On the Period--Luminosity--Metallicity relation of Classical Cepheids

TL;DR

This paper investigates how metallicity affects the period-luminosity and period-Wesenheit relations of Galactic Classical Cepheids, proposing new relations that include metallicity dependence to improve extragalactic distance measurements.

Contribution

It introduces the first metallicity-dependent PL/PW relations for Galactic Cepheids, accounting for both slope and intercept variations, and estimates the Gaia DR2 parallax zero point offset.

Findings

Indications of metallicity dependence in PL and PW relation slopes.

New generalized PL/PW relations including metallicity effects.

Estimated Gaia DR2 parallax zero point offset of 0.0615 mas.

Abstract

Classical Cepheids (DCEPs) are the most important primary indicators for the extragalactic distance scale. Establishing the dependence on metallicity of their period--luminosity and period--Wesenheit (PL/PW) relations has deep consequences on the estimate of the Hubble constant (H$_0$). We aim at investigating the dependence on metal abundance ([Fe/H]) of the PL/PW relations for Galactic DCEPs. We combined proprietary and literature photometric and spectroscopic data, gathering a total sample of 413 Galactic DCEPs (372 fundamental mode -- DCEP\_F and 41 first overtone -- DCEP\_1O) and constructed new metallicity-dependent PL/PW relations in the near infra-red (NIR) adopting the Astrometric Based Luminosity. We find indications that the slopes of the PL$(K_S)$ and PW$(J,K_S)$ relations for Galactic DCEPs might depend on metallicity when compared to the Large Magellanic Cloud relationships. Therefore, we have used a generalized form of the PL/PW relations to simultaneously take into account the metallicity dependence of the slope and intercept of these relations. We calculated PL/PW relations which, for the first time, explicitly include a metallicity dependence of both the slope and intercept terms. Although the insufficient quality of the available data makes our results not yet conclusive, they are relevant from a methodological point of view. The new relations are linked to the geometric measurement of the distance to the Large Magellanic Cloud and allowed us to estimate a {\it Gaia} DR2 parallax zero point offset $\Delta \varpi$=0.0615$\pm$0.004 mas from the dataset of DCEPs used in this work.