The study of RR Lyrae stars has recently been invigorated thanks to the long, uninterrupted, ultra-precise time series data provided by the Kepler and CoRoT space telescopes. We give a brief overview of the new observational findings concentrating on
the connection between period doubling and the Blazhko modulation, and the omnipresence of additional periodicities in all RR Lyrae subtypes, except for non-modulated RRab stars. Recent theoretical results demonstrate that if more than two modes are present in a nonlinear dynamical system such as a high-amplitude RR Lyrae star, the outcome is often an extremely intricate dynamical state. Thus, based on these discoveries, an underlying picture of complex dynamical interactions between modes is emerging which sheds new light on the century-old Blazhko-phenomenon, as well. New directions of theoretical efforts, like multi-dimensional hydrodynamical simulations, future space photometric missions and detailed spectroscopic investigations will pave the way towards a more complete understanding of the atmospheric and pulsation dynamics of these enigmatic touchstone objects.
We search for signs of period doubling in CoRoT RR Lyrae stars. The occurrence of this dynamical effect in modulated RR Lyrae stars might help us to gain more information about the mysterious Blazhko effect. The temporal variability of the additional
frequencies in representatives of all subtypes of RR Lyrae stars is also investigated. We pre-process CoRoT light curves by applying trend and jump correction and outlier removal. Standard Fourier technique is used to analyze the frequency content of our targets and follow the time dependent phenomena. The most comprehensive collection of CoRoT RR Lyrae stars, including new discoveries is presented and analyzed. We found alternating maxima and in some cases half-integer frequencies in four CoRoT Blazhko RR Lyrae stars, as clear signs of the presence of period doubling. This reinforces that period doubling is an important ingredient to understand the Blazhko effect - a premise we derived previously from the Kepler RR Lyrae sample. As expected, period doubling is detectable only for short time intervals in most modulated RRab stars. Our results show that the temporal variability of the additional frequencies in all RR Lyrae sub-types is ubiquitous. The ephemeral nature and the highly variable amplitude of these variations suggest a complex underlying dynamics of and an intricate interplay between radial and possibly nonradial modes in RR Lyrae stars. The omnipresence of additional modes in all types of RR Lyrae - except in non-modulated RRab stars - implies that asteroseismology of these objects should be feasible in the near future (Abridged).
Pulsating stars in binary systems are ideal laboratories to test stellar evolution and pulsation theory, since a direct, model-independent determination of component masses is possible. The high-precision CoRoT photometry allows a detailed view of th
e frequency content of pulsating stars, enabling detection of patterns in their distribution. The object HD 51844 is such a case showing periastron brightening instead of eclipses. We present a comprehensive study of the HD 51844 system, where we derive physical parameters of both components, the pulsation content and frequency patterns. Additionally, we obtain the orbital elements, including masses, and the chemical composition of the stars. Time series analysis using standard tools was mployed to extract the pulsation frequencies. Photospheric abundances of 21 chemical elements were derived by means of spectrum synthesis. We derived orbital elements both by fitting the observed radial velocities and the light curves, and we did asteroseismic modelling as well. We found that HD 51844 is a double lined spectroscopic binary. The determined abundances are consistent with delta Delphini classification. We determined the orbital period (33.498 +- 0.002 d), the eccentricity (0.484 +- 0.020), the mass ratio (0.988 +- 0.02), and the masses to 2.0 +- 0.2 M_sun for both components. Only one component showed pulsation. Two p modes (f_22 and f_36) and one g mode (f_orb) may be tidally excited. Among the 115 frequencies, we detected triplets due to the frequency modulation, frequency differences connected to the orbital period, and unexpected resonances (3:2, 3:5, and 3:4), which is a new discovery for a delta Scuti star.
The star HD 51844 was observed in CoRoT LRa02 as a Seismo target which turned out to be an SB2 system. The 117 days long light curve revealed delta Scuti pulsation in the range of 6 to 15 d^{-1} where four frequencies have amplitudes larger than 1.4
mmag and a rich frequency spectrum with amplitudes lower than 0.6 mmag. Additionally, the light curve exhibits a brightening event recurring every 33.5 days with a maximum of 3 mmag and a duration of about 5 days. Thus, this star can be considered as a heartbeat candidate. The radial velocities from spectroscopy confirmed an eccentric binary system with nearly identical masses and physical parameters. The brightening event of the light curve coincides with the maximum radial velocity separation showing that the brightening is in fact caused by tidal distortion and/or reflected light. One component displays large line profile variations, while the other does not show significant variation. The frequency analysis revealed a quintuplet structure of the four highest-amplitude frequencies, which is due to the orbital motion of the pulsating star.
