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OGLE Study of the Sagittarius Dwarf Spheroidal Galaxy and its M54 Globular Cluster

Hamanowicz A., Pietrukowicz P., Udalski A., Mróz P., Soszyński I., Szymański M. K., Skowron J., Poleski R., Wyrzykowski Ł., Kozłowski S., Pawlak M., Ulaczyk K.

Acta Astronomica

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2016

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Vol. 66, No. 2

197--217

EN

We use the fundamental-mode RR Lyr-type variable stars (RRab) from OGLE-IV survey to draw a 3D picture of the central part of the tidally disrupted Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy. We estimate the line-of-sight thickness of the Sgr dSph stream to be FWHMcen=2.42 kpc. Based on OGLE-IV observations collected in seasons 2011-2014 we conduct a comprehensive study of stellar variability in the field of the globular cluster M54 (NGC 6715) residing in the core of this dwarf galaxy. Among the total number of 268 detected variable stars we report the identification of 174 RR Lyr stars, four Type II Cepheids, 51 semi-regular variable red giants, three SX Phe-type stars, 18 eclipsing binary systems. Eighty-three variable stars are new discoveries. The distance to the cluster determined from RRab stars is dM54=26.7±0.03stat±1.3sys kpc. From the location of RRab stars in the period-amplitude (Bailey) diagram we confirm the presence of two old populations, both in the cluster and the Sgr dSph stream.

2

Gaia and Variable Stars

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

Acta Astronomica

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2016

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Vol. 66, No. 4

433--453

EN

We present a comparison of the Gaia DR1 samples of pulsating variable stars - Cepheids and RR Lyr type - with the OGLE Collection of Variable Stars aiming at the characterization of the Gaia mission performance in the stellar variability domain. Out of 575 Cepheids and 2322 RR Lyr candidates from the Gaia DR1 samples located in the OGLE footprint in the sky, 559 Cepheids and 2302 RR Lyr stars are genuine pulsators of these types. The number of misclassified stars is low indicating reliable performance of the Gaia data pipeline. The completeness of the Gaia DR1 samples of Cepheids and RR Lyr stars is at the level of 60-75% as compared to the OGLE Collection dataset. This level of completeness is moderate and may limit the applicability of the Gaia data in many projects.

3

On the Distribution of the Modulation Frequencies of RR Lyrae Stars

Jurcsik J., Szeidl B., Nagy A., Sódor Á.

Acta Astronomica

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2005

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Vol. 55, No. 3

303--314

EN

For the first time connection between the pulsation and modulation properties of RR Lyr stars has been detected. Based on the available data it is found that the possible range of the modulation frequencies, i.e., the possible maximum value of the modulation frequency depends on the pulsation frequency. Short period variables (P<0.4 d) can have modulation period as short as some days, while longer period variables (P>0.6 d) always exhibit modulation with Pmod>20 d. We interpret this tendency with the equality of the modulation period with the surface rotation period, because similar distribution of the rotational periods is expected if an upper limit of the total angular momentum of stars leaving the RGB exists. The distribution of the projected rotational velocities of red and blue horizontal branch stars at different temperatures shows a similar behavior as vrot derived for RR Lyr stars from their modulation periods. This common behavior gives reason to identify the modulation period with the rotational period of the modulated RR Lyr stars.

4

Metallicity Dependence of the Blazhko Effect

Smolec R.

Acta Astronomica

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2005

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Vol. 55, No. 1

59--84

EN

The microlensing surveys, such as OGLE or MACHO, have led to the discovery of thousands of RR Lyr stars in the Galactic bulge and in the Magellanic Clouds, allowing for detailed investigation of these stars, especially the still mysterious Blazhko phenomenon. Higher incidence rate of Blazhko (BL) variables in the more metal-rich Galactic bulge than in the LMC, suggests that occurrence of Blazhko effect correlates with metallicity. To investigate this problem, we calibrate the photometric method of determining the metallicity of RRab stars in the I-band and apply it to the OGLE Galactic bulge and LMC data. In both systems, metallicities of non Blazhko and Blazhko variables are close to each other. The LMC Blazhko pulsators prefer slightly lower metallicities. The different metallicities of the Galactic bulge and the LMC, cannot explain the observed incidence rates. As a by-product of our metallicity estimates, we investigate the luminosity--metallicity relation, finding a steep dependence of the luminosity on [Fe/H].

