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1

The Period and the Cause of Variability of the Magnetic Ap Star V473 Tauri

Jerzykiewicz M.

Acta Astronomica

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2009

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Vol. 59, No. 3

307--323

EN

The ground based UBV observations and Hipparcos Hp magnitudes of this well known magnetic variable are analyzed. The period of the star's light variation is improved. The improved value, accounting for the available photometric data from the 1963-1964 Lowell observations of Rakosch to the 1990-1993 Hp magnitudes, is equal to 1.4068541±0.0000029 d. The effective temperature and bolometric correction are then derived from the de-reddened B-V color index and the star is plotted in the theoretical H-R diagram using the luminosity obtained from the Hipparcos parallax. Finally, the cause of variability of V473 Tau is discussed in terms of two propositions: (1) the oblique rotator model, and (2) pulsation in a high radial-order g-mode. Hitherto unpublished UBV photometry of V473 Tau is presented in the Appendix.

2

The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. III. RR Lyrae Stars in the Large Magellanic Cloud

Soszyński I., Udalski A., Szymański M. K., Kubiak M., Pietrzyński G., Wyrzykowski Ł., Szewczyk O., Ulaczyk K., Poleski R.

Acta Astronomica

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2009

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Vol. 59, No. 1

1--18

EN

The third part of the OGLE-III Catalog of Variable Stars comprises 24 906 RR Lyr stars in the Large Magellanic Cloud (LMC). This sample consists of 17 693 fundamental-mode (RRab), 4958 first-overtone (RRc), 986 double-mode (RRd) and 1269 suspected second-overtone (RRe) pulsators. 66 objects are foreground Galactic RR Lyr stars. The catalog data include basic photometric and astrometric properties of these RR Lyr stars, multi-epoch VI photometry and finding charts. We detected one new RR Lyr star with additional eclipsing variations. The spatial distribution of RR Lyr stars in the LMC is distinctly non-spherical and it is elongated in the same direction as the LMC bar. The basic statistical features of RR Lyr stars in the LMC are provided. The apparent V-band magnitudes for RRab stars have the modal value at 19.36 mag, and for overtone RR Lyr stars it is about 19.32 mag. The mean periods for RRab, RRc and RRe stars are 0.576, 0.337 and 0.270 days, respectively.

3

The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. IV. Long-Period Variables in the Large Magellanic Cloud

Soszyński I., Udalski A., Szymański M. K., Kubiak M., Pietrzyński G., Wyrzykowski Ł., Szewczyk O., Ulaczyk K., Poleski R.

Acta Astronomica

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2009

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Vol. 59, No. 3

239--253

EN

The fourth part of the OGLE-III Catalog of Variable Stars presents 91 995 long-period variables (LPVs) in the Large Magellanic Cloud (LMC). This sample consists of 79 200 OGLE Small Amplitude Red Giants (OSARGs), 11 128 semiregular variables (SRVs) and 1667 Mira stars. The catalog data include basic photometric and astrometric properties of these stars, long-term multi-epoch VI photometry and finding charts. We describe the methods used for the identification and classification of LPVs. The distribution of I-band amplitudes for carbon-rich stars shows two maxima, corresponding to Miras and SRVs. Such a distinction between Miras and SRVs is not obvious for oxygen-rich stars. We notice additional period-luminosity sequence located between Wood's sequences C and C' and populated by SRVs.

4

The All Sky Automated Survey. The Catalog of Variable Stars in the Kepler Field of View

Pigulski A., Pojmański G., Pilecki B., Szczygieł D. M.

Acta Astronomica

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2009

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Vol. 59, No. 1

33--46

EN

We present the catalog of 947 variable stars located in the field of view of the Kepler satellite. The catalog is a result of the analysis of VI photometry obtained during the first 17-month observations in the ASAS3-North station. The variable stars we present are divided into eleven groups according to the presented variability; the groups are briefly discussed. The catalog is intended to be a source of information for target selection process and follow-up programs.

