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R. Smolec

Publications and source records attributed to R. Smolec.

At least 73 records · Page 4Linked to original sources
arXiv2016

Non-radial Pulsations in RR Lyrae Stars from the OGLE Collection

RR Lyrae stars are classical pulsating stars. They pulsate mostly in the radial fundamental mode (RRab stars), in the radial first overtone mode (RRc stars), or in both modes simultaneously (RRd stars). Collection of variable stars from the Optical Gravitational Lensing Experiment (OGLE) contains more than 38 000 RR Lyrae stars from the Galactic bulge. We analysed these data for RRc and RRd stars. We have found new members of radial-non-radial double-mode RR Lyrae stars, with characteristic period ratio of the two modes around 0.61. We increased the number of known RR Lyrae stars of this type by a factor of 8. We have also discovered another group of double-mode RR Lyrae stars. They pulsate in the first overtone and in another, unidentified mode, which has period longer than period of the undetected fundamental mode. The period ratios tightly cluster around 0.686. These proceedings are focused on this puzzling group. In particular, we report eight new members of the group.

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arXiv2016

The Blazhko Effect

We review the recent progress in our understanding of the Blazhko effect in RR Lyrae stars.

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arXiv2016

Non-radial pulsation in first overtone Cepheids of the Small Magellanic Cloud

We analyse photometry for 138 first overtone Cepheids from the Small Magellanic Cloud, in which Optical Gravitational Lensing Experiment (OGLE) team discovered additional variability with period shorter than first overtone period, and period ratios in the (0.60, 0.65) range. In the Petersen diagram these stars form three well separated sequences. The additional variability cannot correspond to other radial mode. This form of pulsation is still puzzling. We find that amplitude of the additional variability is small, typically 2-4 per cent of the first overtone amplitude, which corresponds to 2-5 mmag. In some stars we find simultaneously two close periodicities corresponding to two sequences in the Petersen diagram. The most important finding is the detection of power excess at half the frequency of the additional variability (at subharmonic) in 35 per cent of the analysed stars. Interestingly, power excess at subharmonic frequency is detected mostly for stars of the middle sequence in the Petersen diagram (74 per cent), incidence rate is much lower for stars of the top sequence (31 per cent), and phenomenon is not detected for stars of the bottom sequence. The amplitude and/or phase of the additional periodicities strongly vary in time. Similar form of pulsation is observed in first overtone RR Lyrae stars. Our results indicate that the nature and cause of this form of pulsation is the same in the two groups of classical pulsators; consequently, a common model explaining this form of pulsation should be searched for. Our results favour the theory of the excitation of non-radial modes of angular degrees 7, 8 and 9, proposed recently by Dziembowski.

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arXiv2015

Survey of non-linear hydrodynamic models of type-II Cepheids

We present a grid on non-linear convective type-II Cepheid models. The dense model grids are computed for 0.6M_Sun and a range of metallicities ([Fe/H]=-2.0,-1.5,-1.0), and for 0.8M_Sun ([Fe/H]=-1.5). Two sets of convective parameters are considered. The models cover the full temperature extent of the classical instability strip, but are limited in luminosity; for the most luminous models violent pulsation leads to the decoupling of the outermost model shell. Hence, our survey reaches only the shortest period RV Tau domain. In the Hertzsprung-Russel diagram we detect two domains in which period doubled pulsation is possible. The first extends through the BL Her domain and low luminosity W Vir domain (pulsation periods ~2-6.5 d). The second domain extends at higher luminosities (W Vir domain; periods >9.5d). Some models within these domains display period-4 pulsation. We also detect very narrow domains (~10 K wide) in which modulation of pulsation is possible. Another interesting phenomenon we detect is double-mode pulsation in the fundamental mode and in the fourth radial overtone. Fourth overtone is a surface mode, trapped in the outer model layers. Single-mode pulsation in the fourth overtone is also possible on the hot side of the classical instability strip. The origin of the above phenomena is discussed. In particular, the role of resonances in driving different pulsation dynamics as well as in shaping the morphology of the radius variation curves is analysed.

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arXiv2015

Double-mode radial-non-radial RR Lyrae stars. OGLE-IV photometry of two high cadence fields in the Galactic bulge

We analyse the OGLE-IV photometry of the first overtone and double-mode RR Lyrae stars (RRc/RRd) in the two fields towards the Galactic bulge observed with high cadence. In 27 per cent of RRc stars we find additional non-radial mode, with characteristic period ratio, P x /P 1O \in (0.6, 0.64). It strongly corroborates the conclusion arising from the analysis of space photometry of RRc stars, that this form of pulsation must be common. In the Petersen diagram the stars form three sequences. In 20 stars we find two or three close secondary modes simultaneously. The additional modes are clearly non-stationary. Their amplitude and/or phase vary in time. As a result, the patterns observed in the frequency spectra of these stars may be very complex. In some stars the additional modes split into doublets, triplets or appear as a more complex bands of increased power. Subharmonics of additional modes are detected in 20 per cent of stars. They also display a complex structure. Including our previous study of the OGLE-III Galactic bulge data, we have discovered 260 RRc and 2 RRd stars with the additional non-radial mode, which is the largest sample of these stars so far. The additional mode is also detected in two Blazhko RRc stars, which shows that the modulation and additional non-radial mode are not exclusive.

