Belinda Kalomeni

New light curve analysis and period changes of the overcontact binary XY Leonis

Light curves and photometric solutions of the contact binary XY Leo are presented. The UB VRlight curves appear to be almost symmetric. The B and V light curves were analysed with the Wilson-Devinney code and the orbital parameters of the system were obtained and compared with those obtained in previous studies. All the times of minimum light were collected and combined with the new ones obtained in this study. The orbital period of the system oscillates with a period of 19:6 years and a semi-amplitude of 0:023 day. This regular change was analysed under the assumption of the third body hypothesis. The time delay and advance due to the orbit of the eclipsing pair around the third component were computed and subtracted from the original residuals obtained with the linear light elements. The remaining residuals also show a parabolic change. Therefore, we assumed that the orbital period of the system changes with time due to at least two dierent causes. The parabolic change of the period may be attributed to either mass transfer from less massive component to the more massive one, or an additional component bounded to the double binary system with a longer period.

Absolute Properties of the Binary System BB Pegasi

We 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 × 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 = 1.42 M⊙, Mh = 0.53 M⊙, Rc = 1.29 R⊙, Rh = 0.83 R⊙, Lc = 1.86 L⊙, and Lh = 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.

Photometric multi-site campaign on the open cluster NGC 884 - I. Detection of the variable stars

Context. Recent progress in the seismic interpretation of field β 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 β Cep stars, we studied the young open cluster NGC 884 to discover new B-type pulsators, besides the two known β 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 )o f 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 .T he noise level in the amplitude spectra is 50 μmag 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.

The short-period low-mass binary system CC Com revisited

In 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� , M h = 0.378(8) M� , Rc = 0.708(12) R� , R h = 0.530(10) R� , Lc = 0.138(12) L� ,a ndL h = 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 c

Cyclotron emission from accretion plasma columns in magnetic cataclysmic variable stars

Pure 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.

Basic physical properties of the close binary V497 Cep in the open cluster NGC 7160

New light and radial-velocity curves of V497 Cep, a binary in the open cluster NGC 7160, were obtained and the linear ephemeris of the system was refined to HJD (Min I) = (2 446 299:1596 0:0064)+ (1: d 2028287 0: d 0000015) E.T he first light and radial-velocity curve solutions allowed us to derive the basic physical properties of this astrophysically important binary. It was found that the observed light variation of V497 Cep consists of a strong ellipticity eect and a small contribution from grazing eclipses. A comparison of masses and radii of V497 Cep with theoretical evolutionary tracks indicates that both binary components are very close to the zero-age main sequence. A comparison of disentangled line profiles of the He I 6678 line with synthetic, rotationally broadened line profiles indicates that the rotation of both stars is synchronized with the orbital revolution as expected. This finding increases the credibility of our solutions. We find E(B V)= 0: 39. The distance to the cluster NGC 7160 was found to be about 760 (100) pc which agrees well with other available estimates.

Close binary system GO Cyg

abstract In this study, we present long term photometric variations of the close binary system GO Cyg. Modellingof the system shows that the primary is filling Roche lobe and the secondary of the system is almost fill-ing its Roche lobe. The physical parameters of the system are M 1 = 3.0 ± 0.2M , M 2 = 1.3 ± 0.1M ,R 1 = 2.50 ± 0.12R , R 2 = 1.75 ± 0.09R , L 1 =64±9L , L 2 = 4.9 ± 0.7L , and a = 5.5 ± 0.3R . Our results showthat GO Cyg is the most massive system near contact binary (NCB). Analysis of times of the minima showsa 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. 2011 Elsevier B.V. All rights reserved. 1. IntroductionStudies of the evolution of late-type close binary systems revealthat the evolution of detached, semi-detached and contact systemsare closely related (Yakut and Eggleton, 2005; Eggleton, 2010 andreference therein). The more massive star in a detached binary sys-tem fills its Roche lobe first because it has shorter evolutionarytimescale before its companion. The system is semi-detached bin-ary. In addition to nuclear evolution and mass loss, mass transferhas a crucial role in driving a binary towards a contact phase ofevolution. The observations of detached, contact and semi-de-tached binaries are crucial to our further understanding of the evo-lution of close binary systems.We therefore, include GO Cyg (HD 196628, GSC 02694-00550,V =8

Towards ensemble asteroseismology of the young open clusters χ Persei and NGC 6910

As a result of the variability survey in χ Persei and NGC 6910, the number of β Cep stars that are members of these two open clusters is increased to twenty stars, nine in NGC 6910 and eleven in χ Persei. We compare pulsational properties, in particular the frequency spectra, of β Cep stars in both clusters and explain the differences in terms of the global parameters of the clusters. We also indicate that the more complicated pattern of the variability among B-type stars in χ Persei is very likely caused by higher rotational velocities of stars in this cluster. We conclude that the sample of pulsating stars in the two open clusters constitutes a very good starting point for the ensemble asteroseismology of β Cep-type stars and maybe also for other B-type pulsators (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Flickering in the Magnetic Cv Star Am Herculis

In this study, we present a photometric study of AM Her, a prototype of a class of magnetic CVs. Optical photometry of AM Her was obtained using the Russian—Turkish 1.5 m telescope at TUBITAK National Observatory (TUG) in August 2003. The R band light curve of the system shows two maxima and two minima during one orbital cycle. In both observing nights the star showed flickering at a significant level. The measured flickering time scale is about 5 min.