The contact binary VW Cephei revisited: surface activity and period variation

Context. Despite the fact that VW Cephei is one of the well-studied contact binaries in the literature, there is no fully consistent model available that can explain every observed property of this system. Aims. Our motivation is to obtain new spectra along with photometric measurements, to analyze what kind of changes may have happened in the system in the past two decades, and to propose new ideas for explaining them. Methods. For the period analysis we determined 10 new times of minima from our light curves, and constructed a new O$-$C diagram of the system. Radial velocities of the components were determined using the cross-correlation technique. The light curves and radial velocities were modelled simultaneously with the PHOEBE code. All observed spectra were compared to synthetic spectra and equivalent widths of the H$\alpha$ line were measured on their differences. Results. We have re-determined the physical parameters of the system according to our new light curve and spectral models. We confirm that the primary component is more active than the secondary, and there is a correlation between spottedness and the chromospheric activity. We propose that flip-flop phenomenon occurring on the primary component could be a possible explanation of the observed nature of the activity. To explain the period variation of VW Cep, we test two previously suggested scenarios: presence of a fourth body in the system, and the Applegate-mechanism caused by periodic magnetic activity. We conclude that although none of these mechanisms can be ruled out entirely, the available data suggest that mass transfer with a slowly decreasing rate gives the most likely explanation for the period variation of VW Cep.Comment: 13 pages, 18 figures, 9 tables, accepted for publication in Astronomy and Astrophysic

Determination of the size, mass, and density of "exomoons" from photometric transit timing variations

Precise photometric measurements of the upcoming space missions allow the size, mass, and density of satellites of exoplanets to be determined. Here we present such an analysis using the photometric transit timing variation ($TTV_p$). We examined the light curve effects of both the transiting planet and its satellite. We define the photometric central time of the transit that is equivalent to the transit of a fixed photocenter. This point orbits the barycenter, and leads to the photometric transit timing variations. The exact value of $TTV_p$ depends on the ratio of the density, the mass, and the size of the satellite and the planet. Since two of those parameters are independent, a reliable estimation of the density ratio leads to an estimation of the size and the mass of the exomoon. Upper estimations of the parameters are possible in the case when an upper limit of $TTV_p$ is known. In case the density ratio cannot be estimated reliably, we propose an approximation with assuming equal densities. The presented photocenter $TTV_p$ analysis predicts the size of the satellite better than the mass. We simulated transits of the Earth-Moon system in front of the Sun. The estimated size and mass of the Moon are 0.020 Earth-mass and 0.274 Earth-size if equal densities are assumed. This result is comparable to the real values within a factor of 2. If we include the real density ratio (about 0.6), the results are 0.010 Earth-Mass and 0.253 Earth-size, which agree with the real values within 20%.Comment: 6 pages, 5 figures, to appear in Astronomy and Astrophysic

The He-shell flash in action: T Ursae Minoris revisited

We present an updated and improved description of the light curve behaviour of T Ursae Minoris, which is a Mira star with the strongest period change (the present rate is an amazing -3.8+/-0.4 days/year corresponding to a relative decrease of about 1% per cycle). Ninety years of visual data were collected from all available databases and the resulting, almost uninterrupted light curve was analysed with the O-C diagram, Fourier analysis and various time-frequency methods. The Choi-Williams and Zhao-Atlas-Marks distributions gave the clearest image of frequency and light curve shape variations. A decrease of the intensity average of the light curve was also found, which is in accordance with a period-luminosity relation for Mira stars. We predict the star will finish its period decrease in the meaningfully near future (c.c. 5 to 30 years) and strongly suggest to closely follow the star's variations (photometric, as well as spectroscopic) during this period.Comment: 6 pages, 7 figures, accepted for publication in A&

Period-doubling events in the light curve of R Cygni: evidence for chaotic behaviour

A detailed analysis of the century long visual light curve of the long-period Mira star R Cygni is presented and discussed. The data were collected from the publicly available databases of the AFOEV, the BAAVSS and the VSOLJ. The full light curve consists of 26655 individual points obtained between 1901 and 2001. The light curve and its periodicity were analysed with help of the O-C diagram, Fourier analysis and time-frequency analysis. The results demonstrate the limitations of these linear methods. The next step was to investigate the possible presence of low-dimensional chaos in the light curve. For this, a smoothed and noise-filtered signal was created from the averaged data and with help of time delay embedding, we have tried to reconstruct the attractor of the system. The main result is that R Cygni shows such period-doubling events that can be interpreted as caused by a repetitive bifurcation of the chaotic attractor between a period 2T orbit and chaos. The switch between these two states occurs in a certain compact region of the phase space, where the light curve is characterized by ~1500-days long transients. The Lyapunov spectrum was computed for various embedding parameters confirming the chaotic attractor, although the exponents suffer from quite high uncertainty because of the applied approximation. Finally, the light curve is compared with a simple one zone model generated by a third-order differential equation which exhibits well-expressed period-doubling bifurcation. The strong resemblance is another argument for chaotic behaviour. Further studies should address the problem of global flow reconstruction, including the determination of the accurate Lyapunov exponents and dimension.Comment: 13 pages, 14 figures, accepted for publication in A&A (some figures are of reduced quality

