Chicago-Carnegie Hubble Program (CCHP) A Multi-Wavelength Search for the Effects of Metallicity on the Cepheid Distance Scale. Part II: Theoretical Models and Synthetic Spectra

TL;DR

This study uses theoretical models to analyze how metallicity variations affect Cepheid stars' spectral energy distributions across multiple wavelengths, clarifying their impact on distance measurements.

Contribution

It provides a detailed theoretical investigation of metallicity effects on Cepheid spectra, explaining the subtle influence on multi-wavelength period-luminosity relations.

Findings

Metallicity affects spectral energy distributions differently across wavelengths.

Effects are largest in the ultraviolet and reverse sign in the optical.

Metallicity impacts are small but systematic, explaining previous observational challenges.

Abstract

This is the second of two papers exploring the effects of metallicity on the multi-wavelength properties of Cepheids in terms of their multi-wavelength period-luminosity (PL) relations, impacting their use as extragalactic distance indicators, underpinning one of the most popular paths to estimating of the expansion rate of the Universe, Ho. In Paper I (Madore & Freedman 2024) we presented five tests for the influence of metallicity on galactic and extragalactic Cepheid PL relations, spanning nearly 2 dex in metallicity, and inspecting PL relations from the optical (BVI), through the near-infrared (JHK) and into mid-infrared (at 3.4 and 4.5 microns). And,in no case were any statistically significant results forthcoming. Here we interrogate published spectral energy distributions constructed from theoretical (static) stellar atmospheres, covering the surface gravity and temperature ranges attributed to classical (supergiant, F and K spectral type) Cepheid variables, and explore the differential effects of changing the atmospheric metallicity, down by 2 dex from solar (a factor of 100 below the average Milky Way value) and then up from solar by 0.5 dex (i.e., factor of 3x above the Milky Way value). The theoretical models clearly show that metallicity systematically impacts each of the bandpasses differentially: the level of this effect is largest in the ultraviolet (where line blanketing is most intense), reversing sign in the optical (due to flux redistribution from the UV), and then asymptotically falling back to zero from the red to the far infrared. The discovered effects of metallicity are systematic, but they are small; and as such they do not contradict the findings of Paper I, but they do explain why the problem has been so hard to resolve given the low level of precision of the photometry for all but the very nearest and apparently brightest Cepheids.