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Udalski A., Soszyński I., Pietrukowicz P., Szymański M. K., Skowron D. M., Skowron J., Mróz P., Poleski R., Kozłowski S., Ulaczyk K., Rybicki K., Iwanek P., Wrona M.

Acta Astronomica

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2018

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Vol. 68, No. 4

315--339

EN

We present here a new major part of the OGLE Collection of Variable Stars - OGLE Collection of Galactic Cepheids. The new dataset was extracted from the Galaxy Variability Survey images - a dedicated large-scale survey of the Galactic disk and outer bulge conducted by the OGLE project since 2013. The OGLE collection contains 2721 Cepheids of all types - classical, type II and anomalous. It more than doubles the number of known Galactic classical Cepheids. Due to the long-term monitoring and large number of epochs the selected sample is very pure, generally free from contaminating stars of other types often mimicking Cepheids. Its completeness is high at 90% level for classical Cepheids – tested using recent samples of Galactic Cepheids: ASAS-SN, ATLAS, Gaia DR2 and Wise catalog of variable stars. Our comparisons indicate that the completeness of the two latter datasets, Gaia DR2 and Wise catalog, is very low, at < I < 19.5 mag). Both these samples are severely contaminated by non-Cepheids (the purity is 67% and 56%, respectively). We also present several interesting objects found in the new OGLE Collection - multi-mode pulsators, first Galactic candidates for eclipsing systems containing Cepheid, a binary Cepheid candidate. New OGLE Collection of Galactic Cepheids is available for the astronomical community from the OGLE Internet Archive in similar form as previous parts of the OGLE Collection of Variable Stars.

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The OGLE Collection of Variable Stars. Classical, Type II, and Anomalous Cepheids toward the Galactic Center

Soszyński I., Udalski A., Szymański M. K., Wyrzykowski Ł., Ulaczyk K., Poleski R., Pietrukowicz P., Kozłowski S., Skowron D. M., Skowron J., Mróz P., Pawlak M., Rybicki K., Jacyszyn-Dobrzeniecka A.

Acta Astronomica

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2017

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Vol. 67, No. 4

297--316

EN

We present a collection of classical, typeII, and anomalous Cepheids detected in the OGLE fields toward the Galactic center. The sample contains 87 classical Cepheids pulsating in one, two or three radial modes, 924 type II Cepheids divided into BL Her, W Vir, peculiar W Vir, and RV Tau stars, and 20 anomalous Cepheids - first such objects found in the Galactic bulge. Additionally, we upgrade the OGLE Collection of RR Lyr stars in the Galactic bulge by adding 828 newly identified variables. For all Cepheids and RRLyr stars, we publish time-series VI photometry obtained during the OGLE-IV project, from 2010 through 2017. We discuss basic properties of our classical pulsators: their spatial distribution, light curve morphology, period-luminosity relations, and position in the Petersen diagram. We present the most interesting individual objects in our collection: a typeII Cepheid with additional eclipsing modulation, WVir stars with the period doubling effect and the RVb phenomenon, a mode-switching RR Lyr star, and a triple-mode anomalous RRd star.

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Over 38 000 RR Lyrae Stars in the OGLE Galactic Bulge Fields

Soszyński I., Udalski A., Szymański M. K., Pietrukowicz P., Mróz P., Skowron J., Kozłowski S., Poleski R., Skowron D., Pietrzyński G., Wyrzykowski L., Ulaczyk K., Kubiak M.

