1,328 research outputs found
The temporal changes of the pulsational periods of the pre-white dwarf PG 1159-035
PG 1159-035, a pre-white dwarf with T=140000 K, is the prototype of the
PG1159 spectroscopic class and the DOV pulsating class. Changes in the star
cause variations in its oscillation periods. The measurement of temporal change
in the oscillation periods, dP/dt, allows us to estimate directly rates of
stellar evolutionary changes, such as the cooling rate and the envelope
contraction rate, providing a way to test and refine evolutionary models for
pre-white dwarf pulsating stars.
We measured 27 pulsation modes period changes. The periods varied at rates of
between 1 and 100 ms/yr, and several can be directly measured with a relative
standard uncertainty below 10%. For the 516.0 s mode (the highest in amplitude)
in particular, not only the value of dP/dt can be measured directly with a
relative standard uncertainty of 2%, but the second order period change,
d(dP/dt)/dt, can also be calculated reliably. By using the (O-C) method we
refined the dP/dt and estimated the d(dP/dt)/dt for six other pulsation
periods. As a first application, we calculated the change in the PG 1559-035
rotation period, dP_rot/dt = -2.13*10^{-6} s/s, the envelope contraction rate
dR/dt = -2.2*10^{-13} solar radius/s, and the cooling rante dT/dt =
-1.42*10^{-3} K/s.Comment: 8 pages; 2 figures; 2 tables; appendix with 2 table
Low mass variable stars in the globular cluster NGC 6397
We have conducted a photometric survey of the globular cluster NGC 6397 in a
search for variable stars. We obtained ~11h of time-resolved photometric images
with one ne European Southern Observatory-Very Large Telescope using the FOcal
Reducer and low dispersion Spectrograph imager distributed over two consecutive
nights. We analyzed 8391 light curves of stars brighter than magnitude 23 with
the 465 nm-filter, and we identified 412 variable stars, reaching ~ 4.8 +- 0.2
per cent of variability with timescales between 0.004 and 2d, with amplitudes
variation greater than +- 0.2 mag.Comment: 9 figures, complementary dat
SDSS White Dwarf mass distribution at low effective temperatures
The DA white dwarfs in the Sloan Digital Sky Survey, as analyzed in the
papers for Data Releases 1 and 4, show an increase in surface gravity towards
lower effective temperatures below 11500 K. We study the various possible
explanations of this effect, from a real increase of the masses to
uncertainties or deficiencies of the atmospheric models. No definite answer is
found but the tentative conclusion is that it is most likely the current
description of convection in the framework of the mixing-length approximation,
which leads to this effect.Comment: to appear in the proceedings of the 16th European Workshop on White
Dwarfs, Barcelona, 200
Multichromatic colour-magnitude diagrams of the globular cluster NGC 6366
We present multichromatic isochrone fits to the colour-magnitude data of the
globular cluster NGC 6366, based on Hubble Space Telescope Advanced Camera for
Surveys/Wide Field Channel and Southern Astrophysical Research photometric
data. We corrected the photometric data for differential reddening and
calculated the mean ridge line of the colour-magnitude diagrams. We compared
the isochrones of Dartmouth Stellar Evolution Database and PAdova and TRieste
Stellar Evolution Code both with microscopic diffusion starting on the main
sequence. Bracketing all previous determinations of this cluster, we tested
metallicities from [Fe/H]=-1.00 to [Fe/H]=-0.50, and ages from 9 to 13 Gyr.
After determining the total to selective extinction ratio only from stars
belonging to this cluster, R_V=3.06+/-0.14, we found the parameters for this
cluster to be E(B-V)=0.69+/-0.02(int)+/-0.04(ext),
(m-M)_V=15.02+/-0.07(int)+/-0.13(ext), Age=11+/-1.15 Gyr. Evolutionary models
fail to reproduce the low-Teff sequence in multiband colour-magnitude diagrams,
indicating that they still have an incomplete physics. We found that the
Dartmouth Stellar Evolution Database isochrones better fit the subgiant branch
and low main sequence than the PAdova and TRieste Stellar Evolution Code.Comment: 9 pages, 5 figure
The rate of cooling of the pulsating white dwarf star G117B15A: a new asteroseismological inference of the axion mass
We employ a state-of-the-art asteroseismological model of G117-B15A, the
archetype of the H-rich atmosphere (DA) white dwarf pulsators (also known as
DAV or ZZ Ceti variables), and use the most recently measured value of the rate
of period change for the dominant mode of this pulsating star to derive a new
constraint on the mass of axion, the still conjectural non-barionic particle
considered as candidate for dark matter of the Universe. Assuming that
G117-B15A is truly represented by our asteroseismological model, and in
particular, that the period of the dominant mode is associated to a pulsation
g-mode trapped in the H envelope, we find strong indications of the existence
of extra cooling in this star, compatible with emission of axions of mass m_a
\cos^2 \beta = 17.4^{+2.3}_{-2.7} meV.Comment: 9 pages, 5 figures and 3 tables. Accepted for publication in MNRA
Axions and the pulsation periods of variable white dwarfs revisited
Axions are the natural consequence of the introduction of the Peccei-Quinn
symmetry to solve the strong CP problem. All the efforts to detect such elusive
