Does Kepler unveil the mystery of the Blazhko effect? First detection of period doubling in Kepler Blazhko RR Lyrae stars

摘要

The first detection of the period doubling phenomenon is reported in theKepler RR Lyrae stars RR Lyr, V808 Cyg and V355 Lyr. Interestingly, all thesepulsating stars show Blazhko modulation. The period doubling manifests itselfas alternating maxima and minima of the pulsational cycles in the light curve,as well as through the appearance of half-integer frequencies located halfwaybetween the main pulsation period and its harmonics in the frequency spectrum.The effect was found to be stronger during certain phases of the modulationcycle. We were able to reproduce the period doubling bifurcation in ournonlinear RR Lyrae models computed by the Florida-Budapest hydrocode. Thisenabled us to trace the origin of this instability in RR Lyrae stars to aresonance, namely a 9:2 resonance between the fundamental mode and a high-order(9th) radial overtone showing strange-mode characteristics. We discuss theconnection of this new type of variation to the mysterious Blazhko effect andargue that it may give us fresh insights to solve this century-old enigma.