Acta Astronomica

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2014

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Vol. 64, No. 3

177--196

EN

We present the most comprehensive picture ever obtained of the central parts of the Milky Way probed with RR Lyr variable stars. This is a collection of 38 257 RR Lyr stars detected over 182 square degrees monitored photometrically by the Optical Gravitational Lensing Experiment (OGLE) in the most central regions of the Galactic bulge. The sample consists of 16 804 variables found and published by the OGLE collaboration in 2011 and 21 453 RR Lyr stars newly detected in the photometric databases of the fourth phase of the OGLE survey (OGLE-IV). 93% of the OGLE-IV variables were previously unknown. The total sample consists of 27 258 RRab, 10 825 RRc, and 174 RRd stars. We provide OGLE-IV I- and V-band light curves of the variables along with their basic parameters. About 300 RR Lyr stars in our collection are plausible members of 15 globular clusters. Among others, we found the first pulsating variables that may belong to the globular cluster Terzan 1 and the first RRd star in the globular cluster M54. Our survey also covers the center and outskirts of the Sagittarius Dwarf Spheroidal Galaxy enabling studies of the spatial distribution of the old stellar population from this galaxy. A group of double-mode RR Lyr stars with period ratios around 0.740 forms a stream in the sky that may be a relic of a cluster or a dwarf galaxy tidally disrupted by the Milky Way. Three of our RR Lyr stars experienced a pulsation mode switching from double-mode to single fundamental mode or vice versa. We also present the first known RRd stars with large-amplitude Blazhko effect.

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The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. Type II Cepheids in the Galactic Bulge - Supplement

Soszyński I., Udalski A., Pietrukowicz P., Szymański M. K., Kubiak M., Pietrzyński G., Wyrzykowski Ł., Ulaczyk K., Poleski R., Kozłowski S.

Acta Astronomica

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2013

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Vol. 63, No. 1

37--40

EN

We report the discovery of additional 22 RV Tau stars located in the OGLE-II and OGLE-III fields toward the Galactic bulge, increasing to 357 objects the OGLE-III catalog of type II Cepheids in the Galactic center. Four of the newly detected RV Tau stars belong to the RVb class, i.e., they show large-amplitude, long-period modulation of the mean luminosity. In the updated catalog, the relative number of RV Tau stars in the whole sample of the Galactic bulge type II Cepheids is similar to the Magellanic Cloud samples.

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The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. XV. Long-Period Variables in the Galactic Bulge

Soszyński I., Udalski A., Szymański M. K., Kubiak M., Pietrzyński G., Wyrzykowski Ł., Ulaczyk K., Poleski R., Kozłowski S., Pietrukowicz P., Skowron J.

Acta Astronomica

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2013

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Vol. 63, No. 1

21--36

EN

The fifteenth part of the OGLE-III Catalog of Variable Stars (OIII-CVS) contains 232406 long-period variables (LPVs) detected in the OGLE-II and OGLE-III fields toward the Galactic bulge. The sample consists of 6528 Mira stars, 33235 semiregular variables and 192643 OGLE small amplitude red giants. The catalog data and data resources that are being published include observational parameters of stars, finding charts, and time-series I- and V-band photometry obtained between 1997 and 2009. We discuss statistical features of the sample and compare it with collections of LPVs in the Magellanic Clouds. The vast majority of red giant stars in the Galactic bulge have an oxygen-rich chemistry. Mira variables form a separate group in the period-amplitude diagram, which was not noticed for oxygen-rich Miras in the Magellanic Clouds. We find a clear deficit of long-secondary period stars toward the Galactic center compared to the sample of Magellanic Clouds' LPVs.

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The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. XIV. Classical and Type II Cepheids in the Galactic Bulge

Soszyński I., Udalski A., Pietrukowicz P., Szymański M. K., Kubiak M., Pietrzyński G., Wyrzykowski Ł., Ulaczyk K., Poleski R., Kozłowski S.

Acta Astronomica

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2011

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Vol. 61, No. 4

285--301

EN

The fourteenth part of the OGLE-III Catalog of Variable Stars (OIII-CVS) contains Cepheid variables detected in the OGLE-II and OGLE-III fields toward the Galactic bulge. The catalog is divided into two main categories: 32 classical Cepheids (21 single-mode fundamental-mode F, four first-overtone 1O, two double-mode F/1O, three double-mode 1O/2O and two triple-mode 1O/2O/3O pulsators) and 335 type II Cepheids (156 BL Her, 128 W Vir and 51 RV Tau stars). Six of the type II Cepheids likely belong to the Sagittarius Dwarf Spheroidal Galaxy. The catalog data include the time-series photometry collected in the course of the OGLE survey, observational parameters of the stars, finding charts, and cross-identifications with the General Catalogue of Variable Stars. We discuss some statistical properties of the sample and compare it with the OGLE catalogs of Cepheids in the Large and Small Magellanic Clouds. Multi-mode classical Cepheids in the Galactic bulge show systematically smaller period ratios than their counterparts in the Magellanic Clouds. BL Her in the Galactic bulge stars seem to be brighter than the linear extension of the period-luminosity relations defined by the longer-period type II Cepheids. We also show individual stars of particular interest, like two BL Her stars with period doubling.

