The Distance Scale for Classical Cepheid Variables

Abstract

New radii, derived from a modified version of the Baade-Wesselink (BW) method that is tied to published KHG narrowband spectrophotometry, are presented for 13 bright Cepheids. The data yield a best-fitting period-radius relation given by log<R/R_solar>=1.071(+/-0.025)+0.747(+/- 0.028)logP₀. In combination with other high-quality radius estimates recently published by Laney & Stobie, the new data yield a period-radius relation described by log<R/R_solar>=1.064(+/-0.0006)+0.750(+/- 0.006)logP₀, which simplifies to <R>~P^3/4. The relationship is used to test the scale of Cepheid luminosities inferred from cluster zero-age main-sequence (ZAMS) fitting, for which we present an updated list of calibrating Cepheids located in stellar groups. The cluster ZAMS-fitting distance scale tied to a Pleiades distance modulus of 5.56 is found to agree closely with the distance scale defined by Hipparcos parallaxes of cluster Cepheids and also yields Cepheid luminosities that are a good match to those inferred from the period-radius relation. The mean difference between absolute visual magnitudes based on cluster ZAMS fitting, <M_V>_C, and those inferred for 23 cluster Cepheids from radius and effective temperature estimates, <M_V>_BW, in the sense of C-BW is +0.019+/-0.029 s.e. There is no evidence to indicate the need for a major revision to the Cepheid cluster distance scale. The absolute magnitude differences are examined using available [Fe/H] data for the cluster Cepheid sample to test the metallicity dependence of the period-luminosity relation. Large scatter and a small range of metallicities hinder a reliable estimate of the exact relationship, although the data are fairly consistent with predictions from stellar evolutionary models. The derived dependence is ΔM_V(C-BW)=+0.06(+/-0.03)-0.43(+/-0.54)[ Fe/H].