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Szabados L.,Konkoly Observatory | Klagyivik P.,Konkoly Observatory
Astronomy and Astrophysics | Year: 2012

Context. Physical and phenomenological properties (radius, luminosity, shape of the light curve, etc.) of Cepheids strongly depend on the pulsation period, with the exception of the pulsation amplitude. A possible factor causing a wide range of pulsation amplitudes might be the different atmospheric metallicities of individual Cepheids. Aims. We studied the influence exerted by the atmospheric iron content, [Fe/H], on the pulsational amplitude of Galactic Cepheids. Methods. We searched for correlations between the [Fe/H] value and both the observed amplitudes and amplitude related parameters. Results. The amplitude of the Cepheid pulsation slightly decreases with increasing iron abundance. This effect is more pronounced for the radial velocity variations and for the shorter pulsation periods. The wavelength dependence of photometric amplitudes is also found to be sensitive to the metallicity. Some of these effects are not consequences of differential line blanketing. Based on the calibrations of the metallicity sensitivity relationships, we derived photometric iron abundance for 21 Galactic Cepheids. The dichotomic behaviour dividing Galactic Cepheids that pulsate in the fundamental mode into short-and long-period groups at the period of 10 d.47 can be noticed in some diagrams that show metallicity-related dependences. Conclusions. We confirm that variety in atmospheric metallicity in Cepheids contributes to the finite range of pulsation amplitudes at a given period. Effects of metallicity on the amplitudes revealed from observational data and the occurrence of the dichotomy also derived from phenomenological data have to be confirmed by appropriate theoretical models of stellar structure and pulsation. © 2012 ESO.


Buchler J.R.,University of Florida | Kollath Z.,Konkoly Observatory
Astrophysical Journal | Year: 2011

The Blazhko effect is a long-term, generally irregular modulation of the light curves that occurs in a sizeable number of RR Lyrae stars. The physical origin of the effect has been a puzzle ever since its discovery over a hundred years ago. We build here upon the recent observational and theoretical work of Szabó etal. on RRab stars who found with hydrodynamical simulations that the fundamental pulsation mode can get destabilized by a 9:2 resonant interaction with the 9th overtone. Alternating pulsation cycles arise, although these remain periodic, i.e., not modulated as in the observations. Here we use the amplitude equation formalism to study this nonlinear, resonant interaction between the two modes. We show that not only does the fundamental pulsation mode break up into a period-two cycle through the nonlinear, resonant interaction with the overtone, but that the amplitudes are modulated, and that in a broad range of parameters the modulations are irregular as in the observations. This irregular behavior is in fact chaotic and arises from a strange attractor in the dynamics. © 2011. The American Astronomical Society. All rights reserved.


Barcza S.,Konkoly Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2010

A photometric calibration of Kurucz static model atmospheres is used to obtain the following parameters of RR Lyrae stars: variation of stellar angular radius θ, effective temperature Te, gravity ge as a function of phase, interstellar reddening E(Ba v- V) towards the star and atmospheric metallicity M. Photometric and hydrodynamic conditions are given to find the phases of pulsation when the quasi-static atmosphere approximation (QSAA) can be applied. The QSAA is generalized to a non-uniformly moving spherical atmosphere, and the distance d, mass and atmospheric motion are derived from the laws of mass and momentum conservation. To demonstrate the efficiency of the method, the UBV(RI)C photometry of SU Dra was used to derive the following parameters: [M] = -1.60 ± 0.10 dex, equilibrium luminosity Leq equals; 45.9 ± 9.3 L⊙ and Teq = 6813 ± 20 K. © 2010 The Author. Journal compilation © 2010 RAS.


Williams D.R.,University College London | Baker D.,University College London | Van Driel-Gesztelyi L.,University College London | Van Driel-Gesztelyi L.,University Paris Diderot | Van Driel-Gesztelyi L.,Konkoly Observatory
Astrophysical Journal | Year: 2013

We present a new method for determining the column density of erupting filament material using state-of-the-art multi-wavelength imaging data. Much of the prior work on filament/prominence structure can be divided between studies that use a polychromatic approach with targeted campaign observations and those that use synoptic observations, frequently in only one or two wavelengths. The superior time resolution, sensitivity, and near-synchronicity of data from the Solar Dynamics Observatory's Advanced Imaging Assembly allow us to combine these two techniques using photoionization continuum opacity to determine the spatial distribution of hydrogen in filament material. We apply the combined techniques to SDO/AIA observations of a filament that erupted during the spectacular coronal mass ejection on 2011 June 7. The resulting "polychromatic opacity imaging" method offers a powerful way to track partially ionized gas as it erupts through the solar atmosphere on a regular basis, without the need for coordinated observations, thereby readily offering regular, realistic mass-distribution estimates for models of these erupting structures. © 2013. The American Astronomical Society. All rights reserved.


Kovacs G.,Konkoly Observatory
Astronomy and Astrophysics | Year: 2015

By using the current photometric rotational data on eight galactic open clusters, we show that the evolutionary stellar model (isochrone) ages of these clusters are tightly correlated with the period shifts applied to the (B - V)0-Prot ridges that optimally align these ridges to the one defined by Praesepe and the Hyades. On the other hand, when the traditional Skumanich-type multiplicative transformation is used, the ridges become far less aligned due to the age-dependent slope change introduced by the period multiplication. Therefore, we employ our simple additive gyro-age calibration on various datasets of Galactic field stars to test its applicability. We show that, in the overall sense, the gyro-ages are systematically greater than the isochrone ages. The difference could exceed several giga years, depending on the stellar parameters. Although the age overlap between the open clusters used in the calibration and the field star samples is only partial, the systematic difference indicates the limitation of the currently available gyro-age methods and suggests that the rotation of field stars slows down with a considerably lower speed than we would expect from the simple extrapolation of the stellar rotation rates in open clusters. © ESO, 2015.