5

Photometric Study of Variable Stars in the Open Cluster NGC 6866

Molenda-Żakowicz J., Kopacki G., Stęślicki M., Narwid A.

Acta Astronomica

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2009

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Vol. 59, No. 2

193--211

EN

We report the discovery of 19 variable stars and two blue stragglers in the field of the open cluster NGC 6866. Three of the variable stars we classify as δ Sct, two as γ Dor, four as W UMa, two as ellipsoidal variables, and one as an eclipsing binary. Seven stars show irregular variability. Two of the pulsators, δ Sct star NGC 6866-29 and γ Dor star NGC 6866-21, are multiperiodic. From an analysis of proper motions, we conclude that the δ Sct stars, one of the γ Dor stars and both blue stragglers are very probable members of the cluster. The position on the color-magnitude diagram of seven other variables suggests that they also belong to the cluster. The eclipsing binary, which we discover to be a new high velocity star, and the seven irregular variables are non-members. We discuss in detail the age and metallicity of open clusters that host γ Dor stars and we show that none of these parameters is correlated with the number of γ Dor stars in cluster.

6

Double-Overtone Cepheids in the Large Magellanic Cloud

Dziembowski W. A., Smolec R.

Acta Astronomica

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2009

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Vol. 59, No. 1

19--31

EN

One of the most interesting results from the OGLE-III study of the LMC Cepheids is the large number of objects that pulsate simultaneously in the first and second overtone (denoted 1O/2O). Double-mode Cepheids yield important constraint on stellar evolution models. We show that great majority of the LMC 1O/2O Cepheids have masses M=3.0±0.5 Msolar. According to current stellar evolution calculations these masses are lower than needed for the blue loop in the helium burning phase to reach the instability strip. On the other hand, we found most of these stars significantly overluminous if they are crossing the instability strip before helium ignition. A possible solution of this discrepancy is to allow for a large overshooting from the convective core in the main sequence phase. We also discuss origin of double-mode pulsation. At the short period range we find two types of resonances that are conducive to this form of pulsation. However, at longer periods, it has a different (non-resonant) origin.

7

The Optical Gravitational Lensing Experiment. Triple-Mode and 1O/3O Double-Mode Cepheids in the Large Magellanic Cloud

Soszyński I., Poleski R., Udalski A., Kubiak M., Szymański M. K., Pietrzyński G., Wyrzykowski Ł., Szewczyk O., Ulaczyk K.

Acta Astronomica

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2008

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Vol. 58, No. 3

153--162

EN

We report the discovery of three new triple-mode classical Cepheids in the Large Magellanic Cloud, two of them with the fundamental, first overtone and second overtone excited, and one pulsating simultaneously in the first three overtones. Thus, the number of triple-mode Cepheids in the LMC is increased to five. We also present two objects belonging probably to a new type of double-mode Cepheids having the first and third overtones excited. We measure the rates of period change in these stars and detect decrease of periods in two of them, what is in conflict with theoretical predictions.

8

The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. I. Classical Cepheids in the Large Magellanic Cloud

Soszyński I., Poleski R., Udalski A., Kubiak M., Szymański M. K., Pietrzyński G., Wyrzykowski Ł., Szewczyk O., Ulaczyk K.

