Blazhko RR Lyrae Research Articles

Period-doubling bifurcation and high-order resonances in RR Lyrae hydrodynamical models

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 was observed only in Blazhko-modulated stars so far, the phenomenon can help in the understanding of the modulation as well. Utilising the Florida-Budapest turbulent convective hydrodynamical code, we identified the phenomenon in radiative and convective models as well. A period-doubling cascade was also followed up to an eight-period solution confirming that the destabilisation 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 display 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 9th overtone and the fundamental mode. Destabilisation 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 sufficiently strong enough to shift the period of overtone with up to 10 percent. 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.

Further Evidence of Convective Cycles in Blazhko RR Lyrae Stars

ABSTRACTThe theory of slow convective cycles in the envelopes of RR Lyrae stars showing the Blazhko effect predicts an anticorrelation between the pulsation period and radius for hot members of the group, contrary to what might be expected from the (period, mean density) relation. This negative correlation has actually been observed by Jurcsik et al. for the hot stars MW Lyr, DM Cyg, and (in part) CZ Lac, the only stars so far measured accurately for radius variations over a Blazhko cycle. Significantly cooler members of the group would be expected to show a normal, positive correlation between pulsation period and radius.

Does Kepler unveil the mystery of the Blazhko effect? First detection of period doubling in Kepler Blazhko RR Lyrae stars

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 nonlinear 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 (9th) 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 to solve this century-old enigma.

Study of globular cluster M53: new variables, distance, metallicity

We study the variable star content of the globular cluster M53 to compute the physical parameters of the constituting stars and the distance of the cluster. Covering two adjacent seasons in 2007 and 2008, new photometric data are gathered for 3048 objects in the field of M53. By using the OIS method and subsequently TFA, we search for variables in the full sample by using DFT and BLS methods. We select variables based on the statistics related to these methods combined with visual inspections. We identified 12 new variables (2 RR Lyrae stars, 7 short periodic stars - 3 of them are SX Phe stars - and 3 long-period variables). No eclipsing binaries were found in the present sample. Except for the 3 (hitherto unknown) Blazhko RR Lyrae stars, no multiperiodic variables were found. We showed that after proper period shift, the PLC relation for the first overtone RR Lyrae sample tightly follows the one spanned by the fundamental stars. Furthermore, the slope is in agreement with the one derived from other clusters. Based on the earlier Baade-Wesselink calibration of the PLC relations, the derived reddening-free distance modulus of M53 is 16.31 +/- 0.04 mag, corresponding to a distance modulus of 18.5 mag for the Large Magellanic Cloud. From the Fourier parameters of the RRab stars we obtained an average iron abundance of -1.58 +/- 0.03. This is ~0.5 dex higher than the overall abundance of the giants as given in the literature and derived in this paper from the three-color photometry of giants. We suspect that the source of this discrepancy (observable also in other, low-metallicity clusters) is the want of sufficient number of low-metallicity objects in the calibrating sample of the Fourier method.

An extensive photometric study of the Blazhko RR Lyrae star DM Cyg

DM Cyg, a fundamental mode RRab star was observed in the 2007 and 2008 seasons in the frame of the Konkoly Blazhko Survey. Very small amplitude light curve modulation was detected with 10.57 d modulation period. The maximum brightness and phase variations do not exceed 0.07 mag and 7 min, respectively. In spite of the very small amplitude of the modulation, beside the frequency triplets characterizing the Fourier spectrum of the light curve two quintuplet components were also identified. The accuracy and the good phase coverage of our observations made it possible to analyse the light curves at different phases of the modulation separately. Utilizing the IP method (S\'odor, Jurcsik and Szeidl, 2009) we could detect very small systematic changes in the global mean physical parameters of DM Cyg during its Blazhko cycle. The detected changes are similar to what we have already found for a large modulation amplitude Blazhko variable MW Lyrae. The amplitudes of the detected changes in the physical parameters of DM Cyg are only about 10% of that what have been found in MW Lyr. This is in accordance with its small modulation amplitude being about one tenth of the modulation amplitude of MW Lyr.

CZ Lacertae – a Blazhko RR Lyrae star with multiperiodic modulation

Die Serie Communications in Asteroseismology (CoAst) stellt eine international gefragte Plattform für wissenschaftliche Publikationen der neuesten Forschungsergebnisse auf dem Gebiet der Asteroseismologie dar. Forschungsgegenstand der Asteroseismologie sind veränderliche und pulsierende Sterne. Neben den halbjährlich erscheinenden regulären Ausgaben mit Beiträgen aus der laufenden Forschung werden auch Sonderausgaben für Konferenzberichte bzw. Programmhandbücher herausgegeben. JENAM (Joint European and National Astronomy Meeting) ist die größte jährliche Zusammenkunft der europäischen Berufsastronomen, die diesmal im September 2008 in Wien stattfand. Die Konferenz beherbergte neun Minisymposien, wobei eines der Asteroseismologie gewidmet war. Die neuesten Entwicklungen auf diesem Gebiet sind im vorliegenden Band im Rahmen von Übersichts- und Detailartikeln zusammengefasst.

An extensive photometric study of the Blazhko RR Lyrae star MW Lyr - I. Light-curve solution

We have obtained the most extensive and most accurate photometric data of a Blazhko variable MW Lyr during the 2006-2007 observing seasons. The data within each 0.05 phase bin of the modulation period (P_m=1/f_m) cover the entire light cycle of the primary pulsation period (P_0=1/f_0), making possible a very rigorous and complete analysis. The modulation period is found to be 16.5462 d, which is about half of that was reported earlier from visual observations. Previously unknown features of the modulation have been detected. Besides the main modulation frequency f_m, sidelobe modulation frequencies around the pulsation frequency and its harmonics appear at +/- 2 f_m, +/- 4 f_m, and +/- 12.5 f_m separations as well. Residual signals in the prewhitened light curve larger than the observational noise appear at the minimum-rising branch-maximum phase of the pulsation, which most probably arise from some stochastic/chaotic behaviour of the pulsation/modulation. The Fourier parameters of the mean light curve differ significantly from the averages of the Fourier parameters of the observed light curves in the different phases of the Blazhko cycle. Consequently, the mean light curve of MW Lyrae never matches its actual light variation. The Phi_21, Phi_31 phase differences in different phases of the modulation show unexpected stability during the Blazhko cycle. A new phenomenological description of the light curve variation is defined that separates the amplitude and phase (period) modulations utilising the phase coherency of the lower order Fourier phases.

Studying Blazhko RR Lyrae stars with the 24‐inch telescope of the Konkoly Observatory

AbstractAbout a dozen field RR Lyrae stars have been observed with the 24‐inch Heyde‐Zeiss telescope of the Konkoly Observatory at Svábhegy, Budapest, since its refurbishment in 2003. Most of the observing time is allocated for the investigation of the Blazhko modulation, a phenomenon that still does not have a satisfactory explanation. The obtained multicolour CCD observations are unique in extent. The accuracy of the measurements makes it possible to detect low amplitude modulation of the light curve as well. The discovery of Blazhko stars with low modulation amplitudes warns that the incidence rate of the Blazhko modulation is, in fact, much larger than it was previously expected. This makes the efforts exploring the cause of the modulation even more important. A summary of our measurements and results achieved during the last 3 years is presented. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)