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1

Classical Cepheids in the Milky Way

Pietrukowicz P., Soszyński I., Udalski A.

Acta Astronomica

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2021

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Vol. 71, No. 3

205--222

EN

We share the most up-to-date, carefully verified list of classical Cepheids residing in the Galaxy. Based on long-term OGLE experience in the field of variable stars, we have inspected candidates for Cepheids from surveys such as ASAS, ASAS-SN, ATLAS, Gaia, NSVS, VVV, WISE, ZTF, among others, and also known sources from the General Catalogue of Variable Stars. Only objects confirmed in the optical range as classical Cepheids are included in the list. We provide Gaia EDR3 identifications of the stars. Purity of the sample exceeds 97 per cent, while its completeness is of about 88 per cent down to a magnitude G = 18. The list contains 3352 classical Cepheids, of which 2140 stars are fundamental-mode pulsators. Basic statistics and comparison between the classical Cepheids from the Milky Way, Andromeda Galaxy (M31), and Magellanic Clouds are provided. The list is available at the OGLE Internet Data Archive.

2

Additional Galactic Cepheids from the OGLE Survey

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

Acta Astronomica

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2020

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Vol. 70, No. 2

101--119

EN

We report on the results of a comprehensive search for Cepheid variable stars in the fields photometrically monitored by the OGLE Galaxy Variability Survey. We identify 742 Cepheids not included in the first release of the OGLE Collection of Galactic Cepheids and reclassify several dozen previously published variables. The upgraded collection comprises 1974 classical, 1625 type II, and 119 anomalous Cepheids located in the area of about 3000 square degrees covering the disk and bulge of the Milky Way. We present the most interesting objects in our sample: an isolated group of long-period double-mode classical Cepheids pulsating in the fundamental mode and first overtone, the first known Galactic double-mode Cepheid with the second and third overtone excited, double-mode type II Cepheids, candidates for single-mode first-overtone BL Her stars, and Cepheids showing simultaneous eclipsing variability. We also discuss type II and anomalous Cepheids that are potential members of globular clusters.

3

Variable Stars in M13. III. The Cepheid Variables and their Relation to Evolutionary Changes in Metal-Poor BL Her Stars

Osborn W., Kopacki G., Smith H. A., Layden A., Pritzl B., Kuehn C., Anderson M.

Acta Astronomica

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2019

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Vol. 69, No. 2

101--133

EN

New CCD photometry has been combined with published and unpublished earlier observations to study the three Cepheid variables in M13: V1, V2 and V6. The light curve characteristics in B, V and IC have been determined and the periods updated. A period change analysis shows all three stars have increasing periods but for V1 and V2 the rate of period increase does not appear to be constant over the 118 years of observation. The observed rates of period increase are in good agreement with the predictions of the Pisa theoretical models with helium abundance Y=0.25. Theory suggests V1 and V6 have masses of ≈0.57 M⊙ and are in the redward-evolving final stage of the "blue loop" evolutionary phase that is produced when helium-shell ignition occurs. The larger period and period change rate for V2 indicate it has a mass of ≈0.52 M⊙. A study of eighteen metal-poor BL Her stars shows the observed period changes for such objects in general can be reasonably well explained using the predictions from horizontal branch evolutionary tracks. BL Her stars with periods less than ≈3 d and relatively large secular period change rates (dP/dt≈5-15 d/Myr) are in the evolutionary stage before He-shell ignition. The remaining cases are stars that have already experienced He-shell ignition. Moreover, an analysis of crossing time through the instability strip indicates that it is likely that few, if any, BL Her stars have a He abundance as large as Y=0.33.

4

Final Release of the OGLE Collection of Cepheids and RR Lyrae Stars in the Magellanic System. The Outer Regions

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

Acta Astronomica

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2019

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Vol. 69, No. 2

87--99

EN

We present the final release of the OGLE collection of classical pulsators (Cepheids and RR Lyr stars) in the Large and Small Magellanic Clouds. The sky coverage has been increased from 670 to 765 square degrees compared to the previous edition of our collection. We also add some Cepheids and RR Lyr stars found by the Gaia team and reclassify three Cepheids. Ultimately, our collection consists of 9650 classical Cepheids, 343 type II Cepheids, 278 anomalous Cepheids, and 47 828 RR Lyr stars inside and toward the Magellanic System.

5

Three-Dimensional Distributions of Type II Cepheids and Anomalous Cepheids in the Magellanic Clouds. Do these Stars Belong to the Old, Young or Intermediate-Age Population?