Possibility of a photometric detection of "exomoons"

We examined which exo-systems contain moons that may be detected in transit. We numerically modeled transit light curves of Earth-like and giant planets that cointain moons with 0.005--0.4 Earth-mass. The orbital parameters were randomly selected, but the entire system fulfilled Hill-stability. We conclude that the timing effect is caused by two scenarios: the motion of the planet and the moon around the barycenter. Which one dominates depends on the parameters of the system. Already planned missions (Kepler, COROT) may be able to detect the moon in transiting extrasolar Earth-Moon-like systems with a 20% probability. From our sample of 500 free-designed systems, 8 could be detected with the photometric accuracy of 0.1 mmag and a 1 minute sampling, and one contains a stony planet. With ten times better accuracy, 51 detections are expected. All such systems orbit far from the central star, with the orbital periods at least 200 and 10 days for the planet and the moon, while they contain K- and M-dwarf stars. Finally we estimate that a few number of real detections can be expected by the end of the COROT and the Kepler missions.Comment: 5 pages, 4 figures, accepted by Astronomy and Astrophysic

Exomoon simulations

We introduce and describe our newly developed code that simulates light curves and radial velocity curves for arbitrary transiting exoplanets with a satellite. The most important feature of the program is the calculation of radial velocity curves and the Rossiter-McLaughlin effect in such systems. We discuss the possibilities for detecting the exomoons taking the abilities of Extremely Large Telescopes into account. We show that satellites may be detected also by their RM effect in the future, probably using less accurate measurements than promised by the current instrumental developments. Thus, RM effect will be an important observational tool in the exploration of exomoons.Comment: 5 pages, 2 figures with 9 figure panels, accepted by EM&

Variability of M giant stars based on Kepler photometry: general characteristics

M giants are among the longest-period pulsating stars which is why their studies were traditionally restricted to analyses of low-precision visual observations, and more recently, accurate ground-based data. Here we present an overview of M giant variability on a wide range of time-scales (hours to years), based on analysis of thirteen quarters of Kepler long-cadence observations (one point per every 29.4 minutes), with a total time-span of over 1000 days. About two-thirds of the sample stars have been selected from the ASAS-North survey of the Kepler field, with the rest supplemented from a randomly chosen M giant control sample. We first describe the correction of the light curves from different quarters, which was found to be essential. We use Fourier analysis to calculate multiple frequencies for all stars in the sample. Over 50 stars show a relatively strong signal with a period equal to the Kepler-year and a characteristic phase dependence across the whole field-of-view. We interpret this as a so far unidentified systematic effect in the Kepler data. We discuss the presence of regular patterns in the distribution of multiple periodicities and amplitudes. In the period-amplitude plane we find that it is possible to distinguish between solar-like oscillations and larger amplitude pulsations which are characteristic for Mira/SR stars. This may indicate the region of the transition between two types of oscillations as we move upward along the giant branch.Comment: 12 pages, 13 figures, accepted for publication in MNRAS. The normalized light curves are available upon reques

Methods for exomoon characterisation: combining transit photometry and the Rossiter-McLaughlin effect

It has been suggested that moons around transiting exoplanets may cause observable signal in transit photometry or in the Rossiter-McLaughlin (RM) effect. In this paper a detailed analysis of parameter reconstruction from the RM effect is presented for various planet-moon configurations, described with 20 parameters. We also demonstrate the benefits of combining photometry with the RM effect. We simulated 2.7x10^9 configurations of a generic transiting system to map the confidence region of the parameters of the moon, find the correlated parameters and determine the validity of reconstructions. The main conclusion is that the strictest constraints from the RM effect are expected for the radius of the moon. In some cases there is also meaningful information on its orbital period. When the transit time of the moon is exactly known, for example, from transit photometry, the angle parameters of the moon's orbit will also be constrained from the RM effect. From transit light curves the mass can be determined, and combining this result with the radius from the RM effect, the experimental determination of the density of the moon is also possible.Comment: 10 pages, 7 figures, accepted for publication in MNRA