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Variable Star Population in the Open Cluster NGC 6791 Observed by the Kepler Spacecraft

Sanjayan S., Baran A. S., Németh P., Kinemuchi K., Ostrowski J., Sahoo S. K.

Acta Astronomica

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2022

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Vol. 72, No. 2

77--102

EN

We present the list of variable stars we found in the Kepler superstamp data covering approximately 9' from the central region of NGC 6791. We classified the variable stars based on the variability type and we established their cluster membership based on the available Gaia Early Data Release 3 astrometry, by means of the Bayesian Gaussian mixture models. In total we found 278 variable objects, among which 17 binaries, 45 pulsators, 62 rotational and five unclassified variables are cluster members. The remaining 28 binaries, 25 pulsators, 83 rotational, four unclassified and nine unidentified variables are either not members or their membership is not established. In the case of eclipsing binaries we calculated the mid-times of eclipses and derived ephemerides. We searched for eclipse timing variation by means of the observed minus calculated diagrams. Only three objects show significant orbital period variation. Independently of a recently published report, we found 119 new variables. We used isochrones calculated within the MIST project and derived the age (8.91 Gyr), average distance (4134 pc) and iron content [Fe/H] (0.26-0.28), of NGC 6791. Using the cluster members with membership probabilities greater than 0.9, we calculated the distance to the cluster of 4123(31) pc, which agrees with the result from our isochrone fitting.

2

Evolution of Cool Close Binaries - Rapid Mass Transfer and Near Contact Binaries

Stępień K., Kiraga M.

Acta Astronomica

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2013

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Vol. 63, No. 2

239--266

EN

We test the evolutionary model of cool close binaries developed by one of us (KS) on the observed properties of near contact binaries (NCBs). These are binaries with one component filling the inner critical Roche lobe and the other almost filling it. Those with a more massive component filling the Roche lobe are SD1 binaries whereas in SD2 binaries the Roche lobe filling component is less massive. Our evolutionary model assumes that, following the Roche lobe overflow by the more massive component (donor), mass transfer occurs until mass ratio reversal. A binary in an initial phase of mass transfer, before mass equalization, is identified with SD1 binary. We show that the transferred mass forms an equatorial bulge around the less massive component (accretor). Its presence slows down the mass transfer rate to the value determined by the thermal time scale of the accretor, once the bulge sticks out above the Roche lobe. It means, that in a binary with a (typical) mass ratio of 0.5 the SD1 phase lasts at least 10 times longer than resulting from the standard evolutionary computations neglecting this effect. This is why we observe so many SD1 binaries. Our explanation is in contradiction to predictions identifying the SD1 phase with a broken contact phase of the Thermal Relaxation Oscillations model. The continued mass transfer, past mass equalization, results in mass ratio reversed. SD2 binaries are identified with this phase. Our model predicts that the time scales of SD1 and SD2 phases are comparable to one another. Analysis of the observations of 22 SD1 binaries, 27 SD2 binaries and 110 contact binaries (CBs) shows that relative number of both types of NCBs favors similar time scales of both phases of mass transfer. Total masses, orbital angular momenta and orbital periods of SD1 and SD2 binaries are indistinguishable from each other whereas they differ substantially from the corresponding parameters of CBs. We conclude that the results of the analysis fully support the model presented in this paper.

3

ASAS Photometry of ROSAT Sources. II. New Variables from the ASAS North Survey

Kiraga M., Stępień K.

Acta Astronomica

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2013

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Vol. 63, No. 1

53--66

EN

We present a catalog of 307 optical counterparts of the bright ROSAT X-ray sources, identified with the ASAS North survey data and showing periodic brightness variations. They all have declination north of -25°. Other data available from the literature for the listed stars are also included. All the tabulated stars are new variables, except for 13 previously known, for which the revised values of periods are given.

4

ASAS Photometry of ROSAT Sources. I. Periodic Variable Stars Coincident with Bright Sources from the ROSAT All Sky Survey

Kiraga M.

Acta Astronomica

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2012

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Vol. 62, No. 1

67--95

EN

Photometric data from the ASAS - South (declination less than 29°) survey have been used for identification of bright stars located near the sources from the ROSAT All Sky Survey Bright Source Catalog (RBSC). In total 6028 stars brighter than 12.5 mag in I- or V-bands have been selected and analyzed for periodicity. Altogether 2302 variable stars have been found with periods ranging from 0.137 d to 193 d. Most of these stars have X-ray emission of coronal origin with a few cataclysmic binaries and early type stars with colliding winds. Whenever it was possible we collected data available in the literature to verify periods and to classify variable objects. The catalog includes 1936 stars (1233 new) considered to be variable due to presence of spots (rotationally variable), 127 detached eclipsing binary stars (33 new), 124 contact binaries (11 new), 96 eclipsing stars with deformed components (19 new), 13 ellipsoidal variables (4 new), 5 miscellaneous variables and one pulsating RR Lyr type star (blended with an eclipsing binary). More than 70% of new variable stars have amplitudes smaller than 0.1 mag, but for ASAS 063656-0521.0 we have found the largest known amplitude of brightness variations due to the presence of spots (up to ΔV=0.8 mag). The table with the compiled data and figures with light curves can be downloaded from Acta Astronomica Archive.

5

Evolution of Cool Close Binaries - Approach to Contact

Stępień K.

Acta Astronomica

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2011

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Vol. 61, No. 2

139--159

EN

As a part of a larger project, a set of 27 evolutionary models of cool close binaries was computed under the assumption that their evolution is influenced by the magnetized winds blowing from both components. Short period binaries with the initial periods of 1.5 d, 2.0 d and 2.5 d were considered. For each period three values of 1.3 Msun, 1.1 Msun and 0.9 Msun were taken as the initial masses of the more massive components. The initial masses of the less massive components were adjusted to avoid extreme mass ratios. Here the results of the computations of the first evolutionary phase are presented, which starts from the initial conditions and ends when the more massive component reaches its critical Roche lobe. In all considered cases this phase lasts for several Gyr. For binaries with the higher total mass and/or longer initial periods this time is equal to, or longer than the main sequence life time of the more massive component. For the remaining binaries it amounts to a substantial fraction of this life time. From the statistical analysis of models, the predicted period distribution of detached binaries with periods shorter than 2 d was obtained and compared to the observed distribution from the ASAS data. An excellent agreement was obtained under the assumption that the period distribution in this range is determined solely by magnetic braking (MB), i.e., the mass and angular momentum loss due to the magnetized winds, as considered in the present paper. This result indicates, in particular, that virtually all cool detached binaries with periods of a few tenths of a day, believed to be the immediate progenitors of W UMa-type stars, were formed from young detached systems with periods around 2-3 d. MB is the dominant formation mechanism of cool contact binaries. It operates on the time scale of several Gyr rendering them rather old, with age of 6-10 Gyr. The results of the present analysis will be used as input data to investigate the subsequent evolution of the binaries, through the mass exchange phase and contact or semi-detached configuration till the ultimate merging of the ultimate merging of the components.

6

Catalog of Projected Rotational Velocities

Głębocki R., Stawikowski A.

Acta Astronomica

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2000

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Vol. 50, No. 4

509--515

EN

The catalog is an up-to-date compilation of the observed projected rotational velocities, Vrotsin i, for over 11000 stars of all spectral types and luminosity classes. Table 1 contains individual Vrotsin i data, together with the errors and the methods of determination. The bibliographic references are provided in Table 2. Tables 1 and 2 are available only in electronic version.