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Szabo R.,Hungarian Academy of Sciences | Kollath Z.,Hungarian Academy of Sciences | Molnar L.,Hungarian Academy of Sciences | Kolenberg K.,University of Vienna | And 14 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

The first detection of the period doubling phenomenon is reported in the Kepler RR Lyrae stars RR Lyr, V808 Cyg and V355 Lyr. Interestingly, all these pulsating stars show Blazhko modulation. The period doubling manifests itself as alternating maxima and minima of the pulsational cycles in the light curve, as well as through the appearance of half-integer frequencies located halfway between the main pulsation period and its harmonics in the frequency spectrum. The effect was found to be stronger during certain phases of the modulation cycle. We were able to reproduce the period-doubling bifurcation in our non-linear RR Lyrae models computed by the Florida-Budapest hydrocode. This enabled us to trace the origin of this instability in RR Lyrae stars to a resonance, namely a 9:2 resonance between the fundamental mode and a high-order (ninth) radial overtone showing strange-mode characteristics. We discuss the connection of this new type of variation to the mysterious Blazhko effect and argue that it may give us fresh insights into solving this century-old enigma. © 2010 The Authors. Journal compilation © 2010 RAS.

Kim C.-H.,Chungbuk National University | Song M.-H.,Chungbuk National University | Song M.-H.,Korea Astronomy and Space Institute | Yoon J.-N.,Chungbuk National University | And 3 more authors.
Astrophysical Journal | Year: 2014

A photometric study of BD And was made through the analysis of two sets of new BVR light curves. The light curves with migrating photometric waves outside eclipse show that BD And is a short-period RS CVn-type binary star. The analysis of all available timings reveals that the orbital period has varied in a strictly cyclical way with a period of 9.2 yr. The periodic variation most likely arises from the light-time effect due to a tertiary moving in a highly elliptical orbit (e 3= 0.76). The Applegate mechanism could not operate properly in the eclipsing pair. The light curves were modeled with two large spots on the hotter star and a large third light amounting to about 14% of the total systemic light. BD And is a triple system: a detached binary system consisting of two nearly equal solar-type stars with an active primary star and a G6-G7 tertiary dwarf. The absolute dimensions of the eclipsing pair and tertiary components were determined. The three components with a mean age of about 5.8 Gyr are located at midpositions in main-sequence bands. The radius of the secondary is about 17% larger than that deduced from stellar models. The orbital and radiometric characteristics of the tertiary are intensively investigated. One important feature is that the mutual inclination between two orbits is larger than 60°, implying that Kozai cycles had occurred very efficiently in the past. The possible past and future evolutions of the BD And system, driven by KCTF and MBTF, are also discussed. © 2014. The American Astronomical Society. All rights reserved..

Bianchi L.,Johns Hopkins University | Kang Y.,Daejeon University | Kang Y.,Korea Astronomy and Space Institute | Hodge P.,University of Washington | And 2 more authors.
Advances in Space Research | Year: 2014

We discuss the relevance of UV data in the detection and characterization of hot massive stars and young stellar populations in galaxies. We show results from recent extensive surveys in M31 and M33 with Hubble Space Telescope (HST) multi-wavelength data including UV filters, which imaged several regions at a linear resolution (projected) of less than half a pc in these galaxies, and from GALEX far-UV and near-UV wide-field, low-resolution imaging of the entire galaxies. Both datasets allow us to study the hierarchical structure of star formation: the youngest stellar groups are the most compact, and are often arranged within broader, sparser structures. The derived recent star-formation rates are rather similar for the two galaxies, when scaled for the respective areas. We show how uncertainties in metallicity and type of selective extinction for the internal reddening may affect the results, and how an appropriate complement of UV filters could reduce such uncertainties, and significantly alleviate some parameter degeneracies. © 2013 The Authors. Published by Elsevier Ltd.

Hachisuka K.,Yamaguchi University | Hachisuka K.,Chinese Academy of Sciences | Hachisuka K.,Ibaraki University | Choi Y.K.,Korea Astronomy and Space Institute | And 6 more authors.
Astrophysical Journal Letters | Year: 2015

We report parallaxes and proper motions of three water maser sources in high-mass star-forming regions in the Outer Spiral Arm of the Milky Way. The observations were conducted with the Very Long Baseline Array as part of Bar and Spiral Structure Legacy Survey and double the number of such measurements in the literature. The Outer Arm has a pitch angle of 14.°9 ± 2.°7 and a Galactocentric distance of 14.1 ± 0.6 kpc toward the Galactic anticenter. The average motion of these sources toward the Galactic center is 10.7 ± 2.1 km s-1 and we see no sign of a significant fall in the rotation curve out to 15 kpc from the Galactic center. The three-dimensional locations of these star-forming regions are consistent with a Galactic warp of several hundred parsecs from the plane. © 2015. The American Astronomical Society. All rights reserved.

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