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1

OGLE-ing the Magellanic System: Three-Dimensional Structure of the Clouds and the Bridge Using Classical Cepheids

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

Acta Astronomica

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2016

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Vol. 66, No. 2

149--196

EN

We analyzed a sample of 9418 fundamental-mode and first-overtone classical Cepheids from the OGLE-IV Collection of Classical Cepheids. The distance to each Cepheid was calculated using the period-luminosity relation for the Wesenheit magnitude, fitted to our data. The classical Cepheids in the LMC are situated mainly in the bar and in the northern arm. The eastern part of the LMC is closer to us and the plane fit to the whole LMC sample yields the inclination i=24.°2 ±0.°7 and position angle P.A.=151.°4±1.°7. We redefined the LMC bar by extending it in the western direction and found no offset from the plane of the LMC contrary to previous studies. On the other hand, we found that the northern arm is offset from a plane by about -0.5 kpc, which was not observed before. The age distribution of the LMC Cepheids shows one maximum at about 100 Myr. We demonstrate that the SMC has a non-planar structure and can be described as an extended ellipsoid. We identified two large ellipsoidal off-axis structures in the SMC. The northern one is located closer to us and is younger, while the south-western is farther and older. The age distribution of the SMC Cepheids is bimodal with one maximum at 110 Myr, and another one at 220 Myr. Younger stars are located in the closer part of this galaxy while older ones are more distant. We classified nine Cepheids from our sample as Magellanic Bridge objects. These Cepheids show a large spread in three-dimensions although five of them form a connection between the Clouds. The closest one is closer than any of the LMC Cepheids, while the farthest one - farther than any SMC Cepheid. All but one Cepheids in the Magellanic Bridge are younger than 300 Myr. The oldest one can be associated with the SMC Wing.

2

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.

3

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

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

Acta Astronomica

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2015

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Vol. 65, No. 4

297--312

EN

We present here a nearly complete census of classical Cepheids in the Magellanic System. The sample extends the set of Cepheids published in the past by the Optical Gravitational Lensing Experiment (OGLE) to the outer regions of the Large (LMC) and Small Magellanic Cloud (SMC). The entire collection consists of 9535 Cepheids of which 4620 belong to the LMC and 4915 are members of the SMC. We provide the I- and V-band time-series photometry of the published Cepheids, their finding charts, and basic observational parameters. Based on this unique OGLE sample of Cepheids we present updated period-luminosity relations for fundamental, first, and second mode of pulsations in the I- and V-bands and for the WI extinction-free Wesenheit index. We also show the distribution of classical Cepheids in the Magellanic System. The OGLE collection contains several classical Cepheids in the Magellanic Bridge - the region of interaction between the Magellanic Clouds. The discovery of classical Cepheids and their estimated ages confirm the presence of young stellar population between these galaxies.

4

The OGLE Collection of Variable Stars. Anomalous Cepheids in the Magellanic Clouds

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

Acta Astronomica

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2015

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Vol. 65, No. 3

233--250

EN

We present a collection of 250 anomalous Cepheids (ACs) discovered in the OGLE-IV fields toward the Large (LMC) and Small Magellanic Cloud (SMC). The LMC sample is an extension of the OGLE-III Catalog of ACs published in 2008, while the SMC sample contains the first known bona fide ACs in this galaxy. The total sample is composed of 141 ACs in the LMC and 109 ACs in the SMC. All these stars pulsate in single modes: fundamental (174 objects) or first overtone (76 objects). Additionally, we report the discovery of four ACs located in the foreground of the Magellanic Clouds. These are the first unambiguously identified fundamental-mode ACs known in the Galactic field. We demonstrate that the coefficients φ21 and φ31 determined by the Fourier light curve decomposition are useful discriminators between classical Cepheids and ACs, at least in the LMC and in the field of the Milky Way. In the SMC, the light curve shapes and mean magnitudes of short-period classical Cepheids make them similar to ACs, which is a source of difficulties in the discrimination of both classes of pulsators. The presence of unidentified ACs in the catalogs of classical Cepheids may be partly responsible for the observed non-linearity of the period--luminosity relation observed for short-period Cepheids in the SMC. We compare spatial distributions of ACs, classical Cepheids and RR Lyr stars. We show that the distribution of ACs resembles that of old stars (RR Lyr variables), although in the LMC there are visible structures typical for young population (classical Cepheids): the bar and spiral arms. This may suggest that ACs are a mixture of relatively young stars and mergers of very old stars.

