Baade-Wesselink distances and the effect of metallicity in classical cepheids

Abstract

Context. The metallicity dependence of the Cepheid -relation is of importance in establishing the extra-galactic distance scale.Aims. The aim of this paper is to investigate the metallicity dependence of the -relation in and based on a sample of 68 Galactic Cepheids with individual Baade-Wesselink distances (some of the stars also have an HST-based parallax) and individually determined metallicities from high-resolution spectroscopy.Methods. Literature values of the -band, -band and radial velocity data have been collected for a sample of 68 classical cepheids that have their metallicity determined in the literature from high-resolution spectroscopy. Based on a surface-brightness relation and a projection factor derived in a previous paper, distances have been derived from a Baade-Wesselink analysis. - and -relations in and are derived.Results. The effect of the adopted dependence of the projection factor on period is investigated. The change from a constant -factor to one recently suggested in the literature with a mild dependence on results in a less steep slope by 0.1 unit, which is about the 1-sigma error bar in the slope itself. The observed slope in the -relation in in the LMC agrees with both hypotheses. In the difference between the Galactic and LMC slope is larger and would favour a mild period dependence of the -factor. The dependence on metallicity in and is found to be marginal, and independent of the choice of -factor on period. This result is severely limited by the small range in metallicity covered by the Galactic Cepheids. . © 2008 ESO.