Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 32Citation - Scopus: 34Photometric Multi-Site Campaign on the Open Cluster Ngc 884: I. Detection of the Variable Stars(EDP Sciences, 2010) Saesen, S.; Carrier, F.; Pigulski, A.; Aerts, C.; Handler, G.; Kalomeni, Belinda; Debosscher, J.Context. Recent progress in the seismic interpretation of field ? Cep stars has resulted in improvements of the physics in the stellar structure and evolution models of massive stars. Further asteroseismic constraints can be obtained from studying ensembles of stars in a young open cluster, which all have similar age, distance and chemical composition. Aims.To improve our comprehension of the ? Cep stars, we studied the young open cluster NGC 884 to discover new B-type pulsators, besides the two known ? Cep stars, and other variable stars. Methods. An extensive multi-site campaign was set up to gather accurate CCD photometry time series in four filters (U, B, V, I) of a field of NGC 884. Fifteen different instruments collected almost 77 500 CCD images in 1286 h. The images were calibrated and reduced to transform the CCD frames into interpretable differential light curves. Various variability indicators and frequency analyses were applied to detect variable stars in the field. Absolute photometry was taken to deduce some general cluster and stellar properties. Results.We achieved an accuracy for the brightest stars of 5.7 mmag in V, 6.9 mmag in B, 5.0 mmag in I and 5.3 mmag in U. The noise level in the amplitude spectra is 50 ìmag in the V band. Our campaign confirms the previously known pulsators, and we report more than one hundred new multi-and mono-periodic B-, A-and F-type stars. Their interpretation in terms of classical instability domains is not straightforward, pointing to imperfections in theoretical instability computations. In addition, we have discovered six new eclipsing binaries and four candidates as well as other irregular variable stars in the observed field. © ESO, 2010.Article Citation - WoS: 6Citation - Scopus: 6Variation of the Light and Period of the Magnetic Cataclysmic Variable Am Her(IOP Publishing, 2008) Kalomeni, Belinda; Yakut, KadriGround-based, long-term optic variability of AM Her, covering the period between 2003-2008, has been conducted to study the features seen in both low and high states of the system. Low-state analysis shows the presence of short-term, low-amplitude light variations of about 0.02-0.03 mag with a mean power time between 16 s and 226 s. Brightness variations on the order of 0.7-2 mag, which could be due to the stellar activity of the component in the system, are also detected. A total of 30 years' times of minimum light given in the literature are combined with nine times of minima obtained in this study. We represented the (observed-calculated) diagram by a parabolic curve and also by two broken lines. Under the assumption of a parabolic variation, we estimate an increase in period, dP/dt = 7.5(1.2) x 10(-9) days yr(-1), with a mass transfer rate of. M = 8(2) x 10(-9) M(circle dot) yr(-1), in agreement with the previous findings by a different method.Article Citation - WoS: 172Citation - Scopus: 181Possible Disintegrating Short-Period Super-Mercury Orbiting Kic 12557548(IOP Publishing Ltd., 2012) Rappaport, S.; Levine, A.; Chiang, E.; El Mellah, I.; Jenkins, J.; Kalomeni, Belinda; Kite, E. S.; Kotson, M.; Nelson, L.; Rousseau-Nepton, L.; Tran, K.We report on the discovery of stellar occultations, observed with Kepler, which recur periodically at 15.685 hr intervals, but which vary in depth from a maximum of 1.3% to a minimum that can be less than 0.2%. The star that is apparently being occulted is KIC 12557548, a V = 16 mag K dwarf with T eff, s ≃ 4400 K. The out-of-occultation behavior shows no evidence for ellipsoidal light variations, indicating that the mass of the orbiting object is less than 3 M J (for an orbital period of 15.7 hr). Because the eclipse depths are highly variable, they cannot be due solely to transits of a single planet with a fixed size. We discuss but dismiss a scenario involving a binary giant planet whose mutual orbit plane precesses, bringing one of the planets into and out of a grazing transit. This scenario seems ruled out by the dynamical instability that would result from such a configuration. We also briefly consider an eclipsing binary, possibly containing an accretion disk, that either orbits KIC 12557548 in a hierarchical triple configuration or is nearby on the sky, but we find such a scenario inadequate to reproduce the observations. The much more likely explanation - but one which still requires more quantitative development - involves macroscopic particles escaping the atmosphere of a slowly disintegrating planet not much larger than Mercury in size. The particles could take the form of micron-sized pyroxene or aluminum oxide dust grains. The planetary surface is hot enough to sublimate and create a high-Z atmosphere; this