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1

Variable Stars in the Field of Young Open Cluster NGC 659

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Pietrzyński G. , Wyrzykowski Ł. , Szewczyk O. , Bielewicz M.

Acta Astronomica

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2001

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tom Vol. 51, No. 1

65--74

EN

We present results of the search for variable stars in young open cluster NGC 659. Based on 35 nights of observations, 7 new variable stars were discovered. Three of them are Be stars. Another three are most probably pulsating variables of γ Dor type. One star is a detached binary system.

2

Variable Stars in the Field of Young Open Cluster NGC 581

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Wyrzykowski Ł. , Pietrzyński G. , Szewczyk O.

Acta Astronomica

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2002

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tom Vol. 52, No. 1

105--114

EN

We present results of the search for variable stars in the field of young open cluster NGC 581. Based on 19 nights of observations, 6 new variable stars were discovered. Two of them turned out to be eclipsing binary systems. Another two detected variable stars are most probably of γ Dor type. During our observations one of the known Be stars located in our field of view showed irregular variations of brightness, typical for this class of stars. The sixth variable star is a pulsating red giant.

3

The Araucaria Project: The Distance to the Small Magellanic Cloud from Near Infrared Photometry of Type II Cepheids

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Ciechanowska A. , Pietrzyński G. , Szewczyk O. , Gieren W. , Soszyński I.

Acta Astronomica

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2010

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tom Vol. 60, No. 3

233--243

EN

We have obtained deep near infrared J- and K-band observations of 14 BL Her and 5 W Vir SMC stars from the OGLE-III survey with the ESO New Technology Telescope equipped with the SOFI infrared camera. From these observations, period-luminosity (P-L) relations in the J and Ks 2MASS bands were derived. The slopes of the K- and J-band relations of -2.15±0.19 and -1.95±0.24, respectively, agree very well with the corresponding slopes derived previously for population II Cepheids in globular clusters, Galactic bulge and in the Large Magellanic Cloud. The distance modulus to the SMC obtained from our data using P-L relation derived for globular cluster Cepheids equals to 18.85±0.07 (statistical) ± 0.07 mag (systematic without including potential metallicity effect), which within the uncertainties agrees well with the results obtained with other methods.

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

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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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tom 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.

5

The Optical Gravitational Lensing Experiment. Cepheids in the Galaxy IC1613: No Dependence of the Period-Luminosity Relation on Metallicity

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

Acta Astronomica

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2001

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tom Vol. 51, No. 3

221--245

EN

We present results of the search for Cepheids in the galaxy IC1613 carried out as a sub-project of the OGLE-II microlensing survey. 138 Cepheids were found in the 14'.2×14'.2 region in the center of the galaxy. We present light curves, VI photometry and basic data for all these objects, as well as color-magnitude diagram of the observed field. The Period-Luminosity (PL) diagrams for IC1613 fundamental mode Cepheids for V, I and interstellar extinction insensitive index WI are constructed. Comparison of PL relations in metal poor galaxy IC1613 ([Fe/H]≈-1.0 dex) with relations in metal richer Magellanic Clouds allows us to study dependence of Cepheid PL relations on metallicity in the wide range of metallicities covered by these three galaxies. The slopes of PL relations in IC1613 are identical as in the Magellanic Clouds. The comparison of brightness of Cepheids with the magnitudes of the tip of the red giant branch stars and RR Lyr stars in all three objects provides information on the stability of zero points of PL relations in the I and V-band, respectively. We find that the zero points of PL relations are constant to better than ±0.03 mag over the entire range of covered metallicities. Thus, the most important conclusion of the paper is that the Cepheid PL relations do not depend on metallicity. Additionally we determine the mean distance to IC1613, based on the common distance scale resulting from four major stellar distance indicators: Cepheids, RR Lyr, TRGB and red clump stars. The distance modulus to IC1613 is equal to (m-M)IC1613=24.20 mag with the standard deviation of ±0.02 mag from six measurements and systematic uncertainty of calibrations of ±0.07 mag.