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The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. XI. RR Lyrae Stars in the Galactic Bulge

Soszyński I., Dziembowski W. A., Udalski A., Poleski R., Szymański M. K., Kubiak M., Pietrzyński G., Wyrzykowski Ł., Ulaczyk K., Kozłowski S., Pietrukowicz P.

Acta Astronomica

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2011

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Vol. 61, No. 1

1--23

EN

The eleventh part of the OGLE-III Catalog of Variable Stars (OIII-CVS) contains 16 836 RR Lyr stars detected in the OGLE fields toward the Galactic bulge. The total sample is composed of 11 756 RR Lyr stars pulsating in the fundamental mode (RRab), 4989 overtone pulsators (RRc), and 91 double-mode (RRd) stars. About 400 RR Lyr stars are members of the Sagittarius Dwarf Spheroidal Galaxy. The catalog includes the time-series photometry collected in the course of the OGLE survey, basic parameters of the stars, finding charts, and cross-identifications with other catalogs of RR Lyr stars toward the Milky Way center. We notice that some RRd stars in the Galactic bulge show unusually short periods and small ratio of periods, down to PF≈0.35 days and P1O/PF≈0.726. In the Petersen diagram double-mode RRLyr stars form a parabola-like structure, which connects shorter- and longer-period RRd stars. We show that the unique properties of the bulge RRd stars may be explained by allowing for the wide range of the metal abundance extending up to [Fe/H]≈-0.36. We report the discovery of an RR Lyr star with additional eclipsing variability with the orbital period of 15.2447 days. Some statistical features of the RRLyr sample are presented. We discuss potential applications of our catalog in studying the structure and history of the central region of the Galaxy, mapping the interstellar extinction toward the bulge, studying globular clusters and the Sagittarius Dwarf Galaxy.

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Concerning the Distance to the Center of the Milky Way and Its Structure

Majaess D.

Acta Astronomica

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2010

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Vol. 60, No. 1

55--74

EN

The distance to the Galactic center inferred from OGLE RR Lyr variables observed in the direction of the bulge is R0=8.1±0.6 kpc. An accurate determination of R0 is hindered by countless effects that include an ambiguous extinction law, a bias for smaller values of R0 because of a preferential sampling of variable stars toward the near side of the bulge owing to extinction, and an uncertainty in characterizing how a mean distance to the group of variable stars relates to R0. A VI-based period-reddening relation for RR Lyr variables is derived to map extinction throughout the bulge. The reddening inferred from RR Lyr variables in the Galactic bulge, LMC, SMC, and IC 1613 match that established from OGLE red clump giants and classical Cepheids. RR Lyr variables obey a period-color (VI) relation that is relatively insensitive to metallicity. Edge-on and face-on illustrations of the Milky Way are constructed by mapping the bulge RR Lyr variables in tandem with cataloged red clump giants, globular clusters, planetary nebulae, classical Cepheids, young open clusters, HII regions, and molecular clouds. The sample of RR Lyr variables do not trace a prominent Galactic bar or triaxial bulge oriented at φ≅25°.

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Binary Lenses in OGLE-III EWS Database. Seasons 2002-2003

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

Acta Astronomica

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2004

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Vol. 54, No. 2

103--128

EN

We present 15 binary lens candidates from OGLE-III Early Warning System database for seasons 2002-2003. We also found 15 events interpreted as single mass lensing of double sources. The candidates were selected by visual light curves inspection. Examining the models of binary lenses of this and our previous study (10 caustic crossing events of OGLE-II seasons 1997-1999) we find one case of extreme mass ratio binary (q≈0.005) and the rest in the range 0.1

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Metal Abundance of Red Clump Stars in Baade's Window

Kubiak M., McWilliam A., Udalski A., Górski K.