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

Acta Astronomica

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2018

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Vol. 68, No. 3

213--236

EN

The nature of type II Cepheids and anomalous Cepheids is still not well known and their evolutionary channels leave many unanswered questions. We use complete collection of classical pulsating stars in the Magellanic Clouds discovered by the OGLE project, to compare their spatial distributions, which are one of the characteristic features directly related to the star formation history. In this analysis we use 9649 classical Cepheids, 262 anomalous Cepheids, 338 type II Cepheids and 46 443 RR Lyr stars from both Magellanic Clouds. We compute three-dimensional Kolmogorov-Smirnov tests for every possible pair of type II and anomalous Cepheids with classical Cepheids, and RR Lyr stars. We confirm that BL Her stars are as old as RR Lyr variable stars - their spatial distributions are similar, and they create a vast halo around both galaxies. We discover that spatial distribution of W Vir stars has attributes characteristic for both young and old stellar populations. Hence, it seems that these similarities are related to the concentration of these stars in the center of the Large Magellanic Cloud, and the lack of a vast halo. This leads to the conclusion that W Vir variables could be a mixture of old and intermediate-age stars. Our analysis of the three-dimensional distributions of anomalous Cepheids shows that they differ significantly from classical Cepheids. Statistical tests of anomalous Cepheids distributions with RR Lyr distributions do not give unambiguous results. We consider that these two distributions can be similar through the vast halos they create. This similarity would confirm anomalous Cepheids evolution scenario that assumes coalescence of a binary system.

6

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.

7

OGLE Collection of Galactic Cepheids

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.

8

New Galactic Multi-Mode Cepheids from the ASAS-SN Survey

Jurcsik J., Hajdu G., Catelani M.

Acta Astronomica

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2018

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Vol. 68, No. 4

341--350

EN

A systematic search for multi-mode Cepheids using the database of the ASAS-SN survey has led to the detection of thirteen new double-mode and two triple-mode Cepheids in the Galactic disk. These discoveries have increased the number of Galactic disk multi-mode Cepheids by 33%. One of the new triple-mode variables pulsates simultaneously in the fundamental and in the first and the second radial overtone modes and the other in the first three radial overtone modes. Overtone triple-mode Cepheids were identified only in the Galactic bulge and in the Large and Small Magellanic Clouds previously.

9

Metallicity Estimates for Double-Mode Cepheids from Photometry

Szilàdi K., Vinkò J., Szabados L.

Acta Astronomica

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2018

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Vol. 68, No. 2

111--123

EN

We investigate the effect of different metallicities on the light curve shape of classical double-mode (beat) Cepheids pulsating in the fundamental and first overtone modes by correlating the light curve Fourier parameters with [Fe/H] ratios. We combine data of known Galactic beat Cepheids with those from both Magellanic Clouds assembled by the OGLE-III and OGLE-IV surveys. The [Fe/H] values for the Galactic sample are based on recent high-resolution spectroscopy. For the Magellanic sample we apply the metallicity-period-period ratio relationship to estimate individual [Fe/H] values. We confirm that the metallicity is most strongly correlated with the amplitude of the light curve and the amplitude ratio of the first and second harmonics (R21), similar to single-mode classical Cepheids found by Klagyivik. We also show that the metallicity is less correlated with φ31 parameter in our sample, as compared with the case of the RR Lyr type variables. We derive empirical formulae that can be used to estimate the [Fe/H] of classical Cepheids that extends well below the metallicity of the Milky Way disk, down to the lower metallicities of the Magellanic Clouds.

10

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.

11

Concluding Henrietta Leavitt's Work on Classical Cepheids in the Magellanic System and Other Updates of the OGLE Collection of Variable Stars

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.

Acta Astronomica

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2017

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Vol. 67, No. 2

103--113

EN

More than a century ago, Henrietta Leavitt discovered the first Cepheids in the Magellanic Clouds together with the famous period-luminosity relationship revealed by these stars, which soon after revolutionized our view of the Universe. Over the years, the number of known Cepheids in these galaxies has steadily increased with the breakthrough in the last two decades thanks to the new generation of large-scale long-term sky variability surveys. Here we present the final upgrade of the OGLE Collection of Cepheids in the Magellanic System which already contained the vast majority of known Cepheids. The updated collection now comprises 9649 classical and 262 anomalous Cepheids. Type-II Cepheids will be updated shortly. Thanks to high completeness of the OGLE survey the sample of classical Cepheids includes virtually all stars of this type in the Magellanic Clouds. Thus, the OGLE survey concludes the work started by Henrietta Leavitt. Additionally, the OGLE sample of RR Lyr stars in the Magellanic System has been updated. It now counts 46 443 variables. A collection of seven anomalous Cepheids in the halo of our Galaxy detected in front of the Magellanic Clouds is also presented. OGLE photometric data are available to the astronomical community from the OGLE Internet Archive. The time-series photometry of all pulsating stars in the OGLE Collection has been supplemented with new observations.

