Evolution and Period Change in RR Lyr Variables of the Globular Cluster M 3

Abstract

The grid of evolutionary tracks of population II stars with initial masses 0.81 M_⊙ ≤ M_ZAMS ≤ 0.85 M_⊙ and chemical composition of the globular cluster M3 is computed. Selected models of horizontal branch stars were used as initial conditions for solution of the equations of radiation hydrodynamics and time–dependent convection describing radial stellar oscillations. The boundaries of the instability strip on the Herztsprung–Russel diagram were determined using ≈100 hydrodynamic models of RR Lyr pulsating variables. For each evolutionary track crossing the instability strip the pulsation period was determined as a function of evolutinary time. The rate of period change of most variables is shown to range within −0.02 ≤ $\dot{\Pi}$ ≤ 0.05 day/10⁶ yr. Theoretical estimate of the mean period change rate obtained by the population synthesis method is 〈$\dot{\Pi}$〉 = 6.0 × 10⁻³ day/10⁶ yr and agrees well with observations of RR Lyr variables of the globular cluster M3.