Classical Cepheid period-Wesenheit-metallicity relation in the Gaia bands

TL;DR

This study quantifies the metallicity dependence of the Gaia-band period-Wesenheit relation for Galactic Cepheids, revealing a significant metallicity term and validating the relation through LMC distance and Galactic metallicity gradient comparisons.

Contribution

The paper provides the first calibration of the Gaia-band period-Wesenheit-metallicity relation for Galactic Cepheids using high-resolution spectroscopy and Gaia DR3 data, highlighting a larger metallicity effect than in NIR bands.

Findings

Significant metallicity term of about -0.5 mag/dex in Gaia bands

Calibrated PWZ relation consistent with LMC distance

Derived Galactic metallicity gradient of -0.0527 dex/kpc

Abstract

Classical Cepheids (DCEPs) represent a fundamental tool to calibrate the extragalactic distance scale. However, they are also powerful stellar population tracers in the context of Galactic studies. The forthcoming Data Release 3 (DR3) of the Gaia mission will allow us to study, with unprecedented detail, the structure, the dynamics, and the chemical properties of the Galactic disc, and in particular of the spiral arms, where most Galactic DCEPs reside. In this paper, we aim to quantify the metallicity dependence of the Galactic DCEPs' period-Wesenheit ($PWZ$) relation in the Gaia bands. We adopted a sample of 499 DCEPs with metal abundances from high-resolution spectroscopy, in conjunction with Gaia Early Data Release 3 parallaxes and photometry to calibrate a $PWZ$ relation in the Gaia bands. We find a significant metallicity term, of the order of $-$0.5 mag/dex, which is larger than the values measured in the near-infrared (NIR) bands by different authors. Our best $PWZ$ relation is $W=(-5.988\pm0.018)-(3.176\pm0.044)(\log P-1.0)-(0.520\pm0.090){\rm [Fe/H]}$. We validated our $PWZ$ relations by using the distance to the Large Magellanic Cloud as a benchmark, finding very good agreement with the geometric distance provided by eclipsing binaries. As an additional test, we evaluated the metallicity gradient of the young Galactic disc, finding $-0.0527 \pm 0.0022$ dex/kpc, which is in very good agreement with previous results.