K. Yakut

Long term photometric monitoring with the Mercator telescope - Frequencies and mode identification of variable O-B stars

Aims. We selected a large sample of O-B stars that were considered as (candidate) slowly pulsating B, beta Cep, and Maia stars after the analysis of their hipparcos data. We analysed our new seven passband geneva data collected for these stars during the first three years of scientific operations of the mercator telescope. We performed a frequency analysis for 28 targets with more than 50 high-quality measurements to improve their variability classification. For the pulsating stars, we tried both to identify the modes and to search for rotationally split modes. Methods: We searched for frequencies in all the geneva passbands and colours by using two independent frequency analysis methods and we applied a 3.6 S/N-level criterion to locate the significant peaks in the periodograms. The modes were identified by applying the method of photometric amplitudes for which we calculated a large, homogeneous grid of equilibrium models to perform a pulsational stability analysis. When both the radius and the projected rotational velocity of an object are known, we determined a lower limit for the rotation frequency to estimate the expected frequency spacings in rotationally split pulsation modes. Results: We detected 61 frequencies, among which 33 are new. We classified 21 objects as pulsating variables (7 new confirmed pulsating stars, including 2 hybrid beta Cep/SPB stars), 6 as non-pulsating variables (binaries or spotted stars), and 1 as photometrically constant. All the Maia candidates were reclassified into other variability classes. We performed mode identification for the pulsating variables for the first time. The most probable l value is 0, 1, 2, and 4 for 1, 31, 9, and 5 modes, respectively, including only 4 unambiguous identifications. For 7 stars we cannot rule out that some of the observed frequencies belong to the same rotationally split mode. For 4 targets we may begin to resolve close frequency multiplets. Based on observations collected with the p7 photometer attached to the Flemish 1.2-m mercator telescope situated at the Roque de los Muchachos observatory on La Palma (Spain). Section [see full text], including Figs. is only available in electronic form at http://www.aanda.org, and Tables 2 and 3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/243

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 early-type close binary CV Velorum revisited

Aims. Our goal was to improve the fundamental parameters of the massive close double-lined eclipsing B2.5V+B2.5V binary CV Velorum. Methods. We gathered new high-resolution echelle spectroscopy on 13 almost consecutive nights covering two orbits. We computed a simultaneous solution to all the available high-quality radial-velocity and light data with the latest version of the Wilson-Deviney code. Results. We obtained the following values for the physical parameters: M1 = 6.066(74) M� , M2 = 5.972(70) M� , R1 = 4.126(24) R� , R2 = 3.908(27) R� ,l ogL1 = 3.20(5) L� ,a nd logL2 = 3.14(5) L� . The quoted errors contain a realistic estimate of systematic uncertainties mainly stemming from the effective temperature estimation. We derived abundances for both components and found them to be compatible with those of B stars in the solar neighbourhood. We discovered low-amplitude periodic line-profile variations with the orbital frequency for both components. Their interpretation requires new data with a longer time span. The primary rotates subsynchronously while the secondary’s v sini and radius are compatible with synchronous rotation. We provide an update of the empirical mass-luminosity relation for main-sequence B stars which can be used for statistical predictions of masses or luminosities.

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.

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)

Photometric multi-site campaign on massive B stars in the open cluster χ Persei (NGC 884)

In 2005 a photometric observation campaign started on the open cluster χ 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 β Cephei stars, four candidate B-type pulsators and other interesting variable stars.

Analysis of MERCATOR data - Part I: variable B stars

We re-classied 31 variable B stars which were observed more than 50 times in the Geneva photometric system with the p7 photometer attached to the mercator telescope (La Palma) during its rst 3 years of scientic observations. HD 89688 is a possible Cephei/slowly pulsating B star hybrid and the main mode of the corot target HD 180642 shows non-linear eects. The Maia candidates are re-classied as either ellipsoidal variables or spotted stars. Although the mode identication is still ongoing, all the well-identied modes so far have ‘ 2.