Hunting for binary Cepheids using lucky imaging technique

Detecting companions to Cepheids is difficult. In most cases the bright pulsator overshines the fainter secondary. Since Cepheids play a key role in determining the cosmic distance scale it is crucial to find binaries among the calibrating stars of the period-luminosity relation. Here we present an ongoing observing project of searching for faint and close companions of Galactic Cepheids using lucky imaging technique.Comment: 4 pages, 2 figures, published in AN. Proceedings for the 6th Workshop of Young Researchers in Astronomy and Astrophysic

The multiperiodic Blazhko modulation of CZ Lacertae

A thorough analysis of the multicolour CCD observations of the RRab-type variable, CZ Lacertae, is presented. The observations were carried out in two consecutive observing seasons in 2004 and 2005 within the framework of the Konkoly Blazhko Survey of bright, northern, short-period RRab variables. The O − C variation of CZ Lac indicated that a significant period decrease took place just around the time of the CCD observations. Our data gave a unique opportunity to study the related changes in the pulsation and modulation properties of a Blazhko star in detail. Two different period components (≈ 14.6 and ≈ 18.6 d) of the Blazhko modulation were identified. Both modulation components had similar strength. The periods and amplitudes of the modulations changed significantly from the first season to the next, while the mean pulsation amplitude decreased slightly. The modulation frequencies were in a 5:4 resonance ratio in the first observing season, and then the frequencies shifted in opposite directions, and their ratio was close to the 4:3 resonance in the next season. The interaction of the two modulations caused beating with a period of 74 d in the first season, which resembled the 4-yr-long cycle of the ≈ 40-d modulation of RR Lyr. The mean values of the global physical parameters and their changes with the Blazhko phase of both modulation components were determined by the inverse photometric metho

Overtone and multi-mode RR Lyrae stars in the globular cluster M3

The overtone and multi-mode RR Lyrae stars in the globular cluster M3 are studied using a 200-d long, $B,V$ and $I_{\mathrm C}$ time-series photometry obtained in 2012. 70\% of the 52 overtone variables observed show some kind of multi-periodicity (additional frequency at ${f_{0.61}}={f_{\mathrm {1O}}}/0.61$ frequency ratio, Blazhko effect, double/multi-mode pulsation, period doubling). A signal at 0.587 frequency ratio to the fundamental-mode frequency is detected in the double-mode star, V13, which may be identified as the second radial overtone mode. If this mode-identification is correct, than V13 is the first RR Lyrae star showing triple-mode pulsation of the first three radial modes. Either the Blazhko effect or the ${f_{0.61}}$ frequency (or both of these phenomena) appear in 7 double-mode stars. The $P_{\mathrm{1O}}/P_{\mathrm{F}}$ period ratio of RRd stars showing the Blazhko effect are anomalous. A displacement of the main frequency component at the fundamental-mode with the value of modulation frequency (or its half) is detected in three Blazhko RRd stars parallel with the appearance of the overtone-mode pulsation. The ${f_{0.61}}$ frequency appears in RRc stars that lie at the blue side of the double-mode region and in RRd stars, raising the suspicion that its occurrence may be connected to double-mode pulsation. The changes of the Blazhko and double-mode properties of the stars are also reviewed using the recent and archive photometric data.Comment: accepted for publication in ApJ Suppl. 26 pages, 25 figure

Exploring the circumstellar environment of the young eruptive star V2492 Cyg

Context. V2492 Cyg is a young eruptive star that went into outburst in 2010. The near-infrared color changes observed since the outburst peak suggest that the source belongs to a newly defined sub-class of young eruptive stars, where time-dependent accretion and variable line-of-sight extinction play a combined role in the flux changes. Aims. In order to learn about the origin of the light variations and to explore the circumstellar and interstellar environment of V2492 Cyg, we monitored the source at ten different wavelengths, between 0.55 \mu m and 2.2 \mu m from the ground and between 3.6 \mu m and 160 \mu m from space. Methods. We analyze the light curves and study the color-color diagrams via comparison with the standard reddening path. We examine the structure of the molecular cloud hosting V2492 Cyg by computing temperature and optical depth maps from the far-infrared data. Results. We find that the shapes of the light curves at different wavelengths are strictly self-similar and that the observed variability is related to a single physical process, most likely variable extinction. We suggest that the central source is episodically occulted by a dense dust cloud in the inner disk, and, based on the invariability of the far-infrared fluxes, we propose that it is a long-lived rather than a transient structure. In some respects, V2492 Cyg can be regarded as a young, embedded analog of UX Orionis-type stars. Conclusions. The example of V2492 Cyg demonstrates that the light variations of young eruptive stars are not exclusively related to changing accretion. The variability provided information on an azimuthally asymmetric structural element in the inner disk. Such an asymmetric density distribution in the terrestrial zone may also have consequences for the initial conditions of planet formation.Comment: 9 pages, 7 figures, 2 online tables, accepted for publication in A&

