Peculiar double-periodic pulsation in RR Lyrae stars of the OGLE collection – I. Long-period stars with dominant radial fundamental mode

Abstract

We present the discovery of a new, peculiar form of double-periodic pulsation in RR Lyrae stars. In four, long-period (P > 0.6 d) stars observed by the Optical Gravitational Lensing Experiment, and classified as fundamental mode pulsators (RRab), we detect additional, low-amplitude variability, with period shorter than fundamental mode period. The period ratios fall in a range similar to double-mode fundamental and first overtone RR Lyrae stars (RRd), with the exception of one star, in which the period ratio is significantly lower and nearly exactly equals 0.7. Although period ratios are fairly different for the four stars, the light curve shapes corresponding to the dominant, fundamental mode are very similar. The peak-to-peak amplitudes and amplitude ratios (Fourier parameters R 21 and R 31 ) are among the highest observed in RRab stars of similar period, while Fourier phases (ϕ 21 and ϕ 31 ) are among the lowest observed in RRab stars. If the additional variability is interpreted as due to radial first overtone, then, the four stars are the most extreme RRd variables of the longest pulsation periods known. Indeed, the observed period ratios can be well modelled with high metallicity pulsation models. However, at such long pulsation periods, first overtone is typically damped. Five other candidates, with weak signature of additional variability, sharing the same characteristics, were also detected and are briefly discussed.

On the Oosterhoff dichotomy in the Galactic bulge: I. spatial distribution

“…In addition, the low-amplitude double-mode variables found by Smolec et al (2016) with a dominant fundamental mode and long periods fall roughly in the region of Oo III variables. Most of these double-mode pulsators can be described using linear pulsation models with very high metallicity (around -0.5 dex).…”

“…The period ratio of this additional peak and the dominant mode is 0.720, therefore it lies in a region of the Petersen diagram (see fig. 1 in Smolec et al 2016) where the aforementioned double-mode pulsators occur. We did not detect any signal at the combination frequency of the dominant mode and the additional signal ( f x ).…”

On the Oosterhoff dichotomy in the Galactic bulge – II. Kinematical distribution

“…Therefore, some of the stars in our sample might have been stripped from NGC 6441 in the past and have mixed with the bulge field population of RR Lyrae stars. In addition, one of these stars is a candidate for the double-mode variables found by Smolec et al (2016), which, based on stellar pulsation models, have a high metallicity despite the low metallicity determined using photometric data. After a careful analysis of the Fourier spectra of the remaining four stars, none of them shows any sign of a double-mode behavior.…”

Milky Way metallicity gradient from Gaia DR2 F/1O double-mode Cepheids

“…Petersen diagrams (Petersen 1973) where the period ratios of mixed-mode Cepheids are plotted versus the longer period are very useful tools to study the properties of these stars. Combined with the huge amount of data provided by the OGLE microlensing survey , they led to numerous discoveries in recent years for both Cepheids and RR Lyrae stars (e.g., Coppola et al 2015;Smolec et al 2016;Prudil et al 2017). 1 A similar number of 1O/2O double-mode Cepheids (that are not relevant for this paper) have also been discovered: CO Aur and V1048 Cen have been known for a long time; V2157 Sgr, V767 Sgr, V363 Cas have been reported by Hajdu et al (2009), and several more stars have been identified by e.g., Soszyński et al (2011); ; Khruslov (2013); Khruslov & Kusakin (2016) in large scale photometric surveys.…”