Acta Astronomica

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2008

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Vol. 58, No. 3

163--185

EN

We present the first part of a new catalog of variable stars (OIII-CVS) compiled from the data collected in the course of the third phase of the Optical Gravitational Lensing Experiment (OGLE-III). In this paper we describe the catalog of 3361 classical Cepheids detected in the ≈40 square degrees area in the Large Magellanic Cloud. The sample consists of 1848 fundamental-mode (F), 1228 first-overtone (1O), 14 second-overtone (2O), 61 double-mode F/1O, 203 double-mode 1O/2O, 2 double-mode 1O/3O, and 5 triple-mode classical Cepheids. This sample is supplemented by the list of 23 ultra-low amplitude variable stars which may be Cepheids entering or exiting instability strip. The catalog data include VI high-quality photometry collected since 2001, and for some stars supplemented by the OGLE-II photometry obtained between 1997 and 2000. We provide basic parameters of the stars: coordinates, periods, mean magnitudes, amplitudes and parameters of the Fourier light curve decompositions. Our sample of Cepheids is cross-identified with previously published catalogs of these variables in the LMC. Individual objects of particular interest are discussed, including single-mode second-overtone Cepheids, multiperiodic pulsators with unusual period ratios or Cepheids in eclipsing binary systems. We discuss the variations of the Fourier coefficients with periods and point out on the sharp feature for periods around 0.35 days of first-overtone Cepheids, which can be explained by the occurrence of 2:1 resonance between the first and fifth overtones. Similar behavior at P≈3 days for 1O Cepheids and P≈10 days for F Cepheids are also interpreted as an effect of resonances between two radial modes. We fit the period-luminosity relations to our sample of Cepheids and compare these functions with previous determinations.

9

The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. II. Type II Cepheids and Anomalous Cepheids in the Large Magellanic Cloud

Soszyński I., Udalski A., Szymański M. K., Kubiak M., Pietrzyński G., Wyrzykowski Ł., Szewczyk O., Ulaczyk K., Poleski R.

Acta Astronomica

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2008

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Vol. 58, No. 4

293--312

EN

In the second part of the OGLE-III Catalog of Variable Stars (OIII-CVS) we present 197 type II Cepheids and 83 anomalous Cepheids in the Large Magellanic Cloud (LMC). The sample of type II Cepheids consists of 64 BL Her stars, 96 W Vir stars and 37 RV Tau stars. Anomalous Cepheids are divided into 62 fundamental-mode and 21 first-overtone pulsators. These are the largest samples of such types of variable stars detected anywhere outside the Galaxy. We present the period-luminosity and color-magnitude diagrams of stars in the sample. If the boundary period between BL Her and W Vir stars is adopted at 4 days, both groups differ significantly in (V-I) colors. We identify a group of 16 peculiar W Vir stars with different appearance of the light curves, brighter and bluer than ordinary stars of that type. Four of these peculiar W Vir stars show additional eclipsing modulation superimposed on the pulsation light curves. Four other stars of that type show long-period secondary variations which may be ellipsoidal modulations. It suggests that peculiar W Vir subgroup may be related to binarity. In total, we identified seven type II Cepheids simultaneously exhibiting eclipsing variations which is a very large fraction compared to classical Cepheids in the LMC. We discuss diagrams showing Fourier parameters of the light curve decomposition against periods. Three sharp features interpreted as an effect of resonances between radial modes are detectable in these diagrams for type II Cepheids.

10

Period Changes of LMC Cepheids in the OGLE and MACHO Data

Poleski R.

Acta Astronomica

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2008

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Vol. 58, No. 4

313--328

EN

Pulsation period of Cepheids should change as stars evolve through the instability strip. Rates of these changes found by other authors based on the decades-long O-C diagrams show rather good agreement with theoretical predictions. We have checked the variability on the scale of a few years on the data recently published by the Optical Gravitational Lensing Experiment (OGLE) for the Large Magellanic Cloud Cepheids and found period changes for 18% of fundamental mode and 41% of first overtone pulsators. It suggest the overtone pulsations are less stable than the fundamental ones. For stars which had the cross-references in the MACHO catalog we have checked if the period change rates derived from the OGLE and the MACHO data are consistent. It was found that there is no correlation and opposite signs of changes in both data sets are more common than the same ones. Many O-C diagrams show nonlinear period changes similarly as for some stars the diagrams derived from the OGLE data only (spanning up to 4100 days) show random fluctuations. These fluctuations are common on the long-term O-C diagrams and we conclude they dominate the diagrams for the timescales of a few thousand of days. The distributions of periods and colors for all Cepheids and for those with statistically significant period changes are the same. Times of maximum light obtained using the MACHO and the OGLE data as well as the examples of O-C diagrams are presented.

