Evolutionary and hydrodynamic models of short--period Cepheids

Abstract

The evolutionary calculations for population I stars with masses on the main sequence 5 M_\odot <= M_0 <= 6.1 M_\odot and initial fractional abundances of helium Y_0=0.28 and heavier elements Z_0=0.02 were carried out to the stage of central helium exhaustion. Selected core helium--burning models were used as initial conditions for solution of the equations of hydrodynamics and time--dependent convection describing radial pulsations of Cepheids. In the Hertzsprung--Russel diagram the evolutionary tracks are shown to cross the red edge of the instability strip for M_0 > 5.1M_\odot. The grid of hydrodynamic models of core helium--burning Cepheids on the stages of the second and the third crossings of the instability strip was computed. The pulsation period Πand the rate of period change \dotΠwere determined as a continuous function of star age for each evolutionary sequence of first--overtone pulsators. Results of calculations agree with observational estimates of \dotΠrecently obtained for the short--period Cepheids V532 Cyg, BG Cru and RT Aur.

15 pages, 5 figures, accepted to Astronomy Letters

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