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تسجيل مستخدم جديدDouble-mode radial-non-radial RR Lyrae stars. OGLE-IV photometry of two high cadence fields in the Galactic bulge
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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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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
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We analyzed photometry for the up-to-date collection of the first-overtone RR Lyrae stars (RRc; 11415 stars) and double-mode RR Lyrae stars (RRd; 148 stars) towards the Galactic bulge from the Optical Gravitational Lensing Experiment. We analyzed fre
quency spectra of these stars in search for additional, low-amplitude signals, beyond the radial modes. We focused on stars from two groups: $RR_{0.61}$ and $RR_{0.68}$. In the first group, additional low-amplitude signals have periods shorter than the first-overtone period; period ratios fall in the 0.60-0.64 range. In the second group, additional low-amplitude signals have periods longer than the first-overtone period; period ratios tightly cluster around 0.68. Altogether we have detected 960 and 147 RR Lyrae stars that belong to $RR_{0.61}$ and $RR_{0.68}$ groups, respectively, which yield the incidence rates of 8.3 and 1.3 per cent of the considered sample. We discuss statistical properties of RR Lyrae stars with additional periodicities. For $RR_{0.61}$ group we provide strong arguments that additional periodicities are connected to non-radial pulsation modes of degrees $ell=8$ and $ell=9$, as proposed by Dziembowski. We have also detected two double-periodic variables, with two close periodicities, similar to RR Lyrae variable V37 in NGC 6362. Properties of these peculiar variables, which may form a new group of double-mode pulsators, are discussed.
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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 G
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