Galactic Cepheid luminosity scale

Comparing MW cluster Cepheids to Cepheids in the LMC, we confirm these independent results and the adequacy of the L21 corrections for the cluster members in this range. Combining cluster and field Cepheids, we calibrate the Galactic Cepheid Leavitt law for a variety of individual optical and near-infrared passbands, as well as using reddening-free Wesenheit magnitudes based on Gaia and HST photometry, while solving for the residual offset applicable to Cepheid parallaxes, {Delta}{varpi}_Cep_.

The most direct comparison of our results with the SH0ES distance ladder yields excellent (0.3{sigma}) agreement for both the absolute magnitude of a 10d Solar metallicity Cepheid in the _IR HST Wesenheit magnitudes, M^W^_H_=-5.915+/-0.017mag, and the residual parallax offset, {varpi}_Cl_=-13+/-5uas. Despite the use of a common set of photometry, this is an important cross-check of the recent Hubble constant measurement by Riess et al. based on independently determined cluster membership and average parallaxes.

Allowing the LL slope to vary yields results consistent to within 1 and a slightly larger residual parallax offsets {Delta}{varpi}_Cep_=-17+/-5uas. Using the larger sample of 26 Gold cluster Cepheids and 225 MW Cepheids with recent Gaia DR3 astrometry and photometry, we determine the absolute magnitude of a 10d fundamental mode Cepheid in the optical Gaia Wesenheit magnitude at the sample average iron abundance of <[Fe=H]>=0.069, M^W^_G_=-6.051+/-0.020mag, and {Delta}{varpi}_Cep_=-22+/-3uas.

Correcting to Solar metallicity yields M^W^_G_=-5.996+/-0.019mag and {Delta}{varpi}_Cep_=-19+/-3uas. These results mark the currently most accurate absolute calibrations of the Cepheid luminosity scale based purely on observations of MW Cepheids, as well as the most precise determination of the residual Cepheid parallax offset at a significance of 6-7{sigma}.