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.Conference Object Citation - WoS: 3Citation - Scopus: 3Photometric Multi-Site Campaign on Massive B Stars in the Open Cluster Chi Persei (ngc 884)(IOP Publishing, 2008) Saesen, S.; Pigulski, A.; Carrier, F.; De Ridder, J.; Aerts, C.; Handler, G.; Drummond, R.; Kalomeni, BelindaIn 2005 a photometric observation campaign started on the open cluster chi Persei, involving 13 telescopes spread over the whole northern hemisphere. After two years we gathered almost 1200 hours of data. We present here preliminary results on the variability search, especially from the 60-cm telescope in Bialkow (Poland), which show seven confirmed beta Cephei stars, four candidate B-type pulsators and other interesting variable stars.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: 3Citation - Scopus: 3Close Binary System Go Cyg(Elsevier Ltd., 2012) Ulaş, Burak; Kalomeni, Belinda; Keskin, Varol; Köse, O.; Yakut, KadriIn this study, we present long term photometric variations of the close binary system GO Cyg. Modelling of the system shows that the primary is filling Roche lobe and the secondary of the system is almost filling its Roche lobe. The physical parameters of the system are M1 = 3.0 ± 0.2M ⊙, M2 = 1.3 ± 0.1M⊙, R 1 = 2.50 ± 0.12R⊙, R2 = 1.75 ± 0.09R⊙, L1 = 64 ± 9L ⊙, L2 = 4.9 ± 0.7L⊙, and a = 5.5 ± 0.3R⊙. Our results show that GO Cyg is the most massive system near contact binary (NCB). Analysis of times of the minima shows a sinusoidal variation with a period of 92.3 ± 0.5 yr due to a third body whose mass is less than 2.3M⊙. Finally a period variation rate of -1.4 × 10-9 d/yr has been determined using all available light curves.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: 30Citation - Scopus: 32The Short-Period Low-Mass Binary System Cc Com Revisited(John Wiley and Sons Inc., 2011) Köse, O; Kalomeni, Belinda; Keskin, Varol; Ulaş, Burak; Yakut, KadriIn this study we determined precise orbital and physical parameters of the very short-period low-mass contact binary system CC Com. The parameters are obtained by analysis of new CCD data combined with archival spectroscopic data. The physical parameters of the cool and hot components are derived as Mc = 0.717(14) M⊙, Mh = 0.378(8) M⊙, Rc = 0.708(12) R⊙, Rh = 0.530(10) R⊙, Lc = 0.138(12) L⊙, and Lh = 0.085(7) L⊙, respectively, and the distance of the system is estimated as 64(4) pc. The times of minima obtained in this study and with those published before enable us to calculate the mass transfer rate between the components which is 1.6 × 10-8 M⊙ yr-1. Finally, we discuss the possible evolutionary scenario of CC Com.Conference Object Citation - WoS: 1Long-Term Monitoring of Polars(Cambridge University Press, 2011) Kalomeni, BelindaWe present long-term observations of magnetic cataclysmic variables AM Her, AN UMa, AR UMa, DP Leo, and V1309 Ori obtained with the ROTSEIIId telescope. All data have been analysed and preliminary results indicate periods of 170 days, 217 days, and 180 days for AM Her, AN UMa, and AR UMa, respectively. © International Astronomical Union 2012.