6

The Optical Gravitational Lensing Experiment. Additional Planetary and Low-Luminosity Object Transits from the OGLE 2001 and 2002 Observational Campaigns

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

Acta Astronomica

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2003

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tom Vol. 56, No. 2

133--149

EN

The photometric data collected by OGLE-III during the 2001 and 2002 observational campaigns aiming at detection of planetary or low-luminosity object transits were corrected for small scale systematic effects using the data pipeline by Kruszewski and Semeniuk and searched again for low amplitude transits. Sixteen new objects with small transiting companions, additional to previously found samples, were discovered. Most of them are small amplitude cases which remained undetected in the original data. Several new objects seem to be very promising candidates for systems containing substellar objects: extrasolar planets or brown dwarfs. Those include OGLE-TR-122, OGLE-TR-125, OGLE-TR-130, OGLE-TR-131 and a few others. Those objects are particularly worth spectroscopic follow-up observations for radial velocity measurements and mass determination. With well known photometric orbit only a few RV measurements should allow to confirm their actual status. All photometric data of presented objects are available to the astronomical community from the OGLE Internet archive.

7

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

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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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tom 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.

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Binary Lenses in OGLE III EWS Database. Season 2005

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Skowron J. , Jaroszyński M. , Udalski A. , Kubiak M. , Szymański M. K. , Pietrzyński G. , Soszyński I. , Szewczyk O. , Wyrzykowski Ł. , Ulaczyk K.

Acta Astronomica

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2007

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tom Vol. 57, No. 4

281--299

EN

We present nine new binary lens candidates from OGLE-III Early Warning System database for the season of 2005. We have also found four events interpreted as single mass lensing of double sources. The candidates have been selected by visual light curves inspection. Examining the models of binary lenses in our previous studies (10 caustic crossing events of OGLE-II seasons 1997-1999 and 34 binary lens events of OGLE-III seasons 2002-2004, including one planetary event), in this work and in three publications concerning planetary events of season 2005, we find four cases of extreme mass ratio binaries (q≤0.01), and almost all other models with mass ratios in the range 0.1

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

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

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2008

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tom 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

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

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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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tom 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.

11

The Optical Gravitational Lensing Experiment. Ellipsoidal Variability of Red Giants in the Large Magellanic Cloud

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

Acta Astronomica

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2004

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tom Vol. 54, No. 4

347--362

EN

We used the OGLE-II and OGLE-III photometry of red giants in the Large Magellanic Cloud to select and study objects revealing ellipsoidal variability. We detected 1546 candidates for long period ellipsoidal variables and 121 eclipsing binary systems with clear ellipsoidal modulation. The ellipsoidal red giants follow a period-luminosity (PL) relationship (sequence E), and the scatter of the relation is correlated with the amplitude of variability: the larger the amplitude, the smaller the scatter. We note that some of the ellipsoidal candidates exhibit simultaneously OGLE Small Amplitude Red Giants pulsations. Thus, in some cases the Long Secondary Period (LSP) phenomenon can be explained by the ellipsoidal modulation. We also select about 1600 red giants with distinct LSP, which are not ellipsoidal variables. We discover that besides the sequence D in the PL diagram known before, the LSP giants form additional less numerous sequence for longer periods. We notice that the PL sequence of the ellipsoidal candidates is a direct continuation of the LSP sequence toward fainter stars, what might suggest that the LSP phenomenon is related to binarity but there are strong arguments against such a possibility. About 10% of the presented light curves reveal clear deformation by the eccentricity of the system orbits. The largest estimated eccentricity in our sample is about 0.4. All presented data, including individual BVI observations and finding charts are available from the OGLE Internet archive.

12

The Optical Gravitational Lensing Experiment. Catalog of RR Lyr Stars in the Large Magellanic Cloud

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Soszyński I. , Udalski A. , Szymański M. , Kubiak M. , Pietrzyński G. , Woźniak P. , Żebruń K. , Szewczyk O. , Wyrzykowski Ł.