Acta Astronomica

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2002

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Vol. 52, No. 2

159--175

EN

Thirteen red clump stars from Baade's window were observed with high resolution in the red part of the optical spectrum with the UVES echelle spectrograph at the Mount Paranal ESO Observatory. The model atmosphere abundance analysis placed their [Fe/H] values in a range from 0.0 to -1.52 dex. Present results, based on direct measurements of iron abundance, confirm former suggestions that the I-band brightness of the red clump giants only weakly depends on [Fe/H]. The determined values of [Fe/H] may contain a slight systematic error connected with still now unexplained difference in colors between stars in the Galactic bulge and in the solar vicinity.

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Difference Image Analysis of the OGLE-II Bulge Data. III. Catalog of 200 000 Candidate Variable Stars

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

Acta Astronomica

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2002

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Vol. 52, No. 2

129--142

EN

We present the first edition of a catalog of variable stars from OGLE-II Galactic bulge data covering 3 years: 1997-1999. Typically 200-300 I band data points are available in 49 fields between -11 and 11 degrees in galactic longitude, totaling roughly 11 square degrees in sky coverage. Photometry was obtained using the Difference Image Analysis (DIA) software and tied to the OGLE data base with the DoPhot package. The present version of the catalog comprises 221 801 light curves. In this preliminary work the level of contamination by spurious detections is still about 10%. Parts of the catalog have only crude calibration, insufficient for distance determinations. The next, fully calibrated, edition will include the data collected in year 2000. The data is accessible via ftp. Due to the data volume, we also distribute DAT tapes upon request.

12

Binary Lenses in OGLE-II 1997-1999 Database. A Preliminary Study

Jaroszyński M.

Acta Astronomica

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2002

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Vol. 52, No. 1

39--60

EN

We present 18 binary lens candidates from OGLE-II database for seasons 1997-1999. The candidates have been selected by visual light curves inspection from the subsample of strong transient events; the same procedure gives 215 single lens candidates. Among the double lenses there are 12 cases interpreted as caustic crossing events. We compare the mass ratio and separation distributions obtained for binary lenses with the predictions based on stellar double systems observations. We take into account the selection bias, which causes over-representation of binary lenses of similar mass and separation close to the Einstein radius. There is no strong discrepancy between the expected and observed distributions of the mass ratio or the binary separations. We find one or two cases of binary lens candidates, SC20_1793 and SC20_3525, with extreme mass ratios, which may suggest presence of planets or brown dwarf companions. Unfortunately, neither case is very strong, as alternative solutions provide fits to the data which are only unsubstantially worse. Binary lenses provide a modest contribution to the overall optical depth to microlensing.

13

Difference Image Analysis of the OGLE-II Bulge Data. II. Microlensing Events

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

Acta Astronomica

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2001

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Vol. 51, No. 3

175--219

EN

We present a sample of microlensing events discovered in the Difference Image Analysis (DIA) of the OGLE-II images collected during three observing seasons, 1997-1999. 4424 light curves pass our criteria on the presence of a brightening episode on top of a constant baseline. Among those, 512 candidate microlensing events were selected visually. We designed an automated procedure, which unambiguously selects up to 237 best events. Including eight candidate events recovered by other means, a total of 520 light curves are presented in this work. In addition to microlensing events, the larger sample contains certain types of transients, but is also strongly contaminated by artifacts. All 4424 light curves in the weakly filtered group are available electronically, with the intent of showing the gray zone between microlensing events and variable stars, as well as artifacts, to some extent inevitable in massive data reductions. We welcome suggestions for improving the selection process before the full analysis of complete four seasons of the OGLE-II bulge data. Selection criteria for binary events can also be investigated with our extended sample.

14

UBVI Color-Magnitude Diagrams in Baade's Window: Metallicity Range, Implications for the Red Clump Method, Color "Anomaly" and the Distances to the Galactic Center and the Large Magellanic Cloud

Stanek K. Z., Kaluzny J., Wysocka A., Thompson I.

