.19 +/- 0.73) x 10(-15) s s(-1) for another similar pulsating DA white dwarf, G 117-B15A. Measuring the cooling rate of ZZ Ceti helps us refine our stellar structure and evolutionary models, as cooling depends mainly on the core composition and stellar mass. Calibrating white dwarf cooling curves with this measurement will reduce the theoretical uncertainties involved in white dwarf cosmochronometry. Should the 213.13 s period be trapped in the hydrogen envelope, then our determination of its drift rate compared to the expected evolutionary rate suggests an additional source of stellar cooling. Attributing the excess cooling to the emission of axions imposes a constraint on the mass of the hypothetical axion particle.NSF AST-1008734, AST-0909107Norman Hackerman Advanced Research Program 003658-0252-2009Astronom
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