The Blazhko behaviour of RR Geminorum I - CCD photometric results in 2004

Extended CCD monitoring of RR Gem revealed that it is a Blazhko type RRab star with the shortest Blazhko period (7.23d) and smallest modulation amplitude (Delta Mmax<0.1 mag) currently known. The short period of the modulation cycle enabled us to obtain complete phase coverage of the pulsation at each phase of the modulation. This is the first multicolour observation of a Blazhko star which is extended enough to define accurate mean magnitudes and colours of the variable at different Blazhko phases. Small, but real, changes in the intensity mean colours at different Blazhko phases have been detected. The Fourier analysis of the light curves shows that, in spite of the mmag and smaller order of the amplitudes, the triplet structure is noticeable up to about the 14th harmonic. The modulation is concentrated to a very narrow, 0.2 phase range of the pulsation, centred on the supposed onset of the H emission during rising light. These observational results raise further complications for theoretical explanation of the long known but poorly understood Blazhko phenomenon.Comment: 10 pages, 12 figures, 3 tables. Accepted for publication in Astronomy and Astrophysic

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 Lyrae (MW Lyr) during the 2006-2007 observing seasons. The data within each 0.05 phase bin of the modulation period (Pm=f−1m) cover the entire light cycle of the primary pulsation period (P0=f−10), 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 fm, sidelobe modulation frequencies around the pulsation frequency and its harmonics appear at ±2fm, ±4fm and ±12.5fm separations as well. Residual signals in the pre-whitened 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 Lyr never matches its actual light variation. The Φ21, Φ31 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 utilizing the phase coherency of the lower order Fourier phase

An extensive photometric study of the Blazhko RR Lyrae star MW Lyr: II. Changes in the physical parameters

The analysis of the multicolour photometric observations of MW Lyr, a large modulation amplitude Blazhko variable, shows for the first time how the mean global physical parameters vary during the Blazhko cycle. About 1-2 percent changes in the mean radius, luminosity and surface effective temperature are detected. The mean radius and temperature changes are in good accordance with pulsation model results, which show that these parameters do indeed vary within this order of magnitude if the amplitude of the pulsation changes significantly. We interpret the phase modulation of the pulsation to be a consequence of period changes. Its magnitude corresponds exactly what one expects from the detected changes of the mean radius assuming that the pulsation constant remains the same during the modulation. Our results indicate that during the modulation the pulsation remains purely radial, and the underlying mechanism is most probably a periodic perturbation of the stellar luminosity with the modulation period.Comment: 10 figures, 2 tables. Accepted for publication in MNRA

Multi-wavelength study of the low-luminosity outbursting young star HBC 722

HBC 722 (V2493 Cyg) is a young eruptive star in outburst since 2010. It is an FU Orionis-type object with an atypically low outburst luminosity. Because it was well characterized in the pre-outburst phase, HBC 722 is one of the few FUors where we can learn about the physical changes and processes associated with the eruption. We monitored the source in the BVRIJHKs bands from the ground, and at 3.6 and 4.5 $\mu$m from space with the Spitzer Space Telescope. We analyzed the light curves and the spectral energy distribution by fitting a series of steady accretion disk models at many epochs. We also analyzed the spectral properties of the source based on new optical and infrared spectra. We also mapped HBC 722 and its surroundings at millimeter wavelengths. From the light curve analysis we concluded that the first peak of the outburst in 2010 September was due to an abrupt increase of the accretion rate in the innermost part of the system. This was followed by a long term process, when the brightening was mainly due to a gradual increase of the accretion rate and the emitting area. Our new observations show that the source is currently in a constant plateau phase. We found that around the peak the continuum was bluer and the H$\alpha$ profile changed significantly between 2012 and 2013. The source was not detected in the millimeter continuum, but we discovered a flattened molecular gas structure with a diameter of 1700 au and mass of 0.3 M$_{\odot}$ centered on HBC 722. While the first brightness peak could be interpreted as a rapid fall of piled-up material from the inner disk onto the star, the later monotonic flux rise suggests the outward expansion of a hot component according to the theory of Bell & Lin (1994). Our study of HBC 722 demonstrated that accretion-related outbursts can occur in young stellar objects even with very low mass disks, in the late Class II phase.Comment: 11 pages, 7 figures, 3 online tables. Accepted for publication in the A&

Pulsational and evolutionary analysis of the double-mode RR Lyrae star BS Com

We derive the basic physical parameters of the field double-mode RR Lyrae star BS Com from its observed periods and the requirement of consistency between the pulsational and evolutionary constraints. By using the current solar-scaled horizontal branch evolutionary models of Pietrinferni et al. and our linear non-adiabatic purely radiative pulsational models, we get M/M⊙= 0.698 ± 0.004, log(L/L⊙) = 1.712 ± 0.005, Teff= 6840 ± 14 K, [Fe/H]=−1.67 ± 0.01, where the errors are standard deviations assuming uniform age distribution along the full range of uncertainty in age. The last two parameters are in a good agreement with the ones derived from the observed BVIC colours and the updated atlas9 stellar atmosphere models. We get Teff= 6842 ± 10 K, [Fe/H]=−1.58 ± 0.11, where the errors are purely statistical ones. It is remarkable that the derived parameters are nearly independent of stellar age at early evolutionary stages. Later stages, corresponding to the evolution towards the asymptotic giant branch, are most probably excluded because the required high temperatures are less likely to satisfy the constraints posed by the colours. We also show that our conclusions are only weakly sensitive to non-linear period shifts predicted by current hydrodynamical model