We analyse photometry for 138 first overtone Cepheids from the Small Magellanic Cloud, in which Optical Gravitational Lensing Experiment team discovered additional variability with period shorter ...than first overtone period, and period ratios in the P/P
1O ∈ (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.
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 ∈ (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.
ABSTRACT
We have analysed Optical Gravitational Lensing Experiment photometry for first overtone classical Cepheids in the Large and Small Magellanic Clouds in search for additional periodicities ...beyond radial modes. We have used standard consecutive pre-whitening technique in some cases followed by time-dependent pre-whitening. We report new candidates for double-mode radial pulsations. However, majority of signals we have detected cannot be interpreted in terms of radial modes. We report 516 double-periodic stars with period ratios, Px/P1O, in the range 0.60–0.65. We study the properties of this class and implications for model explaining these periodicities. We also report 28 stars in which additional variability is of longer period, below radial fundamental mode, with median period ratio, P1O/Px, of 0.684. This class is an analogue of a class known in RR Lyrae stars. Hundreds of other signals were detected that cannot be attributed to radial modes or the above-mentioned classes. Statistical properties of these signals are analysed. We suggest that majority of these signals correspond to non-radial modes. In particular, a significant fraction can be attributed to non-radial modes of moderate degrees, tightly connected to a class with period ratios in between 0.60 and 0.65. In tens of stars, close to radial mode frequency, relatively large-amplitude and coherent signals are observed, which may represent yet another class. In 27 stars periodic modulation of pulsation was detected. Differences in additional frequency content between the two Clouds are discussed.
Abstract
We present a study of the Oosterhoff (Oo) dichotomy in the Galactic bulge using 8141 fundamental-mode RR Lyrae stars. We used public photometric data from the Optical Gravitational Lensing ...Experiment and the Vista Variables in the Vía Láctea survey. We carefully selected fundamental-mode stars without modulation and without association with any globular cluster located towards the Galactic bulge. Subsequently, we identified and separated the Oosterhoff groups I and II on the basis of their period–amplitude distribution and using a relation fitted to the Oosterhoff I locus. Both Oosterhoff groups were then compared to observations of two bulge globular clusters and with models of stellar pulsation and evolution. We found that some of the variables classified as Oo II belong to a third Oo group. The Oosterhoff II variables are more metal-poor on average, more massive, and cooler than their Oosterhoff I counterparts. The analysis of their spatial distribution shows a systematic difference between foreground, central, and background regions in the occurrence of the Oosterhoff II group. The difference between the Oo I and II groups is also seen in their distance distributions with respect to the Galactic bar, but neither group is associated with the bar.
Non-radial modes are excited in classical pulsators, both in Cepheids and in RR Lyrae stars. Firm evidence comes from the first-overtone pulsators, in which an additional shorter period mode is ...detected with characteristic period ratio falling between 0.60 and 0.65. For 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. For first-overtone RR Lyrae stars (RRc stars), the situation is less clear. A dozen or so such stars are known. They 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 and a signature of a 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 our analysis of the OGLE data just touches the tip of the iceberg.
Context
. Close binary central stars of planetary nebulae (PNe) must have formed through a common envelope evolution during the giant phase experienced by one of the stars. Transfer of the angular ...momentum from the binary system to the envelope leads to the shortening of the binary separations from the radius of red giant to the radius of the order of few tenths of AU. Thus, close binary central stars of planetary nebulae are laboratories to study the common envelope phase of evolution. The close binary fraction in the Galaxy has been measured in various sky surveys, but the close binary fraction is not yet well constrained for the Magellanic Clouds, and our results may help the study of common envelope evolution in low-metallicity environments.
Aims
. This paper presents a continuation of our study of variability in the Magellanic Cloud planetary nebulae on the basis of data from the OGLE survey. Previously, we had analysed the OGLE data in the Small Magellanic Cloud. Here, the study is extended to the Large Magellanic Cloud (LMC). In this paper we search for close binary central stars with the aim to constrain the binary fraction and period distribution in the LMC.
Methods
. We identified 290 counterparts of PNe in the LMC in the
I
-band images from the OGLE-III and OGLE-IV surveys. However, the light curves of ten objects were not accessible in the OGLE database, and thus we analysed the time series photometry of 280 PNe.
Results
. In total, 32 variables were found, but 5 of them turned out to be foreground objects. Another 18 objects show irregular or regular variability that is not attributable to the binarity of their central stars. Their status and the nature of their variability will be verified in the follow-up paper. Nine binary central stars of PNe with periods between 0.24 and 23.6 days were discovered. The obtained fraction for the LMC PNe is 3.3
-1.6
+2.6
% without correcting for incompleteness. This number is significantly lower than the 12–21% derived in the analogous search in the Galactic bulge. We discuss this difference, taking into account observational biases. The lower binary fraction suggests a lower efficiency of the common envelope phase in producing close binaries in the LMC compared to the Galaxy.
We report the discovery of a new group of double-periodic RR Lyrae stars from the analysis of the Optical Gravitational Lensing Experiment - IV (OGLE-IV) Galactic bulge photometry. In 11 stars ...identified in the OGLE catalogue 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.
Abstract
We present hydrodynamic BL Herculis type models which display a long-term modulation of pulsation amplitudes and phases. The modulation is either strictly periodic or quasi-periodic, with ...the modulation period and modulation pattern varying from one cycle to another. Such behaviour has not been observed in any BL Herculis variable so far; however, it is a common property of their lower luminosity siblings - RR Lyrae variables showing the Blazhko effect. These models provide support for the recent mechanism proposed by Buchler & Kolláth to explain this still mysterious phenomenon. In their model, a half-integer resonance that causes the period-doubling effect, discovered recently in Blazhko RR Lyrae stars, is responsible for the modulation of pulsation as well. Although our models are more luminous than is appropriate for RR Lyrae stars, they clearly demonstrate, through direct hydrodynamic computation, that the mechanism can indeed be operational.
Of great importance are models that show quasi-periodic modulation - a phenomenon observed in Blazhko RR Lyrae stars. Our models coupled with the analysis of the amplitude equations show that such behaviour may be caused by the dynamical evolution occurring in the close proximity of the unstable single periodic saddle point.
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