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USNO-B1.0 1171-0309158: An RR Lyrae Star that Switched from a Double- to Single-Mode Pulsation

Khruslov A. V., Kusakin A. V., Reva I. V.

Acta Astronomica

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2017

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Vol. 67, No. 4

317--327

EN

We report the discovery of a new case of an RR Lyr star that experienced a switching of its pulsation mode. We detected USNO-B1.0 1171-0309158 as a double-mode RR Lyr star from observations of the Catalina Surveys that showed additional scattering on the light curve. Our analysis of the time-series of CSS data showed gradual increase in scattering and in the amplitude of fundamental pulsation mode. Our CCD observations carried out in 2015-2017 reveal that this object is now a fundamental-mode RRab star, with no sign of the first-overtone pulsation.

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OGLE-BLG-RRLYR-12245: An RR Lyrae Star that Switched from a Double- to Single-Mode Pulsation

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

Acta Astronomica

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2014

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Vol. 64, No. 1

1--9

EN

We report the discovery of an RR Lyr star that experienced a switching of its pulsation mode. OGLE-BLG-RRLYR-12245 was discovered as a double-mode RRd star based on observations conducted in the years 2001-2006 during the third phase of the Optical Gravitational Lensing Experiment (OGLE-III). The OGLE-IV observations carried out since 2010 reveal that this object is now a fundamental-mode RRab star, with no sign of the first-overtone pulsation. The analysis of the OGLE photometry shows that the final stage of the mode switching occurred on a relatively short timescale of a few months in 2005. We study the behavior of the star during this process, showing changes of the pulsational amplitudes and periods. We also discuss possible reasons for the mode switching in RR Lyr stars.

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On the Distribution of the Modulation Frequencies of RR Lyrae Stars

Jurcsik J., Szeidl B., Nagy A., Sódor Á.

Acta Astronomica

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2005

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Vol. 55, No. 3

303--314

EN

For the first time connection between the pulsation and modulation properties of RR Lyr stars has been detected. Based on the available data it is found that the possible range of the modulation frequencies, i.e., the possible maximum value of the modulation frequency depends on the pulsation frequency. Short period variables (P<0.4 d) can have modulation period as short as some days, while longer period variables (P>0.6 d) always exhibit modulation with Pmod>20 d. We interpret this tendency with the equality of the modulation period with the surface rotation period, because similar distribution of the rotational periods is expected if an upper limit of the total angular momentum of stars leaving the RGB exists. The distribution of the projected rotational velocities of red and blue horizontal branch stars at different temperatures shows a similar behavior as vrot derived for RR Lyr stars from their modulation periods. This common behavior gives reason to identify the modulation period with the rotational period of the modulated RR Lyr stars.

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Metallicity Dependence of the Blazhko Effect

Smolec R.

Acta Astronomica

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2005

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Vol. 55, No. 1

59--84

EN

The microlensing surveys, such as OGLE or MACHO, have led to the discovery of thousands of RR Lyr stars in the Galactic bulge and in the Magellanic Clouds, allowing for detailed investigation of these stars, especially the still mysterious Blazhko phenomenon. Higher incidence rate of Blazhko (BL) variables in the more metal-rich Galactic bulge than in the LMC, suggests that occurrence of Blazhko effect correlates with metallicity. To investigate this problem, we calibrate the photometric method of determining the metallicity of RRab stars in the I-band and apply it to the OGLE Galactic bulge and LMC data. In both systems, metallicities of non Blazhko and Blazhko variables are close to each other. The LMC Blazhko pulsators prefer slightly lower metallicities. The different metallicities of the Galactic bulge and the LMC, cannot explain the observed incidence rates. As a by-product of our metallicity estimates, we investigate the luminosity--metallicity relation, finding a steep dependence of the luminosity on [Fe/H].

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UBVI Color-Magnitude Diagrams in Baade's Window: Metallicity Range, Implications for the Red Clump Method, Color "Anomaly" and the Distances to the Galactic Center and the Large Magellanic Cloud

Stanek K. Z., Kaluzny J., Wysocka A., Thompson I.

Acta Astronomica

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2000

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Vol. 50, No. 2

191--210

EN

We analyze the UBVI color-magnitude diagrams towards the Galactic bulge in a relatively low-reddening region of Baade's Window. The dereddened red giant branch is very wide [≈1.0 mag in (U-B)0 and ≈0.4 mag in (B-V) 0 and (V-I) 0], indicating a significant dispersion of stellar metallicities, which by comparison with the theoretical isochrones and data for Galactic clusters we estimate to lie between approximately -0.7<[Fe/H]<+0.3, i.e., spanning about 1 dex in metallicity, in good agreement with earlier spectroscopic studies. We also discuss the metallicity dependence of the red clump I-band brightness and we show that it is between 0.1-0.2 mag/dex. This agrees well with the previous empirical determinations and the models of stellar evolution. The dereddened (V-I)0 color of the red clump in the observed bulge field is ⟨(V-I)0⟩=1.066, σ(V-I)0=0.14, i.e., 0.056 mag redder than the local stars with good parallaxes measured by Hipparcos. It seems that the large "color anomaly" of ≈0.2 mag noticed by Paczyński and Stanek and discussed in many recent papers was mostly due to earlier problems with photometric calibration. When we use our data to re-derive the red clump distance to the Galactic center, we obtain the Galactocentric distance modulus μ0,GC=14.69±0.1 mag (R0=8.67± 0.4 kpc), with error dominated by the systematics of photometric calibration. We then discuss the systematics of the red clump method and how they affect the red clump distance to the Large Magellanic Cloud. We argue that the value of distance modulus μ0,LMC=18.24±0.08 (44.5±1.7 kpc), recently refined by Udalski, is currently the most secure and robust of all LMC distance estimates. This has the effect of increasing any LMC-tied Hubble constant by about 12%, including the recent determinations by the HST Key Project and Sandage et al. The UBVI photometry is available through the anonymous ftp service.