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arXiv2015

An RR Lyrae family portrait: 33 stars observed in Pisces with K2-E2

A detailed analysis is presented of 33 RR Lyrae stars in Pisces observed with the Kepler space telescope over the 8.9-day long K2 Two-Wheel Concept Engineering Test. The sample includes not only fundamental-mode and first overtone (RRab and RRc) stars but the first two double-mode (RRd) stars that Kepler detected and the only modulated first-overtone star ever observed from space so far. The precision of the extracted K2 light curves made it possible to detect low-amplitude additional modes in all subtypes. All RRd and non-modulated RRc stars show the additional mode at P_X/P_1~0.61 that was detected in previous space-based photometric measurements. A periodicity longer than the fundamental mode was tentatively identified in one RRab star that might belong to a gravity mode. We determined the photometric [Fe/H] values for all fundamental-mode stars and provide the preliminary results of our efforts to fit the double-mode stars with non-linear hydrodynamic pulsation models. The results from this short test run indicate that the K2 mission will be, and has started to be, an ideal tool to expand our knowledge about RR Lyrae stars. As a by-product of the target search and analysis, we identified 165 bona-fide double-mode RR Lyrae stars from the Catalina Sky Survey observations throughout the sky, 130 of which are new discoveries.

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arXiv2015

The Araucaria Project. Accurate stellar parameters and distance to evolved eclipsing binary ASAS J180057-2333.8 in Sagittarius Arm

We have analyzed the double-lined eclipsing binary system ASAS J180057-2333.8 from the All Sky Automated Survey (ASAS) catalogue . We measure absolute physical and orbital parameters for this system based on archival $V$-band and $I$-band ASAS photometry, as well as on high-resolution spectroscopic data obtained with ESO 3.6m/HARPS and CORALIE spectrographs. The physical and orbital parameters of the system were derived with an accuracy of about 0.5 - 3%. The system is a very rare configuration of two bright well-detached giants of spectral types K1 and K4 and luminosity class II. The radii of the stars are $R_1$ = 52.12 $\pm$ 1.38 and $R_2$ = 67.63 $\pm$ 1.40 R$_\odot$ and their masses are $M_1$ = 4.914 $\pm$ 0.021 and $M_2$ = 4.875$\pm$ 0.021 M$_\odot$ . The exquisite accuracy of 0.5% obtained for the masses of the components is one of the best mass determinations for giants. We derived a precise distance to the system of 2.14 $\pm$ 0.06 kpc (stat.) $\pm$ 0.05 (syst.) which places the star in the Sagittarius-Carina arm. The Galactic rotational velocity of the star is $Θ_s=258 \pm 26$ km s$^{-1}$ assuming $Θ_0=238$ km s$^{-1}$. A comparison with PARSEC isochrones places the system at the early phase of core helium burning with an age of slightly larger than 100 million years. The effect of overshooting on stellar evolutionary tracks was explored using the MESA star code.

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arXiv2015

Discovery of a new group of double-periodic RR Lyrae stars in the OGLE-IV photometry

We report the discovery of a new group of double-periodic RR Lyrae stars from the analysis of the OGLE-IV Galactic bulge photometry. In 11 stars identified in the OGLE catalog as first overtone pulsators (RRc stars) we detect additional longer period variability of low amplitude, in the mmag regime. One additional star of the same type is identified in a published analysis of the Kepler space photometry. The period ratio between the shorter first overtone period and a new, longer period lies in a narrow range around 0.686. Thus, the additional period is longer than the expected period of the undetected radial fundamental mode. The obvious conclusion that addition periodicity corresponds to a gravity or a mixed mode faces difficulties, however.

