Nonradial mode excitation as the cause of the Blazhko effect in RR Lyrae stars

Abstract

A significant fraction of RR Lyrae stars exhibits amplitude and/or phase modulation known as the the Blazhko effect. The oscillation spectra suggest that, at least in most of the cases, excitation of nonradial modes in addition to the dominant radial modes is responsible for the effect. Though model calculations predict that nonradial modes may be excited, there are problems with explaining their observed properties in terms of finite amplitude development of the linear instability. We propose a scenario, which like some previous, postulates energy transfer from radial to nonradial modes, but avoids those problems. The scenario predicts lower amplitudes in Blazhko stars. We check this prediction with a new analysis of the Galactic bulge RR Lyrae stars from OGLE-II database. The effect is seen, but the amplitude reduction is smaller than predicted.