Acta Astronomica

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2000

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Vol. 50, No. 2

191--210

EN

We analyze the UBVI color-magnitude diagrams towards the Galactic bulge in a relatively low-reddening region of Baade's Window. The dereddened red giant branch is very wide [≈1.0 mag in (U-B)0 and ≈0.4 mag in (B-V) 0 and (V-I) 0], indicating a significant dispersion of stellar metallicities, which by comparison with the theoretical isochrones and data for Galactic clusters we estimate to lie between approximately -0.7<[Fe/H]<+0.3, i.e., spanning about 1 dex in metallicity, in good agreement with earlier spectroscopic studies. We also discuss the metallicity dependence of the red clump I-band brightness and we show that it is between 0.1-0.2 mag/dex. This agrees well with the previous empirical determinations and the models of stellar evolution. The dereddened (V-I)0 color of the red clump in the observed bulge field is ⟨(V-I)0⟩=1.066, σ(V-I)0=0.14, i.e., 0.056 mag redder than the local stars with good parallaxes measured by Hipparcos. It seems that the large "color anomaly" of ≈0.2 mag noticed by Paczyński and Stanek and discussed in many recent papers was mostly due to earlier problems with photometric calibration. When we use our data to re-derive the red clump distance to the Galactic center, we obtain the Galactocentric distance modulus μ0,GC=14.69±0.1 mag (R0=8.67± 0.4 kpc), with error dominated by the systematics of photometric calibration. We then discuss the systematics of the red clump method and how they affect the red clump distance to the Large Magellanic Cloud. We argue that the value of distance modulus μ0,LMC=18.24±0.08 (44.5±1.7 kpc), recently refined by Udalski, is currently the most secure and robust of all LMC distance estimates. This has the effect of increasing any LMC-tied Hubble constant by about 12%, including the recent determinations by the HST Key Project and Sandage et al. The UBVI photometry is available through the anonymous ftp service.

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The Optical Gravitational Lensing Experiment. Catalog of Microlensing Events in the Galactic Bulge

Udalski A., Żebruń K., Szymański M., Kubiak M., Pietrzyński G., Soszyński I., Woźniak P.

Acta Astronomica

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2000

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Vol. 50, No. 1

1--65

EN

We present the Catalog of microlensing events detected toward the Galactic bulge in three observing seasons, 1997-1999, during the OGLE-II microlensing survey. The search for microlensing events was performed using a database of about 4×109 photometric measurements of about 20.5 million stars from the Galactic bulge. The Catalog comprises 214 microlensing events found in the fields covering about 11 square degrees on the sky and distributed in different parts of the Galactic bulge. The sample includes 20 binary microlensing events, 14 of them are caustic crossing. In one case a double star is likely lensed. We present distribution of the basic parameters of microlensing events and show preliminary rate of microlensing in different regions of the Galactic bulge. The latter reveals clear dependence on the Galactic coordinates. The dependence on l indicates that the majority of lenses toward the Galactic bulge are located in the Galactic bar. Models of the Galactic bar seem to reasonably predict the observed spatial distribution of microlensing events in the Galactic bulge. All data presented in the Catalog and photometry of all events are available from the OGLE Internet archive.

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Stellar Kinematics Near the Galactic Center - Possible Test for a Binary Black Hole

Jaroszyński M.

Acta Astronomica

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2000

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Vol. 50, No. 1

67--78

EN

We consider consequences of the presence of a binary black hole system in the center of our Galaxy. We show that the existence of such a system with total mass ≈2.6×106 Msolar and semimajor axis ≈0.001 pc is not excluded on theoretical grounds and it may have characteristic evolution time of a few billions years. We simulate astrometric measurements of proper motion of stars in the vicinity of the black hole system and check whether such observations are sufficient to discover the binary nature of the central mass. We show that such findings require astrometric accuracy about ten times higher than presently available; the Keck Interferometer, will be able to place stringent observational limits on the presence of a black hole binary in the Galactic center.

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The Optical Gravitational Lensing Experiment. UBVI Photometry of Stars in Baade's Window

Paczyński B., Udalski A., Szymański M., Kubiak M., Pietrzyński G., Soszyński I., Woźniak P., Żebruń K.

