108 research outputs found
GD 99 - an unusual, rarely observed DAV white dwarf
New observations of GD 99 are analysed. The unusual pulsation behaviour,
showing both long and short periods, has been confirmed. All the available
periods show a grouping of short and long period modes with roughly regular
spacing. If we interpret the groups separately, a binary nature can be a
possible explanation as in the similar cases of WD 2350-0054 and G29-38.Comment: 2 pages, 1 eps figure; has been accepted for publication in
Communications in Asteroseismology (Vol. 150, 2007), Proceedings of the
Vienna Workshop on the Future of Asteroseismolog
Unexpected series of regular frequency spacing of delta Scuti stars in the non-asymptotic regime -- I. The methodology
A sequence search method was developed to search regular frequency spacing in
delta Scuti stars by visual inspection and algorithmic search. We searched for
sequences of quasi-equally spaced frequencies, containing at least four members
per sequence, in 90 delta Scuti stars observed by CoRoT. We found an
unexpectedly large number of independent series of regular frequency spacing in
77 delta Scuti stars (from 1 to 8 sequences) in the non-asymptotic regime. We
introduce the sequence search method presenting the sequences and echelle
diagram of CoRoT 102675756 and the structure of the algorithmic search. Four
sequences (echelle ridges) were found in the 5-21 d^{-1} region, where the
pairs of the sequences are shifted (between 0.5-0.59 d^{-1}) by twice the value
of the estimated rotational splitting frequency (0.269 d^{-1}). The general
conclusions for the whole sample are also presented in this paper. The
statistics of the spacings derived by the sequence search method, by FT and
that of the shifts are also compared. In many stars, more than one almost
equally valid spacing appeared. The model frequencies of FG Vir and their
rotationally split components were used to reveal a possible explanation that
one spacing is the large separation, while the other is a sum of the large
separation and the rotational frequency. In CoRoT 102675756, the two spacings
(2.249 and 1.977 d^{-1}) agree better with the sum of a possible 1.710 d^{-1}
large separation and two or one times, respectively, the value of the
rotational frequency.Comment: 12 pages, 10 figures, accepted for publication in Ap
Unexpected series of regular frequency spacing of delta Scuti stars in the non-asymptotic regime -- II. Sample -- echelle diagrams and rotation
A sequence search method was developed for searching for regular frequency
spacing in delta Scuti stars by visual inspection and algorithmic search. The
sample contains 90 delta Scuti stars observed by CoRoT. An example is given to
represent the visual inspection. The algorithm (SSA) is described in detail.
The data treatment of the CoRoT light curves, the criteria for frequency
filtering and the spacings derived by two methods (three approaches: VI, SSA
and FT) are given for each target. Echelle diagrams are presented for 77
targets, for which at least one sequence of regular spacing was identified.
Comparing the spacing and the shifts between pairs of echelle ridges revealed
that at least one pair of echelle ridges is shifted to midway between the
spacing for 22 stars. The estimated rotational frequencies compared to the
shifts revealed rotationally split doublets, triplets and multiplets not only
for single frequencies, but for the complete echelle ridges in 31 delta Scuti
stars. Using several possible assumptions for the origin of the spacings, we
derived the large separation (), which are distributed along the
mean density versus large separations relation derived from stellar models
Suarez 2014.Comment: 25 pages, 16 figures, 9 tables. Accepted for publication in ApJ
Amplitude Modulation in the ZZ Ceti Star GD 244
Previous studies of GD 244 revealed seven pulsation frequencies (two doublets
and three single periods) in the light variations of the star. The data
obtained at McDonald Observatory between 2003 and 2006, and our additional
measurements in 2006 and 2007 at Konkoly Observatory, allow the investigation
of the long-term pulsational behaviour of GD 244. We found that the 307.1 s
period component of one of the doublets show long-term, periodic amplitude
modulation with a time scale of ~740 days. Possible explanations are that
nonlinear resonant mode coupling is operating among the rotationally split
frequency components, or two modes, unresolved in the yearly data are excited
at ~307.1 s. This is the first time that such long-term periodic amplitude
modulation is published on a ZZ Ceti star.Comment: 4 pages, 2 figures, appeared in ASP Conference Series vol. 493, 2015
(eds.: P. Dufour, P. Bergeron, G. Fontaine, 19th European Workshop on White
Dwarfs, Montreal, Canada
Revealing the pulsational properties of the V777 Her star KUV 05134+2605 by its long-term monitoring
Context: KUV 05134+2605 is one of the 21 pulsating DB white dwarfs (V777 Her
or DBV variables) known so far. The detailed investigation of the short-period
and low-amplitude pulsations of these relatively faint targets requires
considerable observational efforts from the ground, long-term single-site or
multisite observations. The observed amplitudes of excited modes undergo
short-term variations in many cases, which makes the determination of pulsation
modes difficult.
Methods: We re-analysed the data already published, and collected new
measurements. We compared the frequency content of the different datasets from
the different epochs and performed various tests to check the reliability of
the frequency determinations. The mean period spacings were investigated with
linear fits to the observed periods, Kolmogorov-Smirnov and Inverse Variance
significance tests, and Fourier analysis of different period sets, including a
Monte Carlo test simulating the effect of alias ambiguities. We employed fully
evolutionary DB white dwarf models for the asteroseismic investigations.
Results: We identified 22 frequencies between 1280 and 2530 microHz. These
form 12 groups, which suggests at least 12 possible frequencies for the
asteroseismic investigations. Thanks to the extended observations, KUV
05134+2605 joined the group of rich white dwarf pulsators. We identified one
triplet and at least one doublet with a ~9 microHz frequency separation, from
which we derived a stellar rotation period of 0.6 d. We determined the mean
period spacings of ~31 and ~18 s for the modes we propose as dipole and
quadrupole, respectively. We found an excellent agreement between the stellar
mass derived from the l=1 period spacing and the period-to-period fits, all
providing M_* = 0.84-0.85 M_Sun solutions. Our study suggests that KUV
05134+2605 is the most massive amongst the known V777 Her stars.Comment: 15 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
G 207-9 and LP 133-144: light curve analysis and asteroseismology of two ZZ Ceti stars
G 207-9 and LP 133-144 are two rarely observed ZZ Ceti stars located in the
middle and close to the blue edge of the ZZ Ceti instability domain,
respectively. We aimed to observe them at least during one observing season at
Konkoly Observatory with the purpose of extending the list of known pulsation
modes for asteroseismic investigations and detect any significant changes in
their pulsational behaviour. We determined five and three new normal modes of G
207-9 and LP 133-144, respectively. In LP 133-144, our frequency analysis also
revealed that at least at three modes there are actually triplets with
frequency separations of ~4 microHz. The rotational period of LP 133-144 based
on the triplets is ~42 h. The preliminary asteroseismic fits of G 207-9 predict
Teff=12 000 or 12 400 K and M*=0.855-0.870 MSun values for the effective
temperature and mass of the star, depending on the assumptions on the spherical
degree (l) values of the modes. These results are in agreement with the
spectroscopic determinations. In the case of LP 133-144, the best-fitting
models prefer Teff=11 800 K in effective temperature and M*>=0.71 MSun stellar
masses, which are more than 0.1 MSun larger than the spectroscopic value.Comment: 12 pages, 11 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (2016 June 30
- …