High-resolution Spectroscopic Metallicities of Milky Way Cepheid Standards and Their Impact on the Leavitt Law and the Hubble Constant

Milky Way Cepheid variables with accurate Hubble Space Telescope photometry have been established as standards for primary calibration of the cosmic distance ladder to achieve a percent-level determination of the Hubble constant (H ₀). These 75 Cepheid standards are the fundamental sample for investigation of possible residual systematics in the local H ₀ determination due to metallicity effects on their period-luminosity relations. We obtained new high-resolution (R ∼ 81,000), high-signal-to-noise (S/N ∼ 50-150) multiepoch spectra of 42 out of 75 Cepheid standards using the ESPaDOnS instrument at the 3.6 m Canada-France-Hawaii Telescope. Our spectroscopic metallicity measurements are in good agreement with the literature values with systematic differences up to 0.1 dex due to different metallicity scales. We homogenized and updated the spectroscopic metallicities of all 75 Milky Way Cepheid standards and derived their multiwavelength (GVIJHK _s ) period-luminosity-metallicity and period-Wesenheit-metallicity relations using the latest Gaia parallaxes. The metallicity coefficients of these empirically calibrated relations exhibit large uncertainties due to low statistics and a narrow metallicity range (Δ[Fe/H] = 0.6 dex). These metallicity coefficients are up to 3 times better constrained if we include Cepheids in the Large Magellanic Cloud and range between −0.21 ± 0.07 and −0.43 ± 0.06 mag dex⁻¹. The updated spectroscopic metallicities of these Milky Way Cepheid standards were used in the Cepheid-supernovae distance ladder formalism to determine H ₀ = 72.9 ± 1.0 km s⁻¹ Mpc⁻¹, suggesting little variation (∼0.1 km s⁻¹ Mpc⁻¹) in the local H ₀ measurements due to different Cepheid metallicity scales.