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Hartman J.D.,Harvard - Smithsonian Center for Astrophysics | Bakos G.A.,Harvard - Smithsonian Center for Astrophysics | Kovacs G.,Konkoly Observatory | Noyes R.W.,Harvard - Smithsonian Center for Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2010

Using data from the Hungarian-made Automated Telescope Network (HATNet) survey for transiting exoplanets, we measure photometric rotation periods for 368 Pleiades stars with 0.4 ≤M≤ 1.3. M.. We detect periodic variability for 74 per cent of the cluster members in this mass range that are within our field-of-view, and 93 per cent of the members with 0.7 ≤M≤ 1.0 M.. This increases, by a factor of 5, the number of Pleiades members with measured periods. We compare these data to the rich sample of spectroscopically determined projected equatorial rotation velocities (v sin i) available in the literature for this cluster. Included in our sample are 14 newly identified probable cluster members which have proper motions, photometry and rotation periods consistent with membership. For stars with M≤ 0.85 M. the rotation periods, v sin i and radius estimates are consistent with the stars having an isotropic distribution of rotation axes, if a moderate differential rotation law is assumed. For stars with M≤ 0.85 M., the inferred sin i values are systematically larger than 1.0. These observations imply that the combination of measured parameters P(v sin i)/R is too large by ~ 24 per cent for low-mass stars in this cluster. By comparing our new mass-period relation for the Pleiades to the slightly older cluster M35, we confirm previous indications that the spin-down stalls at ~ 100 Myr for the slowest rotating stars with 0.7 ≤M≤ 1.1 M.- a fact which may indicate that the internal transport of angular momentum is inefficient in slowly rotating solar-mass stars. © 2010 The Authors. Journal compilation © 2010 RAS. Source

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. Source

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. Source

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. Source

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. Source

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