5

Variable Stars in the Archival HST Data of Globular Clusters M13, M30 and NGC 6712

Pietrukowicz P., Kaluzny J.

Acta Astronomica

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2004

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Vol. 54, No. 1

19--31

EN

We have analyzed archival HST/WFPC2 time-series data of the central parts of globular clusters M13, M30 and NGC 6712 in search for variable objects. Among a total of 21 identified variables there are 15 new discoveries. The sample includes nine RR Lyr stars, two SX Phe stars and seven W UMa-type contact binaries. One object is preliminarily classified as a detached eclipsing binary and another as an ellipsoidal variable.

6

Nonradial Mode Excitation as the Cause of the Blazhko Effect in RR Lyrae Stars

Dziembowski W. A., Mizerski T.

Acta Astronomica

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2004

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Vol. 54, No. 4

363--373

EN

A significant fraction of RR Lyr stars exhibits amplitude and/or phase modulation known as the the Blazhko effect. The oscillation spectra suggest that, at least in most of the cases, excitation of a nonradial modes in addition to the dominant radial modes is responsible for the effect. Though model calculations predict that nonradial modes may be excited, there are problems with explaining their observed properties in terms of finite amplitude development of the linear instability. We propose a scenario, which like some previous, postulates energy transfer from radial to nonradial modes, but avoids those problems. The scenario predicts lower amplitudes in Blazhko stars. We check this prediction with a new analysis of the Galactic bulge RR Lyr stars from OGLE-II database. The effect is seen, but the amplitude reduction is smaller than predicted.

7

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

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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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.

8

Reanalysis of the OGLE-I Observations with the Image Subtraction Method. I. Galactic Bar Fields MM1-A, MM1-B, MM7-A, and MM7-B

Pigulski A., Kołaczkowski Z., Kopacki G.

Acta Astronomica

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2003

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Vol. 53, No. 1

27--50

EN

As a result of the reanalysis of the OGLE-I observations by means of the image subtraction method, we present the first part of a catalog, consisting of data for variable stars in four Galactic fields observed by OGLE-I, viz. MM1-A, MM1-B, MM7-A, and MM7-B. In total, 2016 variable stars have been found. This increased the number of known variable stars in these fields more than twofold. We comment on the detectability of the variable stars in previous studies. Some interesting findings are also discussed. Among others, we found 45 δ Sct stars (38 are new) including several multiperiodic objects. Detailed analysis of the light curves of 47 RR Lyr stars (24 are new detections) allowed us to find five stars which exhibit nonradial pulsations and one RRd star. Three RRab stars are members of the Sagittarius dwarf galaxy (Sgr dSph). We also find four objects which are probably Galactic RV Tau stars and one W Vir star which seems to belong to Sgr dSph. This is the first Population II Cepheid found in this satellite galaxy and dSph other than the Fornax one. For eclipsing EW-type binaries, which are the most abundant variables in our catalog, we investigate the amplitude and period distributions. A comparison with the previous OGLE-I catalogs indicates that we found more stars with smaller amplitudes. Finally, in addition to the two microlensing events discovered previously in these fields, we find five more.

9

Multiperiodic RR Lyr Stars in the Center of the Galaxy

Mizerski T.

Acta Astronomica

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2003

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Vol. 56, No. 4

307--320

EN

This paper presents results of a fully automated search for multiperiodic RR Lyr stars in the center of the Galaxy. The search was carried out on OGLE-II database created by means of image subtraction method. I found more than 2700 RR Lyr stars with about 600 showing various kinds of multiperiodic behavior. Previous OGLE-I database contained only about 200 RR Lyr stars in the Galactic bulge, that is twelve times less than the current sample. There are two most interesting outcomes for the Galactic bulge sample: very high percentage of Blazhko stars and very low percentage of RRd stars. Blazhko effect was confirmed in about 25% of all RRab stars. This incidence rate is almost 2.5 times greater than in the Large Magellanic Cloud. On the other hand I discovered only 3 RRd stars which implies that the incidence rate of this type of variables is much smaller in the bulge sample than in the Magellanic Cloud RR Lyr stars.

