Cepheid Parallaxes and the Hubble Constant

TL;DR

This paper refines the measurement of the Hubble constant using Cepheid variable stars' parallaxes, confirming the universality of the period-luminosity relation and revising previous Hubble constant estimates to around 70 km/s/Mpc.

Contribution

It provides a new zero-point calibration for Cepheid relations and updates the Hubble constant, showing insensitivity to metallicity corrections and confirming consistency across different galaxies.

Findings

Hubble constant revised to 70+/-5 km/s/Mpc.

Cepheid period-luminosity relation coefficient is consistent across galaxies.

Distance modulus for LMC is approximately 18.45-18.52.

Abstract

Revised Hipparcos parallaxes for classical Cepheids are analysed together with 10 HST-based parallaxes (Benedict et al.). In a reddening-free V,I relation we find that the coefficient of logP is the same within the uncertainties in our Galaxy as in the LMC, contrary to some previous suggestions. Cepheids in the inner region of NGC4258 with near solar metallicities (Macri et al.) confirm this result. We obtain a zero-point for the reddening-free relation and apply it to Cepheids in galaxies used by Sandage et al. to calibrate the absolute magnitudes of SNIa and to derive the Hubble constant. We revise their result from 62 to 70+/-5 km/s/Mpc. The Freedman et al. 2001 value is revised from 72 to 76+/-8 km/s/Mpc. These results are insensitive to Cepheid metallicity corrections. The Cepheids in the inner region of NGC4258 yield a modulus of 29.22+/-0.03(int) compared with a maser-based modulus of 29.29+/-0.15. Distance moduli for the LMC, uncorrected for any metallicity effects, are; 18.52+/-0.03 from a reddening-free relation in V,I; 18.47+/-0.03 from a period-luminosity relation at K; 18.45+/-0.04 from a period-luminosity-colour relation in J,K. Adopting a metallicity correction in V,I from Marci et al. leads to a true LMC modulus of 18.39+/-0.05.