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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

Concerning the Distance to the Center of the Milky Way and Its Structure

Majaess D.

Acta Astronomica

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2010

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Vol. 60, No. 1

55--74

EN

The distance to the Galactic center inferred from OGLE RR Lyr variables observed in the direction of the bulge is R0=8.1±0.6 kpc. An accurate determination of R0 is hindered by countless effects that include an ambiguous extinction law, a bias for smaller values of R0 because of a preferential sampling of variable stars toward the near side of the bulge owing to extinction, and an uncertainty in characterizing how a mean distance to the group of variable stars relates to R0. A VI-based period-reddening relation for RR Lyr variables is derived to map extinction throughout the bulge. The reddening inferred from RR Lyr variables in the Galactic bulge, LMC, SMC, and IC 1613 match that established from OGLE red clump giants and classical Cepheids. RR Lyr variables obey a period-color (VI) relation that is relatively insensitive to metallicity. Edge-on and face-on illustrations of the Milky Way are constructed by mapping the bulge RR Lyr variables in tandem with cataloged red clump giants, globular clusters, planetary nebulae, classical Cepheids, young open clusters, HII regions, and molecular clouds. The sample of RR Lyr variables do not trace a prominent Galactic bar or triaxial bulge oriented at φ≅25°.

3

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.