The iron and oxygen content of LMC Classical Cepheids and its implications for the extragalactic distance scale and Hubble constant

Equivalent width analysis with Kurucz stellar atmosphere models^⋆,⋆⋆

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85478 Garching bei München, Germany
e-mail: martino.romaniello@eso.org
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
⁴ Institute of Physics, Laboratory of Astrophysics, École Polytechnique Féderale del Lausanne (EPFL), Observatoire de Suverny, 1290 Versoix, Switzerland
⁵ LMU München, Universitätssternwarte, Scheinerstrasse 1, München 81679, Germany
⁶ Institute for Astronomy, University of Hawaii at Manoa, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁷ Mitchell Institute for Fundamental Physics and Astronomy, Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA
⁸ Dipartimento di Fisica e Astronomia Augusto Righi, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy

Received: 14 October 2021
Accepted: 8 November 2021

Abstract

Context. Classical Cepheids are primary distance indicators and a crucial stepping stone in determining the present-day value of the Hubble constant H₀ to the precision and accuracy required to constrain apparent deviations from the ΛCDM Concordance Cosmological Model.

Aims. We measured the iron and oxygen abundances of a statistically significant sample of 89 Cepheids in the Large Magellanic Cloud (LMC), one of the anchors of the local distance scale, quadrupling the prior sample and including 68 of the 70 Cepheids used to constrain H₀ by the SH0ES program. The goal is to constrain the extent to which the luminosity of Cepheids is influenced by their chemical composition, which is an important contributor to the uncertainty on the determination of the Hubble constant itself and a critical factor in the internal consistency of the distance ladder.

Methods. We derived stellar parameters and chemical abundances from a self-consistent spectroscopic analysis based on equivalent width of absorption lines.

Results. The iron distribution of Cepheids in the LMC can be very accurately described by a single Gaussian with a mean [Fe/H] = −0.409 ± 0.003 dex and σ = 0.076 ± 0.003 dex. We estimate a systematic uncertainty on the absolute mean values of 0.1 dex. The width of the distribution is fully compatible with the measurement error and supports the low dispersion of 0.069 mag seen in the near-infrared Hubble Space Telescope LMC period–luminosity relation. The uniformity of the abundance has the important consequence that the LMC Cepheids alone cannot provide any meaningful constraint on the dependence of the Cepheid period–luminosity relation on chemical composition at any wavelength. This revises a prior claim based on a small sample of 22 LMC Cepheids that there was little dependence (or uncertainty) between composition and near-infrared luminosity, a conclusion which would produce an apparent conflict between anchors of the distance ladder with different mean abundance. The chemical homogeneity of the LMC Cepheid population makes it an ideal environment in which to calibrate the metallicity dependence between the more metal-poor Small Magellanic Cloud and metal-rich Milky Way and NGC 4258.