Resonant Excitation of Nonradial Modes in RR Lyr Stars

We study a nonlinear development of radial pulsation instability to a resonant excitation of nonradial modes. Our theory covers the cases of axisymmetric (m=0) modes as well as (m,-m) pairs. Adopting a simplified treatment of the radial and nonradial mode coupling we find that the asymptotic state is a pulsation with constant amplitudes and we evaluate the relative amplitude of the nonradial component. Observable consequence of the m=0 mode excitation is a small period change and a more significant amplitude change, especially in the case of a dipole mode (l=1). Such a mode has a fairly large excitation probability. Significant amplitude and phase modulation is predicted in the case of excitation of a m=±1 pair. We suggest that this may explain Blazhko-type modulation in RR Lyr stars. If this model is correct, the modulation period is determined by the rotation rate and the Brunt-Väisälä frequency in the deepest part of the radiative interior.