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

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

Exploring the pulsational properties of two ZZ Ceti stars

Context. We continued our ground-based observing project with the season-long observations of ZZ Ceti stars at Konkoly Observatory. Our present targets are the newly discovered PM J22299+3024, and the already known LP 119-10 variables. LP 119-10 was also observed by the TESS (Transiting Exoplanet Survey Satellite) space telescope in 120-second cadence mode. Methods. We performed standard Fourier analysis of the daily, weekly, and the whole data sets, together with test data of different combinations of weekly observations. We then performed asteroseismic fits utilising the observed and the calculated pulsation periods. For the calculations of model grids necessary for the fits, we applied the 2018 version of the White Dwarf Evolution Code. Results. We derived six possible pulsation modes for PM J22299+3024, and five plus two TESS pulsation frequencies for LP 119-10. Note that further pulsation frequencies may be present in the data sets, but we found their detection ambiguous, so we omitted them from the final frequency list. Our asteroseismic fits of PM J22299+3024 give 11 400 K and 0.46 Msun for the effective temperature and the stellar mass. The temperature is ~800 K higher, while the mass of the model star is exactly the same as it was earlier derived by spectroscopy. Our model fits of LP~119-10 put the effective temperature in the range of 11 800 - 11 900 K, which is again higher than the spectroscopic 11 290 K value, while our best model solutions give M* = 0.70 Msun mass for this target, near to the spectroscopic value of 0.65 Msun, likewise in the case of PM J22299+3024. The seismic distances of our best-fitting model stars agree with the Gaia astrometric distances of PM J22299+3024 and LP 119-10 within the errors, validating our model results.Comment: 12 pages, 15 figures, accepted for publication in Astronomy & Astrophysic