11

Double-Mode Classical Cepheid Models - Revisited

Smolec R., Moskalik P.

Acta Astronomica

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2008

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Vol. 58, No. 3

233--261

EN

For many years modeling of double-mode pulsation of classical pulsators was a challenging problem. Inclusion of turbulent convection into pulsation hydrocodes finally led to stable double-mode models. However, it was never analyzed, which factor of turbulent convection is crucial. We show that the double-mode behavior displayed in the computed models results from incorrect assumptions adopted in some of the pulsation hydrocodes, namely from the neglect of buoyant forces in convectively stable layers. This leads to significant turbulent energies and consequently to strong eddy-viscous damping in deep, convectively stable layers of the model. Resulting differential reduction of fundamental and first overtone amplitudes favors the occurrence of double-mode pulsation. Once buoyant forces in convectively stable regions are taken into account (as they should), no stable double-mode behavior is found. The problem of modeling double-mode behavior of classical pulsators remains open.

12

Convective Hydrocodes for Radial Stellar Pulsation. Physical and Numerical Formulation

Smolec R., Moskalik P.

Acta Astronomica

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2008

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Vol. 58, No. 3

193--232

EN

In this paper we describe our convective hydrocodes for radial stellar pulsation. We adopt the Kuhfuß (1986) model of convection, reformulated for the use in stellar pulsation hydrocodes. Physical as well as numerical assumptions of the code are described in detail. Described tests show, that our models are numerically robust and reproduce basic observational constraints. We discuss the effects of different treatment of some quantities in other pulsation hydrocodes. Our most important finding concerns the treatment of the turbulent source function in convectively stable regions. In our code we allow for negative values of source function in convectively stable zones, which reflects negative buoyancy. However, some authors restrict the source term to non-negative values. We show that this assumption leads to very high turbulent energies in convectively stable regions. The effect looks like overshooting, but it is not, because turbulence is generated by pulsations. Also, turbulent elements do not carry kinetic nor thermal energy into convectively stable layers. The range of this artificial overshooting (as we shall call it) is as large as six local pressure scale heights, leading to unphysical internal damping through the eddy-viscous forces, in deep, convectively stable parts of the star.

13

δ Ceti from the Ground and from MOST

Jerzykiewicz M.

Acta Astronomica

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2007

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Vol. 57, No. 1

33--47

EN

Strömgren b-light curves of δ Cet, well known β Cep type star, obtained in 1990 December and 1991 January at SAAO and in 2003 October at Lowell are presented. The two epochs of maximum light, derived from these data, cannot be reconciled with the constant rate of secular period change found earlier. By a stroke of luck, the Lowell data were taken at the time when δ Cet was observed by MOST. A comparison of these two sets of data reveals discrepancies between the Lowell and MOST light curves, and between the MOST fluxes available on the net and the MOST magnitudes reported by Aerts et al. (2006). This had motivated us to reanalyze the MOST data. We find that the second strongest periodic signal in the MOST photometry of the star has frequency equal to the reciprocal of synodic month. We show further that the three lowest amplitude periodic terms derived by the above-mentioned authors and taken as a proof of multiperiodicity are significant only if one ignores the increase of the noise level toward lower frequencies, seen in the amplitude spectra of the MOST magnitudes. We conclude that the issue of multiperiodicity of δ Cet is debatable. We warn, therefore, that results of seismic modeling based on frequencies of these low amplitude terms, such as the core overshooting parameter or the inclination of the star's axis of rotation, should be treated with caution.

14

The Optical Gravitational Lensing Experiment. Variable Stars in the Field of Open Cluster NGC 6755

Ciechanowska A., Pietrzyński G., Kołaczkowski Z., Wyrzykowski Ł., Szewczyk O., Kubiak M., Udalski A., Szymański M. K.

