Abstract
The unprecedented photometric precision along with the quasi-continuous sampling provided by the Kepler space telescope revealed new and unpredicted phenomena that reformed and invigorated RR Lyrae star research. The discovery of period doubling and the wealth of low-amplitude modes enlightened the complexity of the pulsation behavior and guided us toward nonlinear and nonradial studies. Searching and providing theoretical explanation for these newly found phenomena became a central question, as well as understanding their connection to the oldest enigma of RR Lyrae stars, the Blazhko effect. We attempt to summarize the highest impact RR Lyrae results based on or inspired by the data of the Kepler space telescope both from the nominal and from the K2 missions. Besides the three most intriguing topics, the period doubling, the low-amplitude modes, and the Blazhko effect, we also discuss the challenges of Kepler photometry that played a crucial role in the results. The secrets of these amazing variables, uncovered by Kepler, keep the theoretical, ground-based, and space-based research inspired in the post-Kepler era, since light variation of RR Lyrae stars is still not completely understood.
Highlights
RR Lyrae stars are large-amplitude, horizontal-branch pulsating stars which serve as tracers and distance indicators of old stellar populations in the Milky Way and neighboring galaxies
In this paper we present a summary of the most intriguing and defining discoveries in RR Lyrae stars achieved with or inspired by the Kepler space telescope
The position of the stars in the CCD changed with every roll, and the differences in pixel sensitivity appeared as mean brightness deviations as well as a bias in the pulsation amplitudes
Summary
RR Lyrae stars are large-amplitude, horizontal-branch pulsating stars which serve as tracers and distance indicators of old stellar populations in the Milky Way and neighboring galaxies. In spite of the enormous efforts from both theoretical and observational sides, the Blazhko effect is still not fully explained Except for this problem, RR Lyrae stars were thought to be a well-studied and well-understood class of pulsating variables before the launch of Kepler. The first additional frequency in a CoRoT light curve was found by Chadid et al (2010) in the Blazhko star V1127 Aql, with a period ratio of ∼ 0.69 with the fundamental mode, which they suggested to be a nonradial mode This short prehistory of additional modes in RR Lyrae stars was followed by the first Kepler results and with these, a new era has begun. At the time of this writing, Kepler and especially K2 data of RR Lyrae stars are far from being fully exploited, but two years after the official retirement of the telescope the time is ripe to review the major results obtained so far
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