5

Multi-Mode and Non-Standard Classical Cepheids in the Magellanic System

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

Acta Astronomica

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2015

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Vol. 65, No. 4

329--339

EN

We present a sample of the most interesting classical Cepheids selected from the OGLE collection of classical Cepheids in the Magellanic System. The main selection criterion for this sample was the presence of non-standard, unique pulsational properties. The sample contains the first known double-mode Cepheid pulsating in the second- and third-overtone modes and a large number of objects with non-radial modes excited. We also found Cepheids revealing Blazhko-like light curve modulation, objects ceasing pulsations or showing atypical shapes of their light curves. Additionally, the status of several triple mode classical Cepheids is updated based on OGLE-IV photometry extending the time baseline to 15 years.

6

Eclipsing Binaries with Classical Cepheid Component in the Magellanic System

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

Acta Astronomica

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2015

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Vol. 65, No. 4

341--350

EN

We present a census of eclipsing binary systems with classical Cepheid as a component. All such systems known were found in the OGLE collection of classical Cepheids in the Magellanic System. We extend the list of potential candidates adding four new objects found in the OGLE-IV photometric data. One of the new Cepheids in the eclipsing system, OGLE-SMC-CEP-3235, revealed only one eclipse during 15 years of the OGLE photometric monitoring. However, it additionally shows very well pronounced light-time effect indicating that the binarity is real and the system is physically bound. We also search for the light-time effect in other known eclipsing Cepheids and we clearly detect it in OGLE-LMC-CEP-1812. We discuss application of this tool for the search for Cepheids in non-eclipsing binary systems.

7

Variable Stars from the OGLE-III Shallow Survey in the Large Magellanic Cloud

Ulaczyk K., Szymański M. K., Udalski A., Kubiak M., Pietrzyński G., Soszyński I., Wyrzykowski Ł., Poleski R., Gieren W., Walker A. R., Garcia-Varel A.

Acta Astronomica

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2013

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Vol. 63, No. 2

159--179

EN

We describe variable stars found in the data collected during the OGLE-III Shallow Survey covering the I-band magnitude range from 9.7 mag to 14.5 mag. The main result is the extension of period-luminosity relations for Cepheids up to 134 days. We also detected 82 binary systems and 110 long-period variables not present in the main OGLE catalogs. Additionally 558 objects were selected as candidates for miscellaneous variables.

8

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

Soszyński I., Poleski R., 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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Vol. 60, No. 1

17--39

EN

The seventh part of the OGLE-III Catalog of Variable Stars (OIII-CVS) consists of 4630 classical Cepheids in the Small Magellanic Cloud (SMC). The sample includes 2626 fundamental-mode (F), 1644 first-overtone (1O), 83 second-overtone (2O), 59 double-mode F/1O, 215 double-mode 1O/2O, and three triple-mode classical Cepheids. For each object basic parameters, multi-epoch VI photometry collected within 8 or 13 years of observations, and finding charts are provided in the OGLE Internet archive. We present objects of particular interest: exceptionally numerous sample of single-mode second-overtone pulsators, five double Cepheids, two Cepheids with eclipsing variations superimposed on the pulsation light curves. At least 139 first-overtone Cepheids exhibit low-amplitude secondary variations with periods in the range 0.60-0.65 of the primary ones. These stars populate three distinct sequences in the Petersen diagram. The origin of this secondary modulation is still unknown. Contrary to the Large Magellanic Cloud (LMC) we found only a few candidates for anomalous Cepheids in the SMC. This fact may be a clue for the explanation of the origin of the anomalous Cepheids. The period and luminosity distributions of Cepheids in both Magellanic Clouds suggest that there are two or three populations of classical Cepheids in each of the galaxies. The main difference between the LMC and SMC lays in different numbers of Cepheids in each group. We fit the period-luminosity (PL) relations of SMC Cepheids and compare them with the LMC PL laws.