Acta Astronomica

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2007

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Vol. 57, No. 3

249--266

EN

We present the results from a search for variable stars in the field of a young open cluster NGC 6755. Altogether seventy one variable stars have been discovered. Thirty one of them are eclipsing systems. This group contains ten EA, four EB and seventeen EW-type systems. Photometric variability of four late type stars is most probably caused by their chromospherical activity. Another seven detected variable stars have light curves typical for pulsating stars from the main instability strip. Four of them we tentatively classified as γ Dor, one as δ Sct and two as Population II Cepheid variable stars, respectively. Star designated as V40 can be either another population II Cepheid or an ellipsoidal binary system. The remaining twenty eight variable stars found in the field of this open cluster are most probably highly obscured background red giants (OSARG, irregular). For all detected variable stars we provide their light curves, preliminary classification, discussion on the possible cluster membership, equatorial coordinates, finding charts and periods when possible.

15

The Optical Gravitational Lensing Experiment. Period-Luminosity Relations of Variable Red Giant Stars

Soszyński I., Dziembowski W. A., Udalski A., Kubiak M., Szymański M. K., Pietrzyński G., Wyrzykowski Ł., Szewczyk O., Ulaczyk K.

Acta Astronomica

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2007

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Vol. 57, No. 3

201--225

EN

Period-luminosity (PL) relations of variable red giants in the Large (LMC) and Small Magellanic Clouds (SMC) are presented. The PL diagrams are plotted in three planes: log P-KS, log P-WJK, and log P-WI, where WJK and WI are reddening free Wesenheit indices. Fourteen PL sequences are distinguishable, and some of them consist of three closely spaced ridges. Each of the sequences is fitted with a linear or quadratic function. The similarities and differences between the PL relations in both galaxies are discussed for four types of red giant variability: OGLE Small Amplitude Red Giants (OSARGs), Miras and Semiregular Variables (SRVs), Long Secondary Periods (LSPs) and ellipsoidal variables. We propose a new method of separating OSARGs from non-variable stars and SRVs. The method employs the position in the reddening-free PL diagrams and the characteristic period ratios of these multiperiodic variables. The PL relations for the LMC OSARG are compared with the calculated relations for RGB models along isochrones of relevant ages and metallicities. We also compare measured periods and amplitudes of the OSARGs with predictions based on the relations valid for less luminous solar-like pulsators. Miras and SRVs seem to follow PL relation of the same slopes in the LMC and SMC, while for LSP and ellipsoidal variables slopes in both galaxies are different. The PL sequences defined by LSP variables and binary systems overlap in the whole range of analyzed wavebands. We put forward new arguments for the binary star scenario as an explanation of the LSP variability and elaborate on it further. The measured pulsation to orbital period ratio implies nearly constant ratio of the star radius to orbital distance, R/A≈0.4, as we find. Combined effect of tidal friction and mass loss enhanced by the low-mass companion may explain why such a value is preferred.

16

Pulsating Components of Eclipsing Binaries in the ASAS-3 Catalog

Pigulski A., Michalska G.

Acta Astronomica

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2007

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Vol. 57, No. 1

61--72

EN

As a result of the search among about 11 000 stars from the public ASAS-3 database, we report detection of pulsating components in eleven eclipsing binaries. In particular, we have found three classical Algols, MX Pav, IZ Tel, and VY Mic, with δ Sct-type primary components. In six other eclipsing binaries, the short-period variability can also be interpreted in terms of δ Sct-type pulsations, but in these systems both components are probably main-sequence stars. In HD 99612, the pulsation mode shows significant amplitude decrease during the time interval covered by observations. In addition, we find variability in one of the components of the eclipsing and double-lined spectroscopic O-type binary ALS 1135 which we interpret as a β Cep-type pulsation. Finally, we find Y Cir to be a good candidate for an SPB star in an eclipsing binary system.

