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

Abstract

We report the discovery of a new group of double-periodic stars in the OGLE Galactic bulge photometry. In 38 stars identified as fundamental mode RR~Lyrae and 4 classified as the first-overtone RR~Lyrae, we detected additional shorter periodicity. Periods of the dominant variability in the newly discovered group are $0.28<P_{\mathrm{D}}<0.41$\,days. Period ratios $(0.68-0.72)$ are smaller than the period ratios of the Galactic bulge RRd stars. The typical amplitude ratio (of the additional to the dominant periodicity) is 20\,\% for the stars identified as fundamental mode RR~Lyrae and 50\,\% for stars classified as the first-overtone RR~Lyrae. Ten stars from our sample exhibit equidistant peaks in the frequency spectrum, that suggest the Blazhko-type modulation of the main pulsation frequency and/or the additional periodicity. The Fourier coefficients $R_{21}$ and $R_{31}$ are one of the lowest among fundamental mode RR Lyrae stars, but among the highest for the first-overtone pulsators. For the phase Fourier coefficients ${\phi }_{21}$ and ${\phi }_{31}$, our stars lie in between RRab and RRc stars. Discussed stars were compared with the radial, linear pulsation models. Their position in the Petersen diagram cannot be reproduced assuming that two radial modes are excited and their physical parameters are similar to that characteristic for RR~Lyrae stars. The non-radial mode scenario also faces difficulties. We conclude that the dominant variability is most likely due to pulsation in the radial fundamental mode including stars classified as the first overtone mode pulsators. At this point, we cannot explain the nature of the additional periodicity. Even more, classification of the stars as RR~Lyrae should be treated as tentative.

Accepted for publication in MNRAS

References in corpus (11)

• RR-Lyrae-type pulsations from a 0.26-solar-mass star in a binary system
• Kepler photometry of RRc stars: peculiar double-mode pulsations and period doubling
• MOST photometry of the RRd Lyrae variable AQ Leo: Two radial modes, 32 combination frequencies, and beyond
• Revisiting CoRoT RR Lyrae stars: detection of period doubling and temporal variation of additional frequencies
• Pulsation models for the 0.26M_sun star mimicking RR Lyrae pulsator. Model survey for the new class of variable stars
• Anomalous Double-Mode RR Lyrae Stars in the Magellanic Clouds
• Blazhko-type modulation in the double-mode RR Lyrae stars of the OGLE Galactic bulge collection
• Double-mode radial-non-radial RR Lyrae stars in the OGLE photometry of the Galactic bulge
• Discovery of a new group of double-periodic RR Lyrae stars in the OGLE-IV photometry
• On the modulation of RR Lyrae stars in the globular cluster M3
• Peculiar double-periodic pulsation in RR Lyrae stars of the OGLE collection. I. Long-period stars with dominant radial fundamental mode

Cited by in corpus (10)

• First results on RR Lyrae stars with the TESS space telescope: untangling the connections between mode content, colors and distances
• Blazhko effect in the Galactic bulge fundamental mode RR~Lyrae stars I: Incidence rate and differences between modulated and non-modulated stars
• Milky Way archaeology using RR Lyrae and type II Cepheids I. The Orphan stream in 7D using RR Lyrae stars
• Frequency analysis of the first-overtone RR Lyrae stars based on the Extended Aperture Photometry from the K2 data
• Time series analysis of bright TESS RRc stars: Additional modes, phase variations and more
• Spectroscopic and photometric time series of the bright RRc star T Sex
• Four 'Peculiar' RRd Stars Observed by K2
• Detection of Gravity Modes in RR Lyrae Stars
• RR Lyrae stars with variable mean magnitudes
• Kepler Pixel Project -- background RR Lyrae stars in the primary Kepler mission field of view