12

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.

13

A Low-Resolution Spectroscopic Exploration of Puzzling OGLE Variable Stars

Pietrukowicz P., Latour M., Angeloni R., di Mille F., Soszyński I., Udalski A., Germanà C.

Acta Astronomica

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2015

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Vol. 65, No. 1

63--79

EN

We present the results of a spectroscopic follow-up of various puzzling variable objects detected in the OGLE-III Galactic disk and bulge fields. The sample includes mainly short-period multi-mode pulsating stars that could not have been unambiguously classified as either δ Sct or β Cep type stars based on photometric data only, also stars with irregular fluctuations mimicking cataclysmic variables and stars with dusty shells, and periodic variables displaying brightenings in their light curves that last for more than half of the period. The obtained low-resolution spectra show that all observed short-period pulsators are of δ Sct type, the stars with irregular fluctuations are young stellar objects, and the objects with regular brightenings are A type stars or very likely Ap stars with strong magnetic field responsible for the presence of bright caps around magnetic poles on their surface. We also took spectra of objects designated OGLE-GD-DSCT-0058 and OGLE-GD-CEP-0013. An estimated effective temperature of 33 000 K in OGLE-GD-DSCT-0058 indicates that it cannot be a δ Sct type variable. This very short-period (0.01962 d) high-amplitude (0.24 mag in the I-band) object remains a mystery. It may represent a new class of variable stars. The spectrum of OGLE-GD-CEP-0013 confirms that this is a classical Cepheid despite a peculiar shape of its light curve. The presented results will help in proper classification of variable objects in the OGLE Galaxy Variability Survey.

14

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.

15

Large Variety of New Pulsating Stars in the OGLE-III Galactic Disk Fields

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

Acta Astronomica

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2013

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Vol. 63, No. 4

379--404

EN

We present the results of a search for pulsating stars in the 7.12 deg2 OGLE-III Galactic disk area in the direction tangent to the Centaurus Arm. We report the identification of 20 Classical Cepheids, 45 RR Lyr type stars, 31 Long-Period Variables, such as Miras and Semi-Regular Variables, one pulsating white dwarf, and 58 very likely δ Sct type stars. Based on asteroseismic models constructed for one quadruple-mode and six triple-mode δ Sct type pulsators, we estimated masses, metallicities, ages, and distance moduli to these objects. The modeled stars have masses in the range 0.9-2.5 MSun and are located at distances between 2.5 kpc and 6.2 kpc. Two triple-mode and one double-mode pulsators seem to be Population II stars of the SX Phe type, probably from the Galactic halo. Our sample also includes candidates for Type II Cepheids and unclassified short-period (P<0.23 d) multi-mode stars which could be either δ Sct or β Cep type stars. One of the detected variables is a very likely δ Sct star with an exceptionally high peak-to-peak I-band amplitude of 0.35 mag at the very short period of 0.0196 d. All reported pulsating variable stars but one object are new discoveries. They are included in the OGLE-III Catalog of Variable Stars. Finally, we introduce the on-going OGLE-IV Galactic Disk Survey, which covers more than half of the Galactic plane. For the purposes of future works on the spiral structure and star formation history of the Milky Way, we have already compiled a list of known Galactic Classical Cepheids.

16

Puzzling Frequencies in First Overtone Cepheids

Dziembowski W. A.

Acta Astronomica

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2012

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Vol. 62, No. 4

323--341

EN

The OGLE project led to the discovery of earlier unknown forms of multiperiodic pulsation in Cepheids. Often, the observed periods may be explained in terms of simultaneous excitation of two or rarely three radial modes. However, a secondary variability at about 0.6 of the dominant period, detected in a number of the first overtone (1O) pulsators inhabiting the Magellanic Clouds, seems to require a different explanation. After reviewing a possibility of explaining this signal in terms of radial and nonradial modes, I find that only unstable modes that may reproduce the observed period ratio are f-modes of high angular degrees (ℓ=42-50). I discuss in detail the driving effect behind the instability and show that it is not the familiar opacity mechanism. Finally, I emphasize the main difficulty of this explanation, which requires high intrinsic amplitudes implying large broadening of spectral line.

