Horizontal-Branch Evolution with Semiconvection II. Theoretical Sequences

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Abstract

A grid of 228 horizontal-branch sequences has been constructed for the following values of the basic model parameters: helium abundance Y = 0.10, 0.20, 0.30, 0.40; heavy-element abundance Z = 0.0001, 0.001, 0.01; initial helium-core mass M0 = 0.425, 0.475, 0.525, 0.575 M0 and total mass M = 0.46(0.04)0.90 M0. The present model computations include a treatment of both convective overshooting and semiconvection within the helium-burning core. Each sequence has been evolved from the zero-age horizontal branch until the helium abundance in the convective core has been depleted to a value of 0.05. In all cases the models develop semiconvective zones which considerably increase the extent of the helium-depleted region within the core. Data on the internal and observable characteristics of the sequences are tabulated for selected cases. With these data the systematic dependence of horizontal-branch properties on the various model parameters can be determined. Both the track lengths in the H-R diagram and the horizontal-branch lifetimes obtained for these sequences can substantially exceed those predicted by calculations in which semiconvecti on is neglected. Subject headings: convection - horizontal-branch stars - interiors, stellar

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