Period doubling in Kepler RR Lyrae stars

TL;DR

This paper reports the discovery of period doubling in Kepler RR Lyrae stars, linking it to the Blazhko effect and explaining it through a high order resonance, advancing understanding of this century-old mystery.

Contribution

It reveals the connection between period doubling and the Blazhko effect and identifies a 9:2 resonance as the cause, supported by hydrodynamic models.

Findings

Period doubling observed in Kepler RR Lyrae stars.

Period doubling is linked to the Blazhko modulation.

Hydrodynamic models confirm the 9:2 resonance as the cause.

Abstract

The origin of the conspicuous amplitude and phase modulation of the RR Lyrae pulsation - known as the Blazhko effect - is still a mystery after more than 100 years of its discovery. With the help of the Kepler space telescope we have revealed a new and unexpected phenomenon: period doubling in RR Lyr - the eponym and prototype of its class - as well as in other Kepler Blazhko RR Lyrae stars. We have found that period doubling is directly connected to the Blazhko modulation. Furthermore, with hydrodynamic model calculations we have succeeded in reproducing the period doubling and proved that the root cause of this effect is a high order resonance (9:2) between the fundamental mode and the 9th radial overtone, which is a strange mode. We discuss the implications of these recent findings on our understanding of the century-old Blazhko problem.