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1

The OGLE Collection of Variable Stars. Type II Cepheids in the Magellanic System

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

Acta Astronomica

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2018

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Vol. 68, No. 2

89--109

EN

We present a nearly complete collection of type II Cepheids in the Magellanic System. The sample consists of 338 objects: 285 and 53 variables in the Large and Small Magellanic Clouds, respectively. Based on the pulsation periods and light-curve morphology, we classified 118 of our type II Cepheids as BL Her, 120 as W Vir, 34 as peculiar W Vir, and 66 as RV Tau stars. For all objects, we publish time-series {\it VI} photometry obtained during the OGLE-IV survey, from 2010 to the end of 2017. We present the most interesting individual objects in our collection: 16 type II Cepheids showing additional eclipsing or ellipsoidal variability, two RV Tau variables more than 2.5 mag fainter than other stars of this type in the LMC, an RVb star that drastically decreased the amplitude of the long-period modulation, type II Cepheids exhibiting significant amplitude and period changes, and an RV Tau star which undergoes interchanges of deep and shallow minima. We show that peculiar W Vir stars have markedly different spatial distribution than other subclasses of type II Cepheids, which indicates different evolutionary histories of these objects.

2

USNO-B1.0 1171-0309158: An RR Lyrae Star that Switched from a Double- to Single-Mode Pulsation

Khruslov A. V., Kusakin A. V., Reva I. V.

Acta Astronomica

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2017

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Vol. 67, No. 4

317--327

EN

We report the discovery of a new case of an RR Lyr star that experienced a switching of its pulsation mode. We detected USNO-B1.0 1171-0309158 as a double-mode RR Lyr star from observations of the Catalina Surveys that showed additional scattering on the light curve. Our analysis of the time-series of CSS data showed gradual increase in scattering and in the amplitude of fundamental pulsation mode. Our CCD observations carried out in 2015-2017 reveal that this object is now a fundamental-mode RRab star, with no sign of the first-overtone pulsation.

3

OGLE-BLG-RRLYR-12245: An RR Lyrae Star that Switched from a Double- to Single-Mode Pulsation

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

Acta Astronomica

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2014

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Vol. 64, No. 1

1--9

EN

We report the discovery of an RR Lyr star that experienced a switching of its pulsation mode. OGLE-BLG-RRLYR-12245 was discovered as a double-mode RRd star based on observations conducted in the years 2001-2006 during the third phase of the Optical Gravitational Lensing Experiment (OGLE-III). The OGLE-IV observations carried out since 2010 reveal that this object is now a fundamental-mode RRab star, with no sign of the first-overtone pulsation. The analysis of the OGLE photometry shows that the final stage of the mode switching occurred on a relatively short timescale of a few months in 2005. We study the behavior of the star during this process, showing changes of the pulsational amplitudes and periods. We also discuss possible reasons for the mode switching in RR Lyr stars.

4

Variable Stars in the Globular Cluster M 80

Kopacki G.

Acta Astronomica

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2013

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Vol. 63, No. 1

91--107

EN

We present results of a search for variable stars in the globular cluster M 80. Application of the image subtraction method to the ground-based times-series of CCD frames resulted in finding nine new RR Lyr stars, six of the RRc and three of the RRab type, and four SX Phe variables. Revised mean period of RRab stars, hPiab=0.68 d, and relative percentage of RRc stars, Nc/(Nab+Nc)=53%, strongly confirm that M 80 belongs to the Oosterhoff II group of globular clusters. The mean V magnitude of the horizontal branch of M 80 based on the ten RR Lyr stars has been estimated to be VHB=hViRR=16.14±0.03 mag. In two pulsating stars, one of the RR Lyr type and the other of the SX Phe type, oscillations with two close frequencies were detected, indicating excitation of nonradial modes in these stars. Moreover, we discovered two W UMa or ellipsoidal systems, two periodic stars of unknown type, one of which is probably a field star, and detected light variations in three red giants of the cluster.

5

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.

6

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.

7

The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. IX. RR Lyr Stars in the Small Magellanic Cloud

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

Acta Astronomica

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2010

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Vol. 60, No. 3

165--178

EN

The ninth part of the OGLE-III Catalog of Variable Stars (OIII-CVS) comprises RR Lyr stars in the Small Magellanic Cloud (SMC). Our sample consists of 2475 variable stars, of which 1933 pulsate in the fundamental mode (RRab), 175 are the first overtone pulsators (RRc), 258 oscillate simultaneously in both modes (RRd) and 109 stars are suspected second-overtone pulsators (RRe). 30 objects are Galactic RR Lyr stars seen in the foreground of the SMC. We discuss some statistical features of the sample. Period distributions show distinct differences between SMC and LMC populations of RR Lyr variable stars, with the SMC stars having on average longer periods. The mean periods for RRab, RRc and RRe stars are 0.596, 0.366 and 0.293 days, respectively. The mean apparent magnitudes of RRab stars are equal to 19.70 mag in the V-band and 19.12 mag in the I-band. Spatial distribution of RRLyr stars shows that the halo of the SMC is roughly round in the sky, however the density map reveals two maxima near the center of the SMC. For each object the multi-epoch V- and I-band photometry collected over 8 or 13 years of observations and finding charts are available to the astronomical community from the OGLE Internet archive.

8

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

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

Acta Astronomica

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2010

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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.

9

SX Phoenicis Stars in the Globular Cluster M13

Kopacki G.

Acta Astronomica

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2005

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Vol. 55, No. 1

85--95

EN

We present results of the search for SX Phe stars in the globular cluster M13. In photometric reductions we used the image subtraction method supplemented with procedure of searching for short-period variable stars, based on the examination of each pixel in the CCD frames. We illustrate also the ability of this approach to find variability with small signal-to-noise ratio. Using this method we were able to derive the light curves for all three SX Phe stars known in the cluster. Moreover, we discovered another star of this type. Except v37, all detected SX Phe stars have full-amplitudes in V smaller than 0.25 mag. Combining our ground-based data and the HST observations we determined the average V magnitudes for these stars. One SX Phe star, v46, shows double-mode pulsations which are very likely the radial fundamental mode and the first overtone.

10

The Optical Gravitational Lensing Experiment. Population II Cepheids in the Galactic Bulge

Kubiak M., Udalski A.

Acta Astronomica

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2003

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Vol. 56, No. 2

117--131

EN

We present I-band light curves of 54 Population II Cepheids identified in the OGLE-II catalog of variable objects in the Galactic bulge fields. Their periods range from a fraction of a day to several days. Their light curves show very close similarity to the light curves of classical Cepheids with periods a few times longer. We analyze location of the newly identified Population II Cepheids in the color-magnitude diagram. The position of these objects confirms recent discovery that the interstellar extinction toward the Galactic bulge might be anomalous. The slope of the reddening line obtained from our sample is in very good agreement with the earlier one derived with red clump stars and the one derived with RR Lyr stars in the Galactic bulge. Our Population II Cepheids follow the same period-luminosity (P-L) relation indicating similar status of these objects. We compare the P-L relation of these stars with the relation of Population II Cepheid detected by the OGLE-II survey in the LMC. Deviations from the P-L relation of the Galactic bulge Cepheids seem to indicate that these objects are located in the Galactic bar similarly to red clump stars.