Revising the Milky Way Cepheid Calibration: Quantifying and Correcting for Previously Undetected Distance Modulus Errors in the Gaia-based Multi-Wavelength Period-Luminosity Relations

TL;DR

This paper revises the calibration of Cepheid period-luminosity relations using multi-wavelength data, correcting for Gaia parallax errors, and improves distance measurements crucial for cosmology.

Contribution

It introduces a physically motivated correction method for Gaia-based Cepheid distances, significantly enhancing the precision of the period-luminosity relation.

Findings

Achromatic residuals indicate parallax errors dominate scatter.

Revised parallaxes improve precision by a factor of two.

No significant metallicity dependence found.

Abstract

We examine the multi-wavelength period-luminosity-color relations for Cepheid variables in the Large and Small magellanic Clouds and the Milky Way. From first-principles stellar physics, the luminosity of a Cepheid is determined by its radius and surface temperature, yielding a fundamental PLC relation whose observational proxies are pulsation period and intrinsic color. Using Cepheids in the Magellanic Clouds, we show that the PLC relation recovers the known geometries and line-of-sight tilts of their disks, confirming its ability to detect true distance-modulus variations that are achromatic and consistent across all filters. Surprisingly, for Milky Way Cepheids with individually determined reddenings and HST and Gaia parallaxes, the residuals from multi-wavelength PL fits are also found to be achromatic, identical in sign and amplitude across all passbands, in this case indicating that parallax errors are the dominant source of scatter. Applying bandpass-averaged corrections to individual Cepheids recovers the theoretically expected wavelength-dependent narrowing of the instability strip, and results in revised parallaxes with a median improvement in precision of roughly a factor of two. In addition, they show no statistically significant correlation with metallicity over the range -0.2 < Fe/H < 0.05 dex. The final extinction- and reddening-corrected PLC relation yields an rms scatter of 0.04 mag, corresponding to 2 percent precision in distance per star. Use of a physically grounded PLC will provide a more robust foundation for the Cepheid-based extragalactic distance scale and the determination of the Hubble constant.