17

On the Prospects for Detection and Identification of Low-Frequency Oscillation Modes in Rotating B Type Stars

Daszyńska-Daszkiewicz J., Dziembowski W. A., Pamyatnykh A. A.

Acta Astronomica

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2007

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Vol. 57, No. 1

11--32

EN

We study how rotation affects observable amplitudes of high-order g- and mixed r/g-modes and examine prospects for their detection and identification. Our formalism, which is described in some detail, relies on a nonadiabatic generalization of the traditional approximation. Numerical results are presented for a number of unstable modes in a model of SPB star, at rotation rates up to 250 km/s. It is shown that rotation has a large effect on mode visibility in light and in mean radial velocity variations. In most cases, fast rotation impairs mode detectability of g-modes in light variation, as Townsend (2003b) has already noted, but it helps detection in radial velocity variation. The mixed modes, which exist only at sufficiently fast rotation, are also more easily seen in radial velocity. The amplitude ratios and phase differences are strongly dependent on the aspect, the rotational velocity and on the mode. The latter dependence is essential for mode identification.

18

Modulation of the Neutron Star Boundary Layer Luminosity by Disk Oscillations

Abramowicz M. A., Horák J., Kluźniak W.

Acta Astronomica

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2007

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Vol. 57, No. 1

1--10

EN

Paczyński (1987) pointed out that any modulation of the rate at which mass is accreted by a neutron star from the innermost part of a relativistic accretion disk will lead to a modulation of the luminosity of the boundary layer. Following this reasoning, we demonstrate that variability of the boundary layer X-ray flux must necessarily show frequencies of certain global accretion disk oscillations. This theoretical paradigm - clock in the disk, modulation at the boundary layer - resolves one puzzling paradox in the neutron star quasi periodic oscillation (QPO) data.

19

The Optical Gravitational Lensing Experiment. Variable Stars in the Field of Open Cluster NGC 6259

Ciechanowska A., Pietrzyński G., Wyrzykowski Ł., Szewczyk O., Kubiak M., Udalski A., Szymański M. K.

Acta Astronomica

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2006

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Vol. 56, No. 2

219--236

EN

We present the results of a search for variable stars in the field of NGC 6259. Altogether 85 variable stars have been discovered. 36 of them are eclipsing systems. This group contains 13 EA, 2 EB and 21 EW-type stars. Light curves of two variable stars resemble those of Miras. The remaining 47 variable stars detected in the field of this open cluster are most probably highly obscured red giants (OSARG, irregular). For all detected variable stars we provide their light curves, preliminary classification, discussion on the possible cluster membership, equatorial coordinates, finding charts and periods when possible.

20

The Optical Gravitational Lensing Experiment. Variable Baseline Microlensing Events in the Galactic Bulge

Wyrzykowski Ł., Udalski A., Mao S., Kubiak M., Szymański M. K., Pietrzyński G., Soszyński I., Szewczyk O.

Acta Astronomica

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2006

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Vol. 56, No. 2

145--169

EN

We present the first systematic search for microlensing events with variability in their baselines using data from the third phase of the Optical Gravitational Lensing Experiment (OGLE-III). A total of 137 candidates (88 new) was discovered toward the Galactic bulge. Among these, 21 have periodic oscillations in their baselines, 111 are irregular variables and 5 are potential long period detached eclipsing binaries. This is about 10% of the total number of constant baseline events. They are hence quite common and can be regarded as a new type of exotic events, which allow the determination of extra parameters of the events. We show that microlensing of variable stars may allow us to break the degeneracy between the blending parameter and magnification. We note that in some cases variability hidden in the baseline due to strong blending may be revealed in highly magnified events and resemble other exotic microlensing behavior, including planetary deviation. A new system (VEWS) for detecting ongoing variable baseline microlensing events is presented.