69 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
Rotation of ZZ Ceti stars as seen by TESS
Context. Knowing the rotation rates and masses of white dwarf stars is an
important step towards characterising the angular momentum transport mechanism
in their progenitors, and coupling the cores of red giants to their envelopes.
However, deriving these rotation rates is not an easy task. One can use the
rotational broadening of spectral lines, but there is another way to gather
reliable information on the stellar rotation periods of pulsators: through
studying the splitting effect of rotation on oscillation frequencies.
Aims. We aim to derive stellar rotation periods in the TESS sample for as
many white dwarf pulsators as possible.
Methods. We rely on light-curve analysis of the TESS observations, and search
for closely spaced frequency multiplets that could be rotationally split
pulsation modes. We work with triplet frequencies, even if one or two triplet
components are only marginally detectable. We also utilise ground-based
observations available from the literature in an attempt to confirm the
presence of several triplets.
Results. We successfully identified rotationally split multiplets and derived
rotation rates for 14 stars. The fastest rotators we identified have rotation
periods of 6.6-10.0 h. The majority of the pulsators rotate with periods of
11.9-47.5 h, while we derived 85.5 and 93.2 h periods for the slowest rotators.
In addition to providing stellar mass estimations, our results confirm previous
findings that larger-mass WDs rotate faster than their lower-mass counterparts.
We determine the rotation periods of four stars for the first time.Comment: 8 pages, 2 figures, accepted for publication in Astronomy &
Astrophysic
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
GD 99: Re-investigation of an old ZZ Ceti companion
Context. Thanks to photometric space missions, we have access to more and
more information on the properties of white dwarf stars, especially pulsating
ones. In the case of pulsators, we have the opportunity to get an insight into
their otherwise hidden interiors by the means of asteroseismology. In addition
to space-based observations, we also take advantage of the opportunity to study
the pulsations of white dwarf stars from the ground, either as observations
that are complementary to space-based measurements or individual observing runs
on selected targets across long timescales.
Aims. We aim to investigate long-term, single-site observations of the
bright, yet scarcely studied ZZ Ceti star, GD 99. Our main goals are to
determine as many eigenmodes for asteroseismology as possible and then to carry
out a seismic analysis of this target.
Methods. We performed a Fourier analysis of the light curves obtained in
different epochs. After finding the normal modes of the pulsation, we ran the
2018 version of the White Dwarf Evolution Code to build model grids for the
period fits. We compared the seismic distance of the best-fit model with the
geometric value provided by Gaia measurements.
Results. We find that GD 99 is rich in pulsation modes in the 200-1100 s
period range, as we detected seven new periods. Together with the literature
data, we were able to use 11 modes for the asteroseismic fits. We accepted an
asteroseismic model solution with Teff = 13 500 K and M* = 0.80 Msun as a best
fit, however, this suggests a hotter and more massive star than we might have
expected based on the spectroscopic values. We also estimated the rotational
rate of the star to be 13.17 h, based on TESS observations.Comment: 9 pages, 8 figures, accepted for publication in Astronomy &
Astrophysics; v2: after modification in the abstract and language editin
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
MOST light-curve analysis of the gamma Dor pulsator HR 8799, showing resonances and amplitude variations
Context The central star of the HR 8799 system is a gamma Doradus-type pulsator. The system harbours four planetary-mass companions detected by direct imaging, and is a good solar system analogue. The masses of the companions are not known accurately, because the estimation depends strongly on the age of the system, which is also not known with sufficient accuracy. Asteroseismic studies of the star might help to better constrain the age of HR\,8799. We organized an extensive photometric and multi-site spectroscopic observing campaign for studying the pulsations of the central star.
Aims The aim of the present study is to investigate the pulsation properties of HR 8799 in detail via the ultra-precise 47-d-long nearly continuous photometry obtained with the MOST space telescope, and to find as many independent pulsation modes as possible, which is the prerequisite of an asteroseismic age determination.
Methods We carried out Fourier analysis of the wide-band photometric time series.
Results e find that resonance and sudden amplitude changes characterize the pulsation of HR 8799. The dominant frequency is always at f_1 = 1.978 d^-1. Many multiples of one ninth of the dominant frequency appear in the Fourier spectrum of the MOST data: n/9 f_1, where n = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 14, 17, 18\}. Our analysis also reveals that many of these peaks show strong amplitude decrease and phase variations even on the 47-d time-scale. The dependencies between the pulsation frequencies of HR 8799 make the planned subsequent asteroseismic analysis rather difficult. We point out some resemblance between the light curve of HR 8799 and the modulated pulsation light curves of Blazhko RR Lyrae stars
More frequencies of KUV 02464+3239
Preliminary results on KUV 02464+3239, a pulsating DA white dwarf are
presented. Located near the red edge of the DAV instability strip, KUV
02464+3239 shows large amplitude and long period pulsation modes. Up to now
only one mode was known from a 50-minute-long light curve. Our more extended
observations allowed the identification of three additional frequencies. The
presence of previously known harmonics were confirmed and weak subharmonics are
also noticeable at some parts of the light curve. This suggests the dominance
of nonlinear pulsation effects from time to time.Comment: 3 pages, 3 eps figures; has been accepted for publication in
Astronomische Nachrichten (Vol. 8, 2007), proceedings of the
British-Hungarian-French N+N+N Workshop for Young Researcher
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