17

New Observations and Period Change Study for the Anomalous Cepheid in M 92

Osborn W., Kopacki G., Haberstroh J.

Acta Astronomica

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2012

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Vol. 62, No. 4

377--388

EN

New observations of the variable star V7 in the globular cluster M 92 have been used to determine the parameters of its B, V and IC light curves. The star's classification as an anomalous Cepheid type is confirmed. The asymmetric shape of the light curve and position in the observed period-luminosity relation for anomalous Cepheids indicate the star is pulsating in the fundamental mode. Data from photographic plates, extending back to 1900, have been combined with published material and recent CCD observations to carry out a period change study. No evidence of a period change is found. Expected period change rates have been calculated using theoretical relations for the pulsational periods of anomalous Cepheids and evolutionary tracks for metal-poor horizontal branch stars with masses 1.0≤M/Msun≤1.6. Only models for stars close to central helium exhaustion show measurable rates, which indicates V7 is in the core helium burning phase of evolution.

18

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.

19

Large Magellanic Cloud Cepheids in the ASAS Data

Karczmarek P., Dziembowski W. A., Lenz P., Pietrukowicz P., Pojmański G.

Acta Astronomica

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2011

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Vol. 61, No. 4

303--318

EN

A catalog of Cepheids in the Large Magellanic Cloud (LMC) from the ASAS project is presented. It contains data on 65 fundamental mode pulsators with periods longer than about 8 days. The period-luminosity (PL) relation in the V-band does not significantly differ from the relation determined from the OGLE-III data extended toward longer periods but shows much larger scatter. For objects with periods longer than 40 d there is an evidence for a shallower PL relation. The rates of long-term period variations significant at 3σ level are found only for seven objects. The rates for 25 objects determined with the 1σ significance are confronted with the values derived from stellar evolution models. The models from various sources yield discrepant predictions. Over the whole data range, a good agreement with measurements is found for certain models but not from the same source.

20

The Cepheids of Centaurus A (NGC 5128) and Implications for H0

Majaess D.

Acta Astronomica

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2010

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Vol. 60, No. 2

121--136

EN

A VI Wesenheit and period-color analysis based on new OGLE observations reaffirms Ferrarese et al. discovery of 5 type II Cepheids in NGC 5128. The distance to that comparatively unreddened population is d=3.8±0.4(σx) Mpc. The classical Cepheids in NGC 5128 are the most obscured in the extragalactic sample (n=30) surveyed, whereas groups of Cepheids tied to several SNe host galaxies feature negative reddenings. Adopting an anomalous extinction law for Cepheids in NGC 5128 owing to observations of SN 1986G (RV≅2.4) is not favored, granted SNe Ia may follow smaller RV. The distances to classical Cepheids in NGC 5128 exhibit a dependence on color and CCD chip, which may arise in part from photometric contamination. Applying a color cut to mitigate contamination yields d≅3.5 Mpc (V-I ≤1.3 mag), while the entire sample's mean is d≅3.1 Mpc. The distance was established via the latest VI Galactic Wesenheit functions that include the 10 HST calibrators, and which imply a shorter distance scale than Sandage et al. (2004) by ≥10% at P≅25 d. HST monitored classical Cepheids in NGC 5128, and the SNe hosts NGC 3021 and NGC 1309, follow a shallower VI Wesenheit slope than ground-based calibrations of the Milky Way, LMC, NGC 6822, SMC, and IC 1613. The discrepancy is unrelated to metallicity since the latter group share a common slope over a sizeable abundance baseline (α=-3.34±0.08(2σ), Δ[Fe/H]≅1). A negligible distance offset between OGLE classical Cepheids and RR Lyr variables in the LMC, SMC, and IC 1613 bolsters assertions that VI-based Wesenheit functions are relatively insensitive to chemical abundance. In sum, a metallicity effect (VI) is not the chief source of uncertainty associated with the Cepheid distance to NGC 5128 or the establishment of the Hubble constant, but rather it may be the admittedly challenging task of obtaining precise, commonly standardized, multiepoch, multiband, comparatively uncontaminated extragalactic Cepheid photometry.