atmosphere may be loaded with dust via cloud condensation or explosive volcanism. Atmospheric gas escapes the planet via a Parker-type thermal wind, dragging dust grains with it. We infer a mass-loss rate from the observations of order 1 M ⊕Gyr-1, with a dust-to-gas ratio possibly of order unity. For our fiducial 0.1 M ⊕ planet (twice the mass of Mercury), the evaporation timescale may be 0.2 Gyr. Smaller mass planets are disfavored because they evaporate still more quickly, as are larger mass planets because they have surface gravities too strong to sustain outflows with the requisite mass-loss rates. The occultation profile evinces an ingress-egress asymmetry that could reflect a comet-like dust tail trailing the planet; we present simulations of such a tail.Article Citation - WoS: 12Citation - Scopus: 12Short and Long Time-Scale Variability in Magnetic Cataclysmic Variables: Long-Term Monitoring of Polars(Oxford University Press, 2012) Kalomeni, BelindaWe present long-term observations of the magnetic cataclysmic variables AM Her, AN UMa, AR UMa, DP Leo and V1309 Ori. Analyses of both short- and long-period light variations are presented. For the first time, as far as we know, long-period variations of AN UMa and AR UMa have been studied and multiple frequencies obtained. Fourier analysis indicates 170-, 218- and 180-d variations for AM Her, AN UMa and AR UMa, respectively. These periodicities may be due to modulation of the mass-transfer rate resulting from magnetic cycles in the secondary stars. In addition, we collect the physical parameters of polars from the literature and estimate their mass-transfer rates and orbital period variation and give a rough estimation of the donor magnetic fields.Article Citation - WoS: 147Citation - Scopus: 147Triple-Star Candidates Among the Kepler Binaries(IOP Publishing Ltd., 2013) Rappaport, S.; Deck, K.; Levine, A.; Borkovits, T.; Carter, J.; El Mellah, I.; Sanchis-Ojeda, R.; Kalomeni, BelindaWe present the results of a search through the photometric database of Kepler eclipsing binaries looking for evidence of hierarchical triple-star systems. The presence of a third star orbiting the binary can be inferred from eclipse timing variations. We apply a simple algorithm in an automated determination of the eclipse times for all 2157 binaries. The "calculated" eclipse times, based on a constant period model, are subtracted from those observed. The resulting O-C (observed minus calculated times) curves are then visually inspected for periodicities in order to find triple-star candidates. After eliminating false positives due to the beat frequency between the ∼1/2 hr Kepler cadence and the binary period, 39 candidate triple systems were identified. The periodic O-C curves for these candidates were then fit for contributions from both the classical Roemer delay and so-called physical delay, in an attempt to extract a number of the system parameters of the triple. We discuss the limitations of the information that can be inferred from these O-C curves without further supplemental input, e.g., ground-based spectroscopy. Based on the limited range of orbital periods for the triple-star systems to which this search is sensitive, we can extrapolate to estimate that at least 20% of all close binaries have tertiary companions. © 2013. The American Astronomical Society. All rights reserved..Article Citation - WoS: 91Citation - Scopus: 88The Anticorrelated Nature of the Primary and Secondary Eclipse Timing Variations for the Kepler Contact Binaries(IOP Publishing Ltd., 2013) Tran, K.; Levine, A.; Rappaport, S.; Borkovits, T.; Csizmadia, Sz.; Kalomeni, BelindaWe report a study of the eclipse timing variations in contact binary systems, using long-cadence lightcurves from the Kepler archive. As a first step, observed minus calculated (O-C) curves were produced for both the primary and secondary eclipses of some 2000 Kepler binaries. We find ∼390 short-period binaries with O-C curves that exhibit (1) random walk-like variations or quasi-periodicities, with typical amplitudes of ±200-300 s, and (2) anticorrelations between the primary and secondary eclipse timing variations. We present a detailed analysis and results for 32 of these binaries with orbital periods in the range of 0.35 ± 0.05 days. The anticorrelations observed in their O-C curves cannot be explained by a model involving mass transfer, which, among other things, requires implausibly high rates of ∼0.01 MȮ yr-1. We show that the anticorrelated behavior, the amplitude of the O-C delays, and the overall random walk-like behavior can be explained by the presence of a starspot that is continuously visible around the orbit and slowly changes its longitude on timescales of weeks to months. The quasi-periods of ∼50-200 days observed in the O-C curves suggest values for k, the coefficient of the latitude dependence of the stellar differential rotation, of ∼0.003-0.013.Article Citation - WoS: 38Citation - Scopus: 37Close Binary