Anomalous Double-Mode RR Lyrae Stars in the Magellanic Clouds

Abstract

We report the discovery of a new subclass of double-mode RR Lyrae stars in the Large and Small Magellanic Clouds. The sample of 22 pulsating stars have been extracted from the latest edition of the OGLE collection of RR Lyrae variables in the Magellanic System. The stars pulsating simultaneously in the fundamental (F) and first-overtone (1O) modes have distinctly different properties than regular double-mode RR Lyrae variables (RRd stars). The $P_{1O}/P_F$ period ratios of our anomalous RRd stars are within a range 0.725-0.738, while "classical" double-mode RR Lyrae variables have period ratios in the range 0.742-0.748. In contrast to the typical RRd stars, in the majority of the anomalous pulsators the F-mode amplitudes are higher than the 1O-mode amplitudes. The light curves associated with the F-mode in the anomalous RRd stars show different morphology than the light curves of, both, regular RRd stars and single-mode RRab stars. Most of the anomalous double-mode stars show long-term modulations of the amplitudes (Blazhko-like effect). Translating the period ratios into the abundance parameter, Z, we find for our stars Z in the range (0.002,0.005) - an order of magnitude higher values than typical for RR Lyrae stars. The mass range of the RRd stars inferred from the $W_I$ vs. $P_F$ diagram is (0.55-0.75) of the solar mass. These parameters cannot be accounted for with single star evolution assuming a Reimers-like mass loss. Much greater mass loss caused by interaction with other stars is postulated. We blame the peculiar pulsation properties of our stars to the parametric resonance instability of the 1O-mode to excitation of the F- and 2O-modes as with the inferred parameters of the stars $2{\omega }_{1O}\approx {\omega }_{\mathrm{F}}+{\omega }_{2O}$.

11 pages, 7 figures, accepted for publication in MNRAS

References in corpus (7)

• An eclipsing binary distance to the Large Magellanic Cloud accurate to 2 per cent
• MOST photometry of the RRd Lyrae variable AQ Leo: Two radial modes, 32 combination frequencies, and beyond
• 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
• Non-linear modelling of beat Cepheids: Resonant and non-resonant models
• On the modulation of RR Lyrae stars in the globular cluster M3
• Amplitude Saturation in Beta Cephei Models

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• Time series analysis of bright TESS RRc stars: Additional modes, phase variations and more
• Four 'Peculiar' RRd Stars Observed by K2
• Possible evidence of hydrogen emission in the first-overtone and multi-mode RR Lyrae variables
• The Changing Lightcurve of the Double-Mode RR Lyrae Variable Star V338 Boo
• Kepler Pixel Project -- background RR Lyrae stars in the primary Kepler mission field of view