9

The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. VIII. Type II Cepheids 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. 2

91--107

EN

The eighth part of the OGLE-III Catalog of Variable Stars (OIII-CVS) contains type II Cepheids in the Small Magellanic Cloud (SMC). The sample consists of 43 objects, including 17 BL Her, 17 W Vir and 9 RV Tau stars (first examples ever found in the SMC). Seven stars have been classified as peculiar W Vir stars - a recently identified subclass of type II Cepheids. These stars have distinctive light curves, are brighter and bluer than the ordinary W Vir variables. We confirm that a large fraction of the peculiar W Vir stars are members of binary systems. Three type II Cepheids exhibit eclipsing variations superimposed on the pulsation light curves, and three other objects show long-period ellipsoidal variability. All stars with the indication of binarity display secondary periods which may be interpreted as amplitude and/or phase modulations of the pulsation light curves with periods equal to the orbital periods or half the orbital periods. We do not have any model for these modulations, however this phenomenon rules out a possibility of the optical blends of a pulsating star and a binary system. 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.

10

Type II Cepheids as Extragalactic Distance Candles

Majaess D., Turner D., Lane D.

Acta Astronomica

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2009

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Vol. 59, No. 4

403--418

EN

Extragalactic type II Cepheids are tentatively identified in photometric surveys of IC 1613, M33, M101, M106, M31, NGC 4603, and the SMC. Preliminary results suggest that type II Cepheids may play an important role as standard candles, in constraining the effects of metallicity on Cepheid parameters, and in mapping extinction.

11

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.

12

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.

13

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.

14

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.

15

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.

16

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.

17

Mean Angular Diameters and Angular Diameter Amplitudes of Bright Cepheids

Moskalik P., Gorynya N. A.

Acta Astronomica

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2005

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Vol. 55, No. 2

247--260

EN

We predict mean angular diameters and amplitudes of angular diameter variations for all monoperiodic Population I Cepheids brighter than (V) = 8.0 mag. The catalog is intended to aid selecting most promising Cepheid targets for future interferometric observations.

18

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.

19

Period Changes in Galactic Classical Cepheids. Slow Evolution of Long-Period Cepheids

Pietrukowicz P.

Acta Astronomica

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2003

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Vol. 53, No. 1

63--71

EN

We compared period changes derived from O-C diagrams for 63 classical Cepheids from our Galaxy with model calculations. We found that for Cepheids with log P>1.0 the observed changes are smaller than predicted values, except variable SZ Cas. However some of the first overtone Cepheids, particularly EU Tau and Polaris, change its period much faster than it follows from theory. Summary of the known data on period changes in Cepheids from the Galaxy and from the Magellanic Clouds (previous papers) leads to conclusion that none of the 999 Cepheids is undergoing the first crossing of the instability strip. Also the observed period changes for long-period Cepheids are a few times slower than predicted by the models. These results imply that much larger fraction of helium is burned in the Cepheid stage than it is predicted by models.

20

Period Changes of the SMC Cepheids from the Harvard, OGLE and ASAS Data

Pietrukowicz P.

Acta Astronomica

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2002

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Vol. 52, No. 2

177--187

EN

Comparison of the old observations of Cepheids in the Small Magellanic Cloud from the Harvard data archive, with the recent OGLE and ASAS observations allows an estimate of their period changes. All of matched 557 Cepheids are still pulsating in the same mode. One of the Harvard Cepheid, HV 11289, has been tentatively matched to a star which is now apparently constant. Cepheids with log P>0.8 show significant period changes, positive as well as negative. We found that for many stars these changes are significantly smaller than predicted by recent model calculations. Unfortunately, there are no models available for Cepheids with periods longer than approximately 80 days, while there are observed Cepheids with periods up to 210 days.