On the Cepheid Metallicity Dichotomy

TL;DR

This study uses stellar atmosphere models and photometry to reveal a metallicity dichotomy between Cepheid types in the Magellanic Clouds, linking metallicity, luminosity, and pulsation characteristics.

Contribution

It demonstrates a correlation between Cepheid metallicity and pulsation mode, supported by pulsation and evolution theories, and suggests improved metallicity estimation methods.

Findings

First overtone Cepheids have lower metallicities than fundamental ones.

Lower luminosity Cepheids tend to have lower metallicities.

The metallicity differences align with pulsation and evolution models.

Abstract

With the aid of stellar atmosphere models, we derive iron abundances [Fe/H] from the OGLE B, V, I photometry on the Magellanic Cloud Cepheids. We show that in both clouds the average metallicities of the first overtone variables are lower than those of the fundamental ones (by ~0.2 and ~0.3 dex in the LMC and SMC, respectively). Consequently, there is a correlation between the overall [Fe/H] and luminosity; the lower luminosity stars tend to be also of lower metallicity. These metallicity dependencies are concordant with the ones derived for the two types of double-mode Cepheids from pulsation theory. Yet another support of this dichotomy comes from the evolution theories that require lower metallicities for blue-looping low-luminosity stars than for high-luminosity ones. We also comment on the possibility of using period-luminosity-color relations to derive more accurate metallicities.