and Other Variable Stars in the Solar-Age Galactic Open Cluster M67(EDP Sciences, 2009) Yakut, Kadri; Zima, W.; Kalomeni, Belinda; Van Winckel, H.; Wealkens, C.; De Cat, P.; Bauwens, E.; Vuckovic, M.; Saesen, S.; Le Guillou, L.; Parmaksızoğlu, M.; Uluç, K.; Khamitov, I.; Raskin, G.; Aerts, C.We present multi-colour time-series CCD photometry of the solar-age galactic open cluster M67 (NGC 2682). About 3600 frames spread over 28 nights were obtained with the 1.5 m Russian-Turkish and 1.2 m Mercator telescopes. High-precision observations of the close binary stars AH Cnc, EV Cnc, ES Cnc, the δ Scuti type systems EX Cnc and EW Cnc, and some long-period variables belonging to M67 are presented. Three full multi-colour light curves of the overcontact binary AH Cnc were obtained during three observing seasons. Likewise we gathered three light curves of EV Cnc, an EB-type binary, and two light curves of ES Cnc, a blue straggler binary. Parts of the light change of long-term variables S1024, S1040, S1045, S1063, S1242, and S1264 are obtained. Period variation analysis of AH Cnc, EV Cnc, and ES Cnc were done using all times of mid-eclipse available in the literature and those obtained in this study. In addition, we analyzed multi-colour light curves of the close binaries and also determined new frequencies for the δ Scuti systems. The physical parameters of the close binary stars were determined with simultaneous solutions of multicolour light and radial velocity curves. Finally we determined the distance of M67 as 857(33) pc via binary star parameters, which is consistent with an independent method from earlier studies.Article Citation - WoS: 37Citation - Scopus: 36Absolute Properties of the Binary System Bb Pegasi(IOP Publishing, 2007) Kalomeni, Belinda; Yakut, Kadri; Keskin, Varol; Değirmenci, Ömer Lütfi; Ulaş, Burak; Köse, OğuzhanWe present ground-based photometry of the low-temperature contact binary BB Peg. We collected all the times of mid-eclipse available in the literature and combined them with those obtained in this study. Analyses of the data indicate a period increase of (3.0 ± 0.1) × 10-8 days yr -1. This period increase of BB Peg can be interpreted in terms of the mass transfer 2.4 × 10-8 M⊙ yr-1 from the less massive to the more massive component. The physical parameters have been determined as Mc = 1.42M⊙, Mh = 0.53 M⊙, Rc = 1.29 R⊙, Rh, = 0.83 R⊙, Lc = 1.86 L⊙, and L h = 0.94 L⊙ through simultaneous solutions of light and of the radial velocity curves. The orbital parameters of the third body, which orbits the contact system in an eccentric orbit, were obtained from the period variation analysis. The system is compared to the similar binaries in the Hertzsprung-Russell and mass-radius diagrams.Article Citation - WoS: 6Citation - Scopus: 6Cyclotron Emission From Accretion Plasma Columns in Magnetic Cataclysmic Variable Stars(EDP Sciences, 2005) Kalomeni, Belinda; Pekünlü, Esat Rennan; Yakut, KadriPure cyclotron spectra of polars produced during their low accretion states are deduced. We used the working hypothesis that the cyclotron emission is produced by electrons spiraling down the dipole magnetic field lines and forming an accretion plasma column on top of the magnetic pole of a white dwarf. The velocity distribution function of electrons emitting cyclotron radiation is assumed to be a bi-Maxwellian. Since the radiating electrons in a million-Gauss magnetic field seek their respective magnetic mirrors, the perpendicular components of their velocity vectors are assumed to be greater than the parallel ones in the radiation region. This assumption implies that the cyclotron radiation is emitted more or less in the perpendicular direction (to the local magnetic field). Then we investigated the contribution of the ordinary and the extraordinary wave modes to the luminosity. The model predictions seem to be consistent with observations. We present the model cyclotron spectra of a randomly chosen polar, UZ For, as a case study.Article Citation - WoS: 28Citation - Scopus: 28Basic Physical Properties of the Low-Temperature Contact Binary System V781 Tau and the Near-Contact Binary System V836 Cyg(Oxford University Press, 2005) Yakut, Kadri; Ulaş, Burak; Kalomeni, Belinda; Gülmen, ÖmürWe present a detailed photometric study of the low-temperature contact binary V781 Tau, and the near-contact binary V836 Cyg. We have combined the parameters obtained from the light-curve analysis with those found by the spectral studies and we have determined the orbital and physical parameters of the stars. We have collected the times of the mid-eclipses obtained so far and combined with the times obtained in this study. By analysing all these data we determined the mass transfer rate from the massive star to the less massive one for V781 Tau, and from the less massive component to the massive one in the case of V836 Cyg. Finally, we have compared the results obtained for V781 Tau and for V836 Cyg with similar systems.