10

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

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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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.

11

Resonant Excitation of Nonradial Modes in RR Lyr Stars

Nowakowski R. M., Dziembowski W. A.

Acta Astronomica

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2001

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Vol. 51, No. 1

5--47

EN

We study a nonlinear development of radial pulsation instability to a resonant excitation of nonradial modes. Our theory covers the cases of axisymmetric (m=0) modes as well as (m,-m) pairs. Adopting a simplified treatment of the radial and nonradial mode coupling we find that the asymptotic state is a pulsation with constant amplitudes and we evaluate the relative amplitude of the nonradial component. Observable consequence of the m=0 mode excitation is a small period change and a more significant amplitude change, especially in the case of a dipole mode (l=1). Such a mode has a fairly large excitation probability. Significant amplitude and phase modulation is predicted in the case of excitation of a m=±1 pair. We suggest that this may explain Blazhko-type modulation in RR Lyr stars. If this model is correct, the modulation period is determined by the rotation rate and the Brunt-Väisälä frequency in the deepest part of the radiative interior.

12

Petersen Diagram for RRd Stars in the Magellanic Clouds

Popielski B. L., Dziembowski W. A., Cassisi S.

Acta Astronomica

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2000

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Vol. 50, No. 4

491--507

EN

RRd stars from the Magellanic Clouds form a well-defined band in the Petersen diagram. We explain this observed band with our evolutionary and pulsation calculations with assumed metallicity [Fe/H]=(-2,-1.3). Vast majority of RRd stars from LMC is confined to a narrower range of (-1.7,-1.3). The width of the band, at specified fundamental mode period, may be explained by mass spread at given metallicity. The shape of the band reflects the path of RRd stars within the RR Lyr instability strip. We regard the success in explaining the Petersen diagram as a support for our evolutionary models, which yield mean absolute magnitude in the mid of the instability strip, ⟨MV⟩, in the range 0.4 to 0.65 mag implying distance modulus to the LMC of 18.4 mag.

13

Identification and Astrometry of Variable Stars in M3

Bakos G. Á., Benkö J. M., Jurcsik J.

Acta Astronomica

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2000

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Vol. 50, No. 2

221--248

EN

The space distribution of orbital poles for 252 visual binaries is analyzed to check a possible tendency towards parallelism. It is confirmed that orbital planes do not show any trend to be parallel to the Galactic plane. No strong evidence is found for a preferential orientation of the orbital planes for subgroups of binaries with similar periods and eccentricities. Asymmetry in the distribution of orbital poles is seen only for a subgroup of 19 binaries lying closer than 10 pc. Small differences in the distribution of orbital poles are also detected for subgroups with different location on HR diagram.

14

Fourier Coefficients of OGLE Variables. IV. RR Lyr Stars in ω Cen and towards 47 Tuc

Morgan S. M., Dickerson C.

Acta Astronomica

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2000

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Vol. 50, No. 2

249--268

EN

Fourier coefficients for RR Lyr stars found by the OGLE survey in ω Cen and in the direction of 47 Tuc are presented. A total of 180 light curves were examined. Physical parameters for the stars including mass, luminosity, effective temperature and metallicity were derived using theoretical relations based upon the Fourier coefficients. The well known metallicity variation is seen in the ω Cen RR Lyr variables. The distance modulus to ω Cen is estimated to be ⟨m-M⟩=13.73±0.07 mag, while the RR Lyr stars seen in the direction of 47 Tuc are located in the Small Magellanic Cloud, and have an estimated distance modulus of ⟨m-M⟩=18.78±0.08 mag.