particles have failed up to now. Nevertheless, it has been recently shown that
the luminosity function of white dwarfs is best fitted if axions with a mass of
a few meV are included in the evolutionary calculations. Our aim is to show
that variable white dwarfs can provide additional and independent evidence
about the existence of axions. The evolution of a white dwarf is a slow cooling
process that translates into a secular increase of the pulsation periods of
some variable white dwarfs, the so-called DAV and DBV types. Since axions can
freely escape from such stars, their existence would increase the cooling rate
and, consequently, the rate of change of the periods as compared with the
standard ones. The present values of the rate of change of the pulsation period
of G117-B15A are compatible with the existence of axions with the masses
suggested by the luminosity function of white dwarfs, in contrast with previous
estimations. Furthermore, it is shown that if such axions indeed exist, the
drift of the periods of pulsation of DBV stars would be noticeably perturbed.Comment: Accepted for publication in Astronomy & Astrophysic
Mode Identification from Combination Frequency Amplitudes in ZZ Ceti Stars
The lightcurves of variable DA stars are usually multi-periodic and
non-sinusoidal, so that their Fourier transforms show peaks at eigenfrequencies
of the pulsation modes and at sums and differences of these frequencies. These
combination frequencies provide extra information about the pulsations, both
physical and geometrical, that is lost unless they are analyzed. Several
theories provide a context for this analysis by predicting combination
frequency amplitudes. In these theories, the combination frequencies arise from
nonlinear mixing of oscillation modes in the outer layers of the white dwarf,
so their analysis cannot yield direct information on the global structure of
the star as eigenmodes provide. However, their sensitivity to mode geometry
does make them a useful tool for identifying the spherical degree of the modes
that mix to produce them. In this paper, we analyze data from eight hot,
low-amplitude DAV white dwarfs and measure the amplitudes of combination
frequencies present. By comparing these amplitudes to the predictions of the
theory of Goldreich & Wu, we have verified that the theory is crudely
consistent with the measurements. We have also investigated to what extent the
combination frequencies can be used to measure the spherical degree (ell) of
the modes that produce them. We find that modes with ell > 2 are easily
identifiable as high ell based on their combination frequencies alone.
Distinguishing between ell=1 and 2 is also possible using harmonics. These
results will be useful for conducting seismological analysis of large ensembles
of ZZ Ceti stars, such as those being discovered using the Sloan Digital Sky
Survey. Because this method relies only on photometry at optical wavelengths,
it can be applied to faint stars using 4 m class telescopes.Comment: 73 pages, 22 figures, accepted in the Ap
A puzzling periodicity in the pulsating DA white dwarf G 117-B15A
We present time-resolved optical spectrophotometry of the pulsating hydrogen
atmosphere (DA) white dwarf G 117-B15A. We find three periodicities in the
pulsation spectrum (215s, 272s, and 304s) all of which have been found in
earlier studies. By comparing the fractional wavelength dependence of the
pulsation amplitudes (chromatic amplitudes) with models, we confirm a previous
report that the strongest mode, at 215s, has l=1. The chromatic amplitude for
the 272s mode is very puzzling, showing an increase in fractional amplitude
with wavelength that cannot be reproduced by the models for any l at optical
wavelengths. Based on archival HST data, we show that while the behaviour of
the 215s mode at ultra-violet wavelengths is as expected from models, the weird
behaviour of the 272s periodicity is not restricted to optical wavelengths in
that it fails to show the expected increase in fractional amplitude towards
shorter wavelengths. We discuss possible causes for the discrepancies found for
the 272s variation, but find that all are lacking, and conclude that the nature
of this periodicity remains unclear.Comment: 9 pages, 9 figures; accepted for publication in A&
New Pulsating DB White Dwarf Stars from the Sloan Digital Sky Survey
We are searching for new He atmosphere white dwarf pulsators (DBVs) based on
the newly found white dwarf stars from the spectra obtained by the Sloan
Digital Sky Survey. DBVs pulsate at hotter temperature ranges than their better
known cousins, the H atmosphere white dwarf pulsators (DAVs or ZZ Ceti stars).
Since the evolution of white dwarf stars is characterized by cooling,
asteroseismological studies of DBVs give us opportunities to study white dwarf
structure at a different evolutionary stage than the DAVs. The hottest DBVs are
thought to have neutrino luminosities exceeding their photon luminosities
(Winget et al. 2004), a quantity measurable through asteroseismology.
Therefore, they can also be used to study neutrino physics in the stellar
interior. So far we have discovered nine new DBVs, doubling the number of
previously known DBVs. Here we report the new pulsators' lightcurves and power
spectra.Comment: 15 pages, 2 figures, 3 tables, ApJ accepte
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