Acta Astronomica

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2003

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tom Vol. 56, No. 2

93--116

EN

We present the catalog of RR Lyr stars discovered in a 4.5 square degrees area in the central parts of the Large Magellanic Cloud (LMC). Presented sample contains 7612 objects, including 5455 fundamental mode pulsators (RRab), 1655 first-overtone (RRc), 272 second-overtone (RRe) and 230 double-mode RR Lyr stars (RRd). Additionally we attach a list of several dozen other short-period pulsating variables. The catalog data include astrometry, periods, BVI photometry, amplitudes, and parameters of the Fourier decomposition of the I-band light curve of each object. We present density map of RR Lyr stars in the observed fields which shows that the variables are strongly concentrated toward the LMC center. The modal values of the period distribution for RRab, RRc and RRe stars are 0.573, 0.339 and 0.276 days, respectively. The period-luminosity diagrams for BVI magnitudes and for extinction insensitive index WI are constructed. We provide the log P-I, log P-V and log P-WI relations for RRab, RRc and RRe stars. The mean observed V-band magnitudes of RR Lyr stars in the LMC are 19.36 mag and 19.31 mag for ab and c types, respectively, while the extinction free values are 18.91 mag and 18.89 mag. We found a large number of RR Lyr stars pulsating in two modes closely spaced in the power spectrum. These stars are believed to exhibit non-radial pulsating modes. We discovered three stars which simultaneously reveal RR Lyr-type and eclipsing-type variability. If any of these objects were an eclipsing binary system containing RR Lyr star, then for the first time the direct determination of the mass of RR Lyr variable would be possible. We provide a list of six LMC star clusters which contain RR Lyr stars. The richest cluster, NGC 1835, hosts 84 RR Lyr variables. The period distribution of these stars suggests that NGC 1835 shares features of Oosterhoff type I and type II groups. All presented data, including individual BVI observations and finding charts are available from the OGLE Internet archive.

13

The Optical Gravitational Lensing Experiment. OGLE-III Long Term Monitoring of the Gravitational Lens QSO 2237+0305

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Udalski A. , Szymański M. K. , Kubiak M. , Pietrzyński G. , Soszyński I. , Żebruń K. , Szewczyk O. , Wyrzykowski Ł. , Ulaczyk K. , Więckowski T.

Acta Astronomica

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2006

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tom Vol. 56, No. 4

293--305

EN

We present results of the long term monitoring of the gravitational lens QSO 2237+0305 conducted during the OGLE survey. Light curves of all four components of the lens obtained during the second phase of the OGLE project (OGLE-II; 1997-2000) are supplemented with the data collected in the OGLE-III phase in the observing seasons 2001-2006. Calibration procedures to tie the new OGLE-III data with already calibrated OGLE-II light curves are described. The resulting homogeneous OGLE data set is the most extensive photometric coverage of the gravitational lens QSO 2237+0305, spanning now one decade - the seasons from 1997 to 2006 - and revealing unique microlensing activity of this spectacular object. All photometric data of the gravitational lens QSO 2237+0305 collected by OGLE are available to the astronomical community from the OGLE Internet} archive.

14

The Optical Gravitational Lensing Experiment. OGLE-III Photometric Maps of the Large Magellanic Cloud

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

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2008

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tom Vol. 58, No. 2

89--102

EN

We present the OGLE-III Photometric Maps of the Large Magellanic Cloud. They cover about 40 square degrees of the LMC and contain mean, calibrated VI photometry and astrometry of about 35 million stars observed during seven observing seasons of the third phase of the Optical Gravitational Lensing Experiment - OGLE-III. We discuss the quality of data and present color-magnitude diagrams of selected fields. The OGLE-III Photometric Maps of the LMC are available to the astronomical community from the OGLE Internet archive.

15

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

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

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2008

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tom 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.

16

The Optical Gravitational Lensing Experiment. Optical Monitoring of the Gravitationally Lensed Quasar HE1104-1805 in 1997-2002

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Wyrzykowski Ł. , Udalski A. , Schechter P. L. , Szewczyk O. , Szymański M. , Kubiak M. , Pietrzyński G. , Soszyński I. , Żebruń K.

Acta Astronomica

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2003

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tom Vol. 56, No. 3

229--240

EN

We present results of the long term monitoring of the gravitationally lensed quasar HE1104-1805. The photometric data were collected between August 1997 and January 2002 as a subproject of the OGLE survey. We determine the time delay in the light curves of images A and B of HE1104-1805 to be equal to 157±21 days with the variability in the image B light curve leading variability of the image A. The result is in excellent agreement with the earlier determination by Ofek and Maoz. OGLE photometry of HE1104-1805 is available to the astronomical community from the OGLE Internet archive.

17

The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. VI. Delta Scuti Stars in the Large Magellanic Cloud

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

Acta Astronomica

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2010

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tom Vol. 60, No. 1

1--16

EN

The sixth part of the OGLE-III catalog of Variable Stars presents δ Sct pulsators in the Large Magellanic Cloud. Altogether 2786 variable stars were found and amongst them 92 are multi-mode objects, including 67 stars pulsating in the fundamental mode and the first overtone (F/1O), nine double-mode pulsators with various combinations of the first three overtones excited (1O/2O, 2O/3O and 1O/3O pulsators), and two triple mode (F/1O/2O) δ Sct stars. In total 1490 of stars are marked as uncertain, due to scattered photometry and small amplitudes. For single-mode objects it was not possible to unambiguously identify pulsation mode, however we suggest the most of the single-mode variable stars pulsate in the first overtone.