Molnar L.,Konkoly Observatory | Kollath Z.,Konkoly Observatory | Szabo R.,Konkoly Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2012

The Blazhko phenomenon, the modulation of the pulsation of RR Lyrae stars, remains one of the most stubborn unsolved problems of stellar pulsation. The recent idea of Stothers proposes that periodic variations in the properties of the convective envelope may be behind the amplitude and phase modulation. In this work, we approximated the mechanism by introducing variations in the convective parameters of the Florida-Budapest hydrodynamic code and also by means of amplitude equations. We found that the process is only effective for long modulation periods, typically for more than 100 d, in agreement with the thermal time-scales of the pulsation in RR Lyrae stars. Due to the slow response of the pulsation to the structure changes, short-period, high-amplitude Blazhko modulation cannot be reproduced with this mechanism or would require implausible variations in the convective parameters on short time-scales. We also found that the modulation of the mixing length results in strong differences between both the luminosity and radius variations and the respective phase modulations of the two quantities, suggesting notable differences between the energy output of the photosphere and the mechanical variations of the layers. The findings suggest that the convective cycle model is not well suited as a stand-alone mechanism behind the Blazhko effect. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Barcza S.,Konkoly Observatory | Benko J.M.,Konkoly Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2012

Our photometric-hydrodynamic method is generalized to determine the fundamental parameters of multiperiodic radially pulsating stars. We report 302UBV(RI) C Johnson-Kron-Cousins observations of GSC 4868-0831. Using these and the published photometric data of V372 Ser, we determine the metallicity, reddening, distance, mass, radius, equilibrium luminosity and effective temperature. The results underline the necessity of using multicolour photometry, including an ultraviolet band, to classify the subgroups of RR Lyrae stars properly. OurUobservations might reveal that GSC 4868-0831 is a subgiant star pulsating in two radial modes and that V372 Ser is a giant star with the size and mass of an RRd star. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Kollath Z.,Konkoly Observatory
Journal of Physics: Conference Series | Year: 2010

One of the first 'International Dark-sky Parks' in Europe was established at the Zselic Landscape Protection Area in Hungary. A special monitoring program has been carrying on to survey the quality of the night sky using 'Sky Quality Meters' and DSLR cameras. The main conclusion of our measurements is that the local villages have only a minimal effect on the quality of the sky. There are light-domes due to the neighbouring cities only close to the horizon, the main source of obtrusive light is the city of Kaposvár. The anthropogenic component of zenith luminance of the night sky is obtained as the function of the distance from the city centre of Kaposvár. Our data were modelled by radiation transfer calculations. These results can help to draw attention to the energy emitted useless to the space and to protect our nocturnal landscape of nature parks for the next generations. © 2010 IOP Publishing Ltd.


Kollath Z.,Konkoly Observatory | Molnar L.,Konkoly Observatory | Szabo R.,Konkoly Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2011

We investigated period doubling, a well-known phenomenon in dynamical systems, for the first time in RR Lyrae models. These studies provide theoretical background for the recent discovery of period doubling in some Blazhko RR Lyrae stars with the Kepler space telescope. Since period doubling has been observed only in Blazhko-modulated stars so far, the phenomenon can help in understanding the modulation as well. Utilizing the Florida-Budapest turbulent convective hydrodynamical code, we have identified the phenomenon in both radiative and convective models. A period-doubling cascade was also followed up to an eight-period solution, confirming that destabilization of the limit cycle is indeed the underlying phenomenon. Floquet stability roots were calculated to investigate the possible causes and occurrences of the phenomenon. A two-dimensional diagnostic diagram was constructed to illustrate the various resonances between the fundamental mode and the different overtones. Combining the two tools, we confirmed that the period-doubling instability is caused by a 9:2 resonance between the ninth overtone and the fundamental mode. Destabilization of the limit cycle by a resonance of a high-order mode is possible because the overtone is a strange mode. The resonance is found to be strong enough to shift the period of overtone by up to 10 per cent. Our investigations suggest that a more complex interplay of radial (and presumably non-radial) modes could happen in RR Lyrae stars that might have connections with the Blazhko effect as well. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Benko J.M.,Konkoly Observatory | Szabo R.,Konkoly Observatory | Paparo M.,Konkoly Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2011

We present an analytical formalism for the description of Blazhko RRLyrae light curves. In this formalism the amplitude and frequency modulations are treated in a manner similar to the theory of electronic signal transmission. We consider monoperiodic RR Lyrae light curves to be carrier waves, and modulate their amplitude (AM), frequency (FM) and phase (PM); as a general case we discuss simultaneous AM and FM. The main advantages of this method are the following: (i) the mathematical formalism naturally explains numerous light-curve characteristics found in the Blazhko RRLyrae stars such as mean brightness variations, complicated envelope curves and non-sinusoidal frequency variations; (ii) our elucidation also explains the properties of the Fourier spectra such as apparent higher order multiplets, amplitude distribution of the sidepeaks, the appearance of the modulation frequency itself and its harmonics. In addition, compared to the traditional methods, our light-curve solutions reduce the number of necessary parameters. This formalism can be applied to any type of modulated light curves, not just to Blazhko RRLyrae star light curves. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

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