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arXiv2015

The Araucaria Project: The First-Overtone Classical Cepheid in the Eclipsing System OGLE-LMC-CEP-2532

We present here the first spectroscopic and photometric analysis of the double-lined eclipsing binary containing the classical, first-overtone Cepheid OGLE-LMC-CEP-2532 (MACHO 81.8997.87). The system has an orbital period of 800 days and the Cepheid is pulsating with a period of 2.035 days. Using spectroscopic data from three high-class telescopes and photometry from three surveys spanning 7500 days we are able to derive the dynamical masses for both stars with an accuracy better than 3%. This makes the Cepheid in this system one of a few classical Cepheids with an accurate dynamical mass determination (M_1=3.90 +/- 0.10 M_sun). The companion is probably slightly less massive (3.82 +/- 0.10 M_sun), but may have the same mass within errors (M_2/M_1= 0.981 +/- 0.015). The system has an age of about 185 million years and the Cepheid is in a more advanced evolutionary stage. For the first time precise parameters are derived for both stars in this system. Due to the lack of the secondary eclipse for many years not much was known about the Cepheid's companion. In our analysis we used extra information from the pulsations and the orbital solution from the radial velocity curve. The best model predicts a grazing secondary eclipse shallower than 1 mmag, hence undetectable in the data, about 370 days after the primary eclipse. The dynamical mass obtained here is the most accurate known for a first-overtone Cepheid and will contribute to the solution of the Cepheid mass discrepancy problem.

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arXiv2014

Intriguing triple-mode RR Lyrae star with period doubling

We report the discovery of an intriguing triple-mode RR Lyrae star found in the OGLE Galactic bulge collection, OGLE-BLG-RRLYR-24137. In the OGLE catalog the star was identified as RRd star - double-mode pulsator, pulsating simultaneously in the fundamental and in the first overtone modes. We find that third mode is excited and firmly detect its period doubling. Period ratios are not far from that expected for triple-mode - fundamental, first and third overtone - pulsation. Unfortunately, we cannot reproduce period ratios of the three modes with a consistent set of pulsation models. Therefore the other interpretation, that additional mode is non-radial, is also likely.

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arXiv2014

Blazhko-type modulation in the double-mode RR Lyrae stars of the OGLE Galactic bulge collection

We present the analysis of Blazhko-type modulation in double-mode RR Lyrae (RRd) stars found in the OGLE photometry of the Galactic bulge. Modulation is detected in 15 stars. Most of them have non-typical period ratio of the radial modes. In the Petersen diagram, at a given period of the fundamental mode, they are located significantly below or above the sequence formed by the majority of RRd stars. Multi-periodic modulation is very frequent; two or three modulation periods are detected in 8 stars. Modulation periods vary from ~20 to more than 300 days. Radial mode amplitudes can be modulated by a few to nearly hundred per cent. Both radial modes may be modulated with the same period. More commonly however, dominant modulation for the fundamental mode has different period than dominant modulation for the first overtone. Quite often modulation of only one mode is detected in the data. We find a clear feedback between pulsation amplitude of the dominant mode and mean stellar brightness: lower the pulsation amplitude, brighter the star. At phases of low pulsation amplitude, the mode periods are prone to fast changes. All the stars share the common feature: their pulsation properties are non-stationary. Amplitudes and phases of the radial modes vary irregularly on a long time-scale of a few hundred or thousand days. The short-term modulations are also irregular. One of the stars has switched the pulsation mode recently: from single-mode fundamental mode pulsation to RRd state. In other star the non-radial mode with characteristic ~0.61 period ratio to the first overtone is detected. This non-radial mode is likely modulated with the same period as the radial modes.

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arXiv2014

Kepler photometry of RRc stars: peculiar double-mode pulsations and period doubling

We present the analysis of four first overtone RR Lyrae stars observed with the Kepler space telescope, based on data obtained over nearly 2.5yr. All four stars are found to be multiperiodic. The strongest secondary mode with frequency f_2 has an amplitude of a few mmag, 20 - 45 times lower than the main radial mode with frequency f_1. The two oscillations have a period ratio of P_2/P_1 = 0.612 - 0.632 that cannot be reproduced by any two radial modes. Thus, the secondary mode is nonradial. Modes yielding similar period ratios have also recently been discovered in other variables of the RRc and RRd types. These objects form a homogenous group and constitute a new class of multimode RR Lyrae pulsators, analogous to a similar class of multimode classical Cepheids in the Magellanic Clouds. Because a secondary mode with P_2/P_1 ~ 0.61 is found in almost every RRc and RRd star observed from space, this form of multiperiodicity must be common. In all four Kepler RRc stars studied, we find subharmonics of f_2 at ~1/2 f_2 and at ~3/2 f_2. This is a signature of period doubling of the secondary oscillation, and is the first detection of period doubling in RRc stars. The amplitudes and phases of f_2 and its subharmonics are variable on a timescale of 10 - 200d. The dominant radial mode also shows variations on the same timescale, but with much smaller amplitude. In three Kepler RRc stars we detect additional periodicities, with amplitudes below 1mmag, that must correspond to nonradial g-modes. Such modes never before have been observed in RR Lyrae variables.

