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

TL;DR

This study examines how metallicity affects the period-luminosity relation of classical Cepheids using Baade-Wesselink distances and finds minimal dependence, with implications for distance measurements.

Contribution

It provides new insights into the metallicity dependence of the Cepheid $PL$-relation using a sample of 68 Galactic Cepheids with high-resolution spectroscopic metallicities.

Findings

Metallicity dependence in $V$ and $K$ bands is marginal.

Adopting a period-dependent $p$-factor slightly alters the $PL$-relation slope.

Galactic and LMC $PL$-relation slopes are consistent within errors.

Abstract

The aim of this paper is to investigate the metallicity dependence of the $PL$-relation in $V$ and $K$ 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. Literature values of the $V$-band, $K$-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 $(V-K)$ surface-brightness relation and a projection factor derived in a previous paper, distances have been derived from a Baade-Wesselink analysis. $PL$- and $PLZ$-relations in $V$ and $K$ are derived. The effect of the adopted dependence of the projection factor on period is investigated. The change from a constant $p$-factor to one recently suggested in the literature with a mild dependence on $\log P$ 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 $PL$-relation in $V$ in the LMC agrees with both hypotheses. In $K$ the difference between the Galactic and LMC slope is larger and would favour a mild period dependence of the $p$-factor. The dependence on metallicity in $V$ and $K$ is found to be marginal, and independent of the choice of $p$-factor on period. This result is severely limited by the small range in metallicity covered by the Galactic Cepheids.