Semiconvection and period changes in RR Lyrae stars
The possibility is considered that convective mixing inherently associated with composition redistribution inward through a semiconvective zone (SCZ) and into the convective core by the end of the horizontal-branch (HB) phase may be primarily responsible for the observed period changes in RR Lyrae stars. Specifically, it is proposed that the period changes in RR Lyrae stars are caused mainly by small random mixing events within the SCZ and at the edge of the convective core. Tests of several HB models that include semiconvection are presented which demonstrate that stellar evolution cannot satisfactorily account for the observed period changes but that mixing events involving only a very small part of the SCZ or a very small extension of the convective core are sufficient to change the pulsation period of an RR Lyrae star by the amount typically observed. The frequency of the period changes is predicted theoretically and found to be in good agreement with the observed value within the substantial uncertainties involved. A procedure is outlined for future period-change testing of HB models.