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1

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.

2

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.

3

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.

4

The Optical Gravitational Lensing Experiment. Stellar Distance Indicators in the Magellanic Clouds and Constraints on the Magellanic Cloud Distance Scale

Udalski A.

Acta Astronomica

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2000

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Vol. 50, No. 3

279--306

EN

BVI photometry of the Magellanic Clouds collected during the OGLE-II microlensing experiment makes it possible to study in detail photometric properties of the "major" stellar distance indicators in the Magellanic Clouds. In addition to Cepheids, RR Lyr and red clump stars, which photometry was presented in the earlier OGLE papers, we present the so far most accurate determination of brightness of the tip of the red giant branch in the LMC and SMC. We analyze the ratios of brightness of all four distance indicators in the LMC and SMC. Additionally, we include in our analysis, when possible, photometric data of the distance indicators in the metal poor Carina dwarf galaxy for which photometry was also collected during the OGLE-II experiment. The analysis is largely differential, free from zero point and extinction uncertainties. The main conclusion is that the distance scales resulting from all four stellar distance indicators are fully consistent with each other. Therefore the distance scale problem is not a problem resulting from different distances yielded by distance indicators, but rather a problem of the proper zero point of this common distance scale. All four stellar distance indicators have to be treated as an ensemble, and any determination of the zero point for one of them must predict reasonable luminosities of the others. This puts strong constraints on many proposed calibrations of distance indicators. For example, very bright calibrations of Cepheids or RR Lyr can practically be ruled out. At present, the most likely calibration of the zero point of the common distance scale, which would be consistent with observations of all four distance indicators, is that resulting from faint calibration of RR Lyr stars or calibration of the red clump stars. With the OGLE-II photometry it leads to the distance moduli of (m-M)LMC=18.24 mag, (m-M)SMC=18.75 mag and (m-M)CAR=19.94 mag for the LMC, SMC and Carina dwarf galaxy, respectively. The systematic uncertainty is of the order of 0.07 mag, while the standard deviation of four determinations of only 0.02 mag. We also analyze the difference of distance moduli between the SMC and LMC. The average value is Δ(m-M)SMC-LMC=0.50±0.03 mag from four independent measurements. This very good agreement allows us to draw conclusions on the interstellar extinction in the Magellanic Clouds. Our photometric data also provide constraints on the properties of red clump stars. Finally, the differential comparison of brightness of Cepheids with brightness of other distance indicators in the Magellanic Clouds, and preliminary observations of Cepheids in the IC1613 galaxy indicate no dependence of the zero point of Cepheid Period-Luminosity relation on metallicity.

5

The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. III. Period-Luminosity-Color and Period-Luminosity Relations of Classical Cepheids

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

Acta Astronomica

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1999

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Vol. 49, No. 3

201--211

EN

We present Period-Luminosity-Color and Period-Luminosity relations of classical Cepheids constructed for about 1280 Cepheids from the LMC and 2140 from the SMC. High quality BVI observations (120-360 epochs in the I-band and 15-40 in the BV-bands) were collected during the OGLE-II microlensing experiment. The I-band diagrams of the LMC show very small scatter, σ=0.074 mag, indicating that Cepheid variables can potentially be a very good standard candle. We compare relations of fundamental mode Cepheids from the LMC and SMC and we do not find significant differences of slopes of the Period-Luminosity-Color and Period-Luminosity relations in these galaxies. For the first overtone Cepheids a small change of the slope of Period-Luminosity relation is possible. We determine the difference of distance moduli between the SMC and LMC with Cepheid relations and compare the result with the difference obtained with other standard candles: RR Lyr and red clump stars. Results are very consistent and indicate that the values of zero points of the fundamental mode Cepheid relations are similar in these galaxies. The mean difference of distance moduli between the SMC and LMC is equal to μSMC-μLMC=0.51±0.03 mag. We calibrate the Period-Luminosity-Color and Period-Luminosity relations for classical, fundamental mode Cepheids using the observed LMC relations and adopting the short LMC distance modulus, μLMC=18.22±0.05 mag, resulting from the recent determinations with eclipsing system HV2274, RR Lyr and red clump stars and consistent with observations of Cepheids in NGC 4258 galaxy, to which precise geometric distance is known. Finally, we determine a constraint on the absolute magnitude of Cepheids by comparison of their mean V-band magnitude with that of RR Lyr stars in both Magellanic Clouds. The 10-day period, fundamental mode Cepheid is on average 4.63±0.05 mag brighter than RR Lyr stars of LMC metallicity which with the most likely calibration of the brightness of RR Lyr stars yields MVC,10=-3.92±0.09 mag.

6

The Optical Gravitational Lensing Experiment. Population Effects on the Mean Brightness of the Red Clump Stars

Udalski A.

Acta Astronomica

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1998

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Vol. 48, No. 3

383--404

EN

We present an empirical test indicating that the mean I-band magnitude of the red clump stars, used as the standard candle in the recent distance determinations to the Magellanic Clouds and other objects, is age independent for intermediate age (2-10 Gyr) stars. Fifteen star clusters of age ≈1.5-12 Gyr from the LMC and SMC (ESO121SC03, SL663, NGC 2155, NGC 2121, SL388, SL862, NGC 121, L1, KRON3, NGC 416, L113, NGC 339, L11, NGC 419, NGC 411) were observed and their color-magnitude diagrams are presented. The mean I-band brightness of the red clump in these clusters is constant and its mean extinction-free magnitude is: I0=17.88±0.05 mag and I0=18.31±0.07 mag at the mean metallicity of -0.8 dex and -1.2 dex for the LMC and SMC clusters, respectively. For older objects (>10 Gyr) the brightness of the red clump, which converts into the red part of the horizontal branch, fades by about 0.3-0.4 mag, setting an important limitation on the red clump stars method of distance determination. The red clump distance moduli to the Magellanic Clouds from the new independent data set are: m-M=18.18±0.06 mag and m-M=18.65±0.08 mag for the LMC and SMC, respectively, in very good agreement with previous determinations. Weak dependence of the mean I-band brightness of the red clump on metallicity and its independence of age for intermediate age population (2-10 Gyr) of stars as well as the most precise calibration as compared to other standard candle candidates makes the red clump stars method one of the most accurate steps in the distance scale ladder. Key words: Magellanic Clouds - Galaxies: distances and redshifts - distance scale - clusters individual: ESO121SC03, SL663, NGC 2155, NGC 2121, SL388, SL862, NGC 121, L1,