The star CoRoT102781750 reveals a puzzle, showing a very complex and altering variation in different `CoRoT colours. We established without doubt that more than a single star was situated within the CoRoT mask. Using a search for periodicity as a too
l, our aim is to disentangle the composite light curve and identify the type of sources behind the variability. Both flux and magnitude light curves were used. Conversion was applied after a jump- and trend-filtering algorithm. We applied different types of period-finding techniques including MuFrAn and Period04. The amplitude and phase peculiarities obtained from the independent analysis of CoRoT r, g, and b colours and ground-based follow-up photometric observations ruled out the possibility of either a background monoperiodic or a Blazhko type RR Lyrae star being in the mask. The main target, an active star, shows at least two spotted areas that reveal a $P_rot = 8.8$ hours $(f_0 = 2.735$ c d$^{-1})$ mean rotation period. The evolution of the active regions helped to derive a period change of $dP/dt = 1.6cdot 10^{-6}$ (18 s over the run) and a differential rotation of $alpha = DeltaOmega/Omega = 0.0074$. The $0fm 015$ linear decrease and a local $0fm 005$ increase in the dominant periods amplitude are interpreted as a decay of the old spotted region and an appearance of a new one, respectively. A star that is detected only in the CoRoT b domain shows a $f_1 = 7.172$ c d$^{-1}$ pulsation connected to a $14fd 83$ periodicity via an equidistant triplet structure. The best explanation for our observation is a $beta$ Cep star with a corotating dust disk.
The high accuracy of space data increased the number of the periodicities determined for pulsating variable stars, but the mode identification is still a critical point in the non-asymptotic regime. We use regularities in frequency spacings for ident
ifying the pulsation modes of the recently discovered delta Sct star ID 102749568. In addition to analysing CoRoT light curves (15252 datapoints spanning 131 days), we obtained and analysed both spectroscopic and extended multi-colour photometric data. We applied standard tools (MUFRAN, Period04, SigSpec, and FAMIAS) for time-series analysis. A satisfactory light-curve fit was obtaining by means of 52 independent modes and 15 combination terms. The frequency spacing revealed distinct peaks around large (25.55-31.43 microHz), intermediate (9.80, 7.66 microHz), and low (2.35 microHz) separations. We directly identified 9 modes, and the l and n values of other three modes were extrapolated. The combined application of spectroscopy, multi-colour photometry, and modelling yielded the precise physical parameters and confirmed the observational mode identification. The large separation constrained the log g and related quantities. The dominant mode is the radial first overtone.
We present the results of a comparative period search on different time-scales and modelling of the ZZ Ceti (DAV) star GD 154. We determined six frequencies as normal modes and four rotational doublets around the ones having the largest amplitude. Tw
o normal modes at 807.62 and 861.56 microHz have never been reported before. A rigorous test revealed remarkable intrinsic amplitude variability of frequencies at 839.14 and 861.56 microHz over a 50 d time-scale. In addition, the multimode pulsation changed to monoperiodic pulsation with an 843.15 microHz dominant frequency at the end of the observing run. The 2.76 microHz average rotational split detected led to a determination of a 2.1 d rotational period for GD 154. We searched for model solutions with effective temperatures and log g close to the spectroscopically determined ones. The best-fitting models resulting from the grid search have M_H between 6.3 x 10^-5 and 6.3 x 10^-7 M*, which means thicker hydrogen layer than the previous studies suggested. Our investigations show that mode trapping does not necessarily operate in all of the observed modes and the best candidate for a trapped mode is at 2484 microHz.
We present the results on period search and modeling of the cool DAV star KUV 02464+3239. Our observations resolved the multiperiodic pulsational behaviour of the star. In agreement with its position near the red edge of the DAV instability strip, it
shows large amplitude, long period pulsation modes, and has a strongly non-sinusoidal light curve. We determined 6 frequencies as normal modes and revealed remarkable short-term amplitude variations. A rigorous test was performed for the possible source of amplitude variation: beating of modes, effect of noise, unresolved frequencies or rotational triplets. Among the best-fit models resulting from a grid search, we selected 3 that gave l=1 solutions for the largest amplitude modes. These models had masses of 0.645, 0.650 and 0.680 M_Sun. The 3 `favoured models have M_H between 2.5x10^-5 - 6.3x10^-6 M_* and give 14.2 - 14.8 mas seismological parallax. The 0.645 M_Sun (11400 K) model also matches the spectroscopic log g and T_eff within 1 sigma. We investigated the possibility of mode trapping and concluded that while it can explain high amplitude modes, it is not required.
The ZZ Ceti star KUV 02464+3239 was observed over a whole season at the mountain station of Konkoly Observatory. A rigorous frequency analysis revealed 6 certain periods between 619 and 1250 seconds, with no shorter period modes present. We use the o
bserved periods, published effective temperature and surface gravity, along with the model grid code of Bischoff-Kim, Montgomery and Winget (2008) to perform a seismological analysis. We find acceptable model fits with masses between 0.60 and 0.70 M_Sun. The hydrogen layer mass of the acceptable models are almost always between 10^-4 and 10^-6 M_*. In addition to our seismological results, we also show our analysis of individual light curve segments. Considering the non-sinusoidal shape of the light curve and the Fourier spectra of segments showing large amplitude variations, the importance of non-linear effects in the pulsation is clearly seen.
New observations of GD 99 are analysed. The unusual pulsation behaviour, showing both long and short periods, has been confirmed. All the available periods show a grouping of short and long period modes with roughly regular spacing. If we interpret t
he groups separately, a binary nature can be a possible explanation as in the similar cases of WD 2350-0054 and G29-38.
