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⊙ = 1.071(±0.025) + 0.747(±0.028) log P0. 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⊙ = 1.064(±0.0006) + 0.750(±0.006) log P0, which simplifies to R ∝ P3/4. 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, MVC, and those inferred for 23 cluster Cepheids from radius and effective temperature estimates, MVBW, 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 ΔMV(C - BW) = +0.06(±0.03) - 0.43(±0.54)[ Fe/H].