The Blazhko effect and additional excited modes in RR Lyrae stars

Recent photometric space missions, such as CoRoT and Kepler revealed that many RR Lyrae stars pulsate -- beyond their main radial pulsation mode -- in low amplitude modes. Space data seem to indicate a clear trend, namely overtone (RRc) stars and modulated fundamental (RRab) RR Lyrae stars ubiquitously show additional modes, while non-Blazhko RRab stars never do. Two Kepler stars (V350 Lyr and KIC 7021124), however, apparently seemed to break this rule: they were classified as non-Blazhko RRab stars showing additional modes. We processed Kepler pixel photometric data of these stars. We detected small amplitude, but significant Blazhko effect for both stars by using the resulted light curves and O$-$C diagrams. This finding strengthens the apparent connection between the Blazhko effect and the excitation of additional modes. In addition, it yields a potential tool for detecting Blazhko stars through the additional frequency patterns even if we have only short but accurate time series observations. V350 Lyr shows the smallest amplitude multiperiodic Blazhko effect ever detected.Comment: 6 pages, 4 figures and 2 tables. Accepted for publication in ApJ Letter

Fundamental parameters of RR Lyrae stars from multicolour photometry and Kurucz atmospheric models -- III. SW And, DH Peg, CU Com, DY Peg

We report the most comprehensive UBV(RI)_C observations of the bright, radially pulsating field stars SW And, DH Peg, CU Com, DY Peg. Long term variation has been found in the ultraviolet colour curves of SW And and DH Peg. We apply our photometric-hydrodynamic method to determine the fundamental parameters of these stars: metallicity, reddening, distance, mass, radius, equilibrium luminosity and effective temperature. Our method works well for SW And, CU Com and DY Peg. A very small mass 0.26+/-0.04 M_Sun of SW And has been found. The fundamental parameters of CU Com are those of a normal double-mode RR Lyrae (RRd) star. DY Peg has been found to have paradoxical astrophysical parameters: the metallicity, mass and period are characteristic for a high-amplitude Delta Sct star while the luminosity and radius place it in the group of RR Lyrae stars. DH Peg has been found to be peculiar: the definite instability in the colour curves towards ultraviolet, the dynamical variability of the atmosphere during the shocked phases suggests that the main assumptions of our photometric-hydrodynamic method, the quasi-static atmosphere approximation (QSAA) and the exclusive excitation of radial modes are probably not satisfied in this star. The fundamental parameters of all stars studied in this series of papers are summarized in tabular and graphical form.Comment: 15 pages, 10 figures; accepted for publication in MNRA

Unexpected series of regular frequency spacing of delta Scuti stars in the non-asymptotic regime -- I. The methodology

A sequence search method was developed to search regular frequency spacing in delta Scuti stars by visual inspection and algorithmic search. We searched for sequences of quasi-equally spaced frequencies, containing at least four members per sequence, in 90 delta Scuti stars observed by CoRoT. We found an unexpectedly large number of independent series of regular frequency spacing in 77 delta Scuti stars (from 1 to 8 sequences) in the non-asymptotic regime. We introduce the sequence search method presenting the sequences and echelle diagram of CoRoT 102675756 and the structure of the algorithmic search. Four sequences (echelle ridges) were found in the 5-21 d^{-1} region, where the pairs of the sequences are shifted (between 0.5-0.59 d^{-1}) by twice the value of the estimated rotational splitting frequency (0.269 d^{-1}). The general conclusions for the whole sample are also presented in this paper. The statistics of the spacings derived by the sequence search method, by FT and that of the shifts are also compared. In many stars, more than one almost equally valid spacing appeared. The model frequencies of FG Vir and their rotationally split components were used to reveal a possible explanation that one spacing is the large separation, while the other is a sum of the large separation and the rotational frequency. In CoRoT 102675756, the two spacings (2.249 and 1.977 d^{-1}) agree better with the sum of a possible 1.710 d^{-1} large separation and two or one times, respectively, the value of the rotational frequency.Comment: 12 pages, 10 figures, accepted for publication in Ap

Unexpected series of regular frequency spacing of delta Scuti stars in the non-asymptotic regime -- II. Sample -- echelle diagrams and rotation

A sequence search method was developed for searching for regular frequency spacing in delta Scuti stars by visual inspection and algorithmic search. The sample contains 90 delta Scuti stars observed by CoRoT. An example is given to represent the visual inspection. The algorithm (SSA) is described in detail. The data treatment of the CoRoT light curves, the criteria for frequency filtering and the spacings derived by two methods (three approaches: VI, SSA and FT) are given for each target. Echelle diagrams are presented for 77 targets, for which at least one sequence of regular spacing was identified. Comparing the spacing and the shifts between pairs of echelle ridges revealed that at least one pair of echelle ridges is shifted to midway between the spacing for 22 stars. The estimated rotational frequencies compared to the shifts revealed rotationally split doublets, triplets and multiplets not only for single frequencies, but for the complete echelle ridges in 31 delta Scuti stars. Using several possible assumptions for the origin of the spacings, we derived the large separation ($\Delta\nu$), which are distributed along the mean density versus large separations relation derived from stellar models Suarez 2014.Comment: 25 pages, 16 figures, 9 tables. Accepted for publication in ApJ

Kepler RR Lyrae stars: beyond period doubling

We examined the complete short cadence sample of Kepler RR Lyrae stars to further investigate the recently discovered dynamical effects such as period doubling and additional modes. Here we present the findings on four stars. V450 Lyr may be a non-classical double-mode RR Lyrae star pulsating in the fundamental mode and the second overtone. In three cases we observe the interaction of three different modes. Since the period ratios are close to resonant values, we observe quasi-repetiting patterns in the pulsation cycles in the stars. These findings support the mode-resonance explanations of the Blazhko effect.Comment: 2 pages, 1 figure. Proceedings of the 301st IAU Symposium, Precision Asteroseismology, August 2013, Wroc{\l}aw. V2: small corrections to the wordin

On the interchange of alternating-amplitude pulsation cycles

We characterized the time intervals between the interchanges of the alternating high- and low-amplitude extrema of three RV Tauri and three RR Lyrae stars.Comment: 2 pages, Proceedings of the 301st IAU Symposium, Precision Asteroseismology, August 201