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SUBMITTED TO APJ Preprint typeset using L ATEX style emulateapj v. 10/09/06 BEAT CEPHEIDS AS PROBES OF STELLAR AND GALACTIC METALLICITY: II. OPACITIES WITH THE AGS MIXTURE

By J. Robert Buchler

Abstract

It is well known that the mere location of a Beat Cepheid model in a Period Ratio vs. Period diagram (Petersen diagram) puts constraints on its metallicity Z. But these bounds are sensitive to the mixture of elements that are lumped into the parameter Z. In this short paper we update the previous results that were based on the Grevesse-Noels solar mixture to the recent, revised Asplund, Grevesse & Sauval (2005) (AGS) solar mixture. We also examine the effect of the envelope depth on the accuracy of the computed pulsation periods. We find that for low period Cepheids with high Z the customary approximation of envelope pulsation breaks down. It is necessary to compute stellar models that extend to the center and to include burning and composition inhomogeneities in the modeling. Fortunately, however, most Beat Cepheids that have been observed so far seem to avoid that regime

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