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arXiv2014

Double-mode radial-non-radial RR Lyrae stars in the OGLE photometry of the Galactic bulge

Non-radial modes are excited in classical pulsators, both in Cepheids and in RR Lyrae stars. Firm evidence come from the first overtone pulsators, in which additional shorter period mode is detected with characteristic period ratio falling in between 0.60 and 0.65. In the case of first overtone Cepheids three separate sequences populated by nearly 200 stars are formed in the Petersen diagram, i.e. the diagram of period ratio versus longer period. In the case of first overtone RR Lyrae stars (RRc stars) situation is less clear. A dozen or so such stars are known which form a clump in the Petersen diagram without any obvious structure. Interestingly, all first overtone RR Lyrae stars for which precise space-borne photometry is available show the additional mode, which suggests that its excitation is common. Motivated by these results we searched for non-radial modes in the OGLE-III photometry of RRc stars from the Galactic bulge. We report the discovery of 147 stars, members of a new group of double-mode, radial-non-radial mode pulsators. They form a clear and tight sequence in the Petersen diagram, with period ratios clustering around 0.613 with a signature of possible second sequence with higher period ratio (0.631). The scatter in period ratios of the already known stars is explained as due to population effects. Judging from the results of space observations this still mysterious form of pulsation must be common among RRc stars and with our analysis of the OGLE data we just touch the tip of the iceberg.

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arXiv2014

Chaos in hydrodynamic BL Herculis models

We present non-linear, convective, BL Her-type hydrodynamic models that show complex variability characteristic for deterministic chaos. The bifurcation diagram reveals a rich structure, with many phenomena detected for the first time in hydrodynamic models of pulsating stars. The phenomena include not only period doubling cascades en route to chaos (detected in earlier studies) but also periodic windows within chaotic band, type-I and type-III intermittent behaviour, interior crisis bifurcation and others. Such phenomena are known in many textbook chaotic systems, from the simplest discrete logistic map, to more complex systems like Lorenz equations. We discuss the physical relevance of our models. Although except of period doubling such phenomena were not detected in any BL Her star, chaotic variability was claimed in several higher luminosity siblings of BL Her stars - RV Tau variables, and also in longer-period, luminous irregular pulsators. Our models may help to understand these poorly studied stars. Particularly interesting are periodic windows which are intrinsic property of chaotic systems and are not necessarily caused by resonances between pulsation modes, as sometimes claimed in the literature.

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arXiv2014

The Araucaria Project. OGLE-LMC-CEP-1718: An exotic eclipsing binary system composed of two classical overtone Cepheids in a 413-day orbit

We have obtained extensive high-quality spectroscopic observations of the OGLE-LMC-CEP-1718 eclipsing binary system in the Large Magellanic Cloud which Soszynski et al. (2008) had identified as a candidate system for containing two classical Cepheids in orbit. Our spectroscopic data clearly demonstrate binary motion of the Cepheids in a 413-day eccentric orbit, rendering this eclipsing binary system the first ever known to consist of two classical Cepheid variables. After disentangling the four different radial velocity variations in the system we present the orbital solution and the individual pulsational radial velocity curves of the Cepheids. We show that both Cepheids are extremely likely to be first overtone pulsators and determine their respective dynamical masses, which turn out to be equal to within 1.5 %. Since the secondary eclipse is not observed in the orbital light curve we cannot derive the individual radii of the Cepheids, but the sum of their radii derived from the photometry is consistent with overtone pulsation for both variables. The existence of two equal-mass Cepheids in a binary system having different pulsation periods (1.96 and 2.48 days, respectively) may pose an interesting challenge to stellar evolution and pulsation theories, and a more detailed study of this system using additional datasets should yield deeper insight about the physics of stellar evolution of Cepheid variables. Future analysis of the system using additional near-infrared photometry might also lead to a better understanding of the systematic uncertainties in current Baade-Wesselink techniques of distance determinations to Cepheid variables.

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arXiv2013

Mode selection in pulsating stars

In this review we focus on non-linear phenomena in pulsating stars the mode selection and amplitude limitation. Of many linearly excited modes only a fraction is detected in pulsating stars. Which of them and why (the problem of mode selection) and to what amplitude (the problem of amplitude limitation) are intrinsically non-linear and still unsolved problems. Tools for studying these problems are briefly discussed and our understanding of mode selection and amplitude limitation in selected groups of self-excited pulsators is presented. Focus is put on classical pulsators (Cepheids and RR Lyrae stars) and main sequence variables (delta Scuti and beta Cephei stars). Directions of future studies are briefly discussed.

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arXiv2013

Order and chaos in hydrodynamic BL Her models

Many dynamical systems of different complexity, e.g. 1D logistic map, the Lorentz equations, or real phenomena, like turbulent convection, show chaotic behaviour. Despite huge differences, the dynamical scenarios for these systems are strikingly similar: chaotic bands are born through the series of period doubling bifurcations and merge through interior crises. Within chaotic bands periodic windows are born through the tangent bifurcations, preceded by the intermittent behaviour. This is a universal behaviour of dynamical systems (Feigenbaum 1983). We demonstrate such behaviour in models of pulsating stars.

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