18

The Optical Gravitational Lensing Experiment. Eclipsing Binary Stars in the Large Magellanic Cloud

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Wyrzykowski Ł. , Udalski A. , Kubiak M. , Szymański M. , Żebruń K. , Soszyński I. , Woźniak P. R. , Pietrzyński G. , Szewczyk O.

Acta Astronomica

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2003

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tom Vol. 53, No. 1

1--25

EN

We present the catalog of 2580 eclipsing binary stars detected in 4.6 square degree area of the central parts of the Large Magellanic Cloud. The photometric data were collected during the second phase of the OGLE microlensing search from 1997 to 2000. The eclipsing objects were selected with the automatic search algorithm based on an artificial neural network. Basic statistics of eclipsing stars are presented. Also, the list of 36 candidates of detached eclipsing binaries for spectroscopic study and for precise LMC distance determination is provided. The full catalog is accessible from the OGLE Internet archive.

19

The Optical Gravitational Lensing Experiment. Catalog of RR Lyr Stars from the Small Magellanic Cloud

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Soszyński I. , Udalski A. , Szymański M. , Kubiak M. , Pietrzyński G. , Woźniak P. , Żebruń K. , Szewczyk O. , Wyrzykowski Ł.

Acta Astronomica

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2002

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tom Vol. 52, No. 4

369--396

EN

We present the catalog of RR Lyrae stars from 2.4 square degrees of central parts of the Small Magellanic Cloud (SMC). The photometric data were collected during four years of the OGLE-II microlensing survey. Photometry of each star was obtained using the Difference Image Analysis (DIA) method. The catalog contains 571 objects, including 458 RRab, 56 RRc variables, and 57 double mode RR Lyr stars (RRd). Additionally we attach a list of variables with periods between 0.18-0.26 days, which are probably δ Sct stars. Period, BVI photometry, astrometry, amplitude, and parameters of the Fourier decomposition of the I-band light curve are provided for each object. We also present the Petersen diagram for double mode pulsators. We found that the SMC RR Lyr stars are fairly uniformly distributed over the studied area, with no clear correlation to any location. The most preferred periods for RRab and RRc stars are 0.589 and 0.357 days, respectively. We noticed significant excess of stars with periods of about 0.29 days, which might be second-overtone RR Lyr stars (RRe). The mean extinction free magnitudes derived for RRab stars are 18.97, 19.45 and 19.73 mag for the I, V and B-band, respectively. All presented data, including individual BVI observations, are available from the OGLE Internet archive.

20

The Optical Gravitational Lensing Experiment. Planetary and Low-Luminosity Object Transits in the Carina Fields of the Galactic Disk

63%

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

Acta Astronomica

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2002

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tom Vol. 52, No. 4

317--359

EN

We present results of the second "planetary and low-luminosity object transit" campaign conducted by the OGLE-III survey. Three fields (35'×35' each) located in the Carina regions of the Galactic disk (l≈290°) were monitored continuously in February-May 2002. About 1150 epochs were collected for each field. The search for low depth transits was conducted on about 103 000 stars with photometry better than 15 mmag. In total, we discovered 62 objects with shallow depth (≤0.08 mag) flat-bottomed transits. For each of these objects several individual transits were detected and photometric elements were determined. Also lower limits on radii of the primary and companion were calculated. The 2002 OGLE sample of stars with transiting companions contains considerably more objects that may be Jupiter-sized (R<1.6 RJup) compared to our 2001 sample. There is a group of planetary candidates with the orbital periods close to or shorter than one day. If confirmed as planets, they would be the shortest period extrasolar planetary systems. In general, the transiting objects may be extrasolar planets, brown dwarfs, or M-type dwarfs. One should be, however, aware that in some cases unresolved blends of regular eclipsing stars can mimic transits. Future spectral analysis and eventual determination of the amplitude of radial velocity should allow final classification. High resolution spectroscopic follow-up observations are, therefore, strongly encouraged. All photometric data are available to the astronomical community from the OGLE Internet archive.