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The Optical Gravitational Lensing Experiment. Cepheids in the Galaxy IC1613: No Dependence of the Period-Luminosity Relation on Metallicity

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

Acta Astronomica

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2001

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Vol. 51, No. 3

221--245

EN

We present results of the search for Cepheids in the galaxy IC1613 carried out as a sub-project of the OGLE-II microlensing survey. 138 Cepheids were found in the 14'.2×14'.2 region in the center of the galaxy. We present light curves, VI photometry and basic data for all these objects, as well as color-magnitude diagram of the observed field. The Period-Luminosity (PL) diagrams for IC1613 fundamental mode Cepheids for V, I and interstellar extinction insensitive index WI are constructed. Comparison of PL relations in metal poor galaxy IC1613 ([Fe/H]≈-1.0 dex) with relations in metal richer Magellanic Clouds allows us to study dependence of Cepheid PL relations on metallicity in the wide range of metallicities covered by these three galaxies. The slopes of PL relations in IC1613 are identical as in the Magellanic Clouds. The comparison of brightness of Cepheids with the magnitudes of the tip of the red giant branch stars and RR Lyr stars in all three objects provides information on the stability of zero points of PL relations in the I and V-band, respectively. We find that the zero points of PL relations are constant to better than ±0.03 mag over the entire range of covered metallicities. Thus, the most important conclusion of the paper is that the Cepheid PL relations do not depend on metallicity. Additionally we determine the mean distance to IC1613, based on the common distance scale resulting from four major stellar distance indicators: Cepheids, RR Lyr, TRGB and red clump stars. The distance modulus to IC1613 is equal to (m-M)IC1613=24.20 mag with the standard deviation of ±0.02 mag from six measurements and systematic uncertainty of calibrations of ±0.07 mag.

2

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

Pietrukowicz P.

Acta Astronomica

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2001

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Vol. 51, No. 3

247--257

EN

Observations of Cepheids in the Large Magellanic Cloud, made over the last several decades, allow us to search for evolutionary period changes. None of the Cepheid from our sample of 378 stars stopped pulsating. Also none of them showed a large period change which could indicate mode switching. However for Cepheids with log P>0.9 we found significant period changes, positive as well as negative. A comparison between the observed period changes and theoretical predictions shows moderate agreement with some models (Bono et al. 2000), and a very large disagreement with others (Alibert et al. 1999). The large differences between the models are likely caused by the very high sensitivity of stellar evolution during core helium burning phase to even small changes in the input physics, as discovered by Lauterborn, Refsdal and Weigert (1971).

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The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. VI. Double-Mode Cepheids in the Large Magellanic Cloud

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

Acta Astronomica

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2000

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Vol. 50, No. 4

451--490

EN

We present a sample of 76 double-mode Cepheids detected in the 4.5 square degree area in the central part of the LMC. 19 stars from the sample pulsate in the fundamental mode and the first overtone while 57 objects are the first and second overtone pulsators. We analyze the period ratio of double-mode Cepheids and Fourier parameters of decomposition of the light curves of these objects. We also present location of different type Cepheids from the LMC in the color-magnitude diagram and show the distribution of their V-I color indices.

4

Fourier Analysis of Hipparcos Photometry of Cepheid Variables

Zakrzewski B., Ogłoza W., Moskalik P.

Acta Astronomica

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2000

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Vol. 50, No. 3

387--397

EN

Fourier parameters have been computed for 240 field Cepheids observed by the Hipparcos satellite. We have identified three new Population I overtone Cepheids: V411 Lac, V898 Cen and V572 Aql. We have compared Fourier progressions of Population I and Population II Cepheids. The possibility of using Fourier parameters to distinguish between the two types is discussed.

5

The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. V. Catalog of Cepheids from the Small Magellanic Cloud

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

Acta Astronomica

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1999

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Vol. 49, No. 4

437--520

EN

We present the Catalog of Cepheids from the SMC which contains data for 2049 objects detected in the 2.4 square degree area of central parts of the SMC. For each object period, BVI photometry, astrometry, and R21, φ21 parameters of the Fourier decomposition of I-band light curve are provided. The Catalog is based on observations collected during the OGLE-II microlensing survey. Tests of completeness performed in overlapping parts of adjacent fields indicate that completeness of the Catalog is very high: ≈92%. Statistics and distributions of basic parameters of Cepheids are also presented. All presented data, including individual BVI observations (≈4.7×105 BVI measurements), are available from the OGLE Internet archive.

6

The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. II. Single-Mode Second Overtone Cepheids in the Small Magellanic Cloud

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

Acta Astronomica

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1999

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Vol. 49, No. 1

45--57

EN

We present a sample of 13 candidates for single-mode second overtone Cepheids discovered in the 2.4 square degree area of the SMC bar during the OGLE microlensing search. All candidates passed four photometric tests: analysis of Fourier parameters of the light curve, R21 and φ21, position in the period-luminosity diagram and location in the color-magnitude diagram. The stars form a very homogeneous group of objects with very similar light curves of low amplitude and almost sinusoidal shape. Their color indices B-V and V-I are also very similar and the objects determine the observational blue edge of the instability strip of the first overtone Cepheids. This is the first firm sample of the single-mode second overtone Cepheids detected so far.

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The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. I. Double-Mode Cepheids in the Small Magellanic Cloud

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

Acta Astronomica

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1999

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Vol. 49, No. 1

1--44

EN

We present a sample of 93 double-mode Cepheids detected in the 2.4 square degree area in the central part of the SMC. 23 stars from the sample pulsate in the fundamental mode and the first overtone while 70 objects are the first and second overtone pulsators. This is the largest sample of such type Cepheids detected in one environment so far. We analyze period ratio of double-mode Cepheids and Fourier parameters of decomposition of the light curves of these objects. We also present location of different type Cepheids from the SMC in the color-magnitude diagram and show their distribution of V-I color indices. We find one object which is probably a blend, either physical or optical, of two Cepheids pulsating in the fundamental mode.

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The Optical Gravitational Lensing Experiment. Cepheids in Star Clusters from the Magellanic Clouds

Pietrzyński G., Udalski A.

Acta Astronomica

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1999

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Vol. 49, No. 4

543--550

EN

We present Cepheids located in the close neighborhood of star clusters from the Magellanic Clouds. 204 and 132 such stars were found in the LMC and SMC, respectively. The lists of objects were constructed based on catalogs of Cepheids and star clusters, recently published by the OGLE-II collaboration. Location of selected Cepheids on the sky indicates that many of them are very likely cluster members. Photometric data of Cepheids and clusters are available from the OGLE Internet archive.

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Study of Neglected Variable Stars Classified as Type II Cepheids

Berdnikov L. N., Szabados L.

Acta Astronomica

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1998

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Vol. 48, No. 4

763--774

EN

Four variable stars classified as Type II Cepheids have been studied partly based on new photometry and the Hipparcos photometric data. The correct period is derived for V341 Ara and NW Pav and it is pointed out that RR Mic and V1711 Sgr do not belong to Type II Cepheids, instead they may be semi-regular variables. The cycle length of their variability is also determined.