Acta Astronomica

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1999

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Vol. 49, No. 3

319--339

EN

We present UBVI photometry for 8530 stars in Baade's Window obtained during the OGLE-II microlensing survey. Among these are over one thousand red clump giants. 1391 of them have photometry with errors smaller than 0.04, 0.06, 0.12, and 0.20 mag in the I, V, B, and U-band, respectively. We constructed a map of interstellar reddening. The corrected colors of the red clump giants: (U-B)0, (B-V)0, and (V-I)0 are very well correlated, indicating that a single parameter determines the observed spread of their values, reaching almost 2 mag in the (U-B)0. It seems most likely that heavy element content is the dominant parameter, but it is possible that another parameter: the age (or mass) of a star moves it along the same trajectory in the color-color diagram as the metallicity. The current ambiguity can be resolved with spectral analysis, and our catalog may be useful as a finding list of red clump giants. We point out that these K giants are more suitable for a fair determination of the distribution of metallicity than brighter M giants. We also present a compilation of UBVI data for 308 red clump giants near the Sun, for which Hipparcos parallaxes are more accurate than 10%. Spectral analysis of their metallicity may provide information about the local metallicity distribution as well as the extent to which mass (age) of these stars affects their colors. It is remarkable that in spite of a number of problems, stellar models agree with observations at the 0.1-0.2 mag level, making red clump giants not only the best calibrated but also the best understood standard candle.

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Erratum: Relativistic Effects in Proper Motions of Stars Surrounding the Galactic Center

Jaroszyński M.

Acta Astronomica

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1999

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Vol. 49, No. 1

117

EN

In the paper "Relativistic Effects in Proper Motions of Stars Surrounding the Galactic Center" (Acta Astron., 48, p. 653) the highest astrometric accuracy of the Keck Interferometer (≈ 20 μas) was assumed to objects as faint as K ≈ 22 mag.

19

The Optical Gravitational Lensing Experiment. The Distance Scale: Galactic Bulge - LMC - SMC

Udalski A.

Acta Astronomica

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1998

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Vol. 48, No. 2

113--145

EN

We analyze the mean luminosity of three samples of field RRab Lyr stars observed in the course of the OGLE microlensing experiment: 73 stars from the Galactic bulge and 110 and 128 stars from selected fields in the LMC and SMC, respectively. The fields are the same as in the recent distance determination to the Magellanic Clouds with the red clump stars method by Udalski et al. (1998). We determine the relative distance scale dGBRR:dLMCRR:dSMCRR equal to: (0.194±0.010):1.00:(1.30±0.08). We calibrate our RR Lyr distance scale with the recent calibration of Gould and Popowski (1998) based on statistical parallaxes. We obtain the following distance moduli to the Galactic bulge, LMC and SMC: m-M=14.53±0.15, m-M=18.09±0.16 and m-M=18.66±0.16 mag. We use the RR Lyr mean V-band luminosity at the Galactic bulge metallicity as the reference brightness and analyze the mean, I-band luminosity of the red clump stars in objects with different ages and metallicities. We add to our analysis the metal poor Carina dwarf galaxy which contains old RR Lyr stars and intermediate age red clump population. We find a weak dependence of the mean red clump brightness on metallicity and we calibrate its zero point with the nearby local red clump stars measured by Hipparcos: MRCI=(0.09±0.03)×[Fe/H]RC-0.23±0.03. Our revised red clump distance moduli to the Galactic bulge, LMC and SMC are m-M=14.53±0.06, m-M=18.13±0.07 and m-M=18.63±0.07 mag, respectively. The distance modulus to the Carina galaxy is m-M=19.84±0.07 mag. Excellent agreement of RR Lyr and red clump distances which have independent absolute calibrations confirms the short distance scale to the LMC.

20

Relativistic Effects in Proper Motions of Stars Surrounding the Galactic Center

Jaroszyński M.

Acta Astronomica

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1998

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Vol. 48, No. 4

653--665

EN

We simulate the astrometric observations of stars moving close to the black hole in the Galactic center. We show that for orbits ≤103 a.u. and position measurements with the accuracy of the Keck Interferometer, the periastron motion of elliptical orbits will be measurable. The models of star trajectories neglecting the periastron motion will be easy to reject with the high confidence level. The measurement of orbital elements and the periastron motion can be effectively used as an independent estimate the distance to the Galactic center. The effects of orbit precession may be visible in some cases. The effects of gravitational radiation are completely negligible as well as influence of the black hole rotation on the propagation of light.