Cepheid parallaxes and the Hubble constant

Abstract

ABSTRACT Revised Hipparcos parallaxes for classical Cepheids are analysed together with 10 Hubble Space Telescope (HST)-based parallaxes. In a reddening-free V, I relation we find that the coefficient of log P is the same within the uncertainties in our Galaxy as in the Large Magellanic Cloud (LMC), contrary to some previous suggestions. Cepheids in the inner region of NGC 4258 with near solar metallicities confirm this result. We obtain a zero-point for the reddening-free relation and apply it to the Cepheids in galaxies used by Sandage et al. to calibrate the absolute magnitudes of Type Ia supernova (SNIa) and to derive the Hubble constant. We revise their result for H0 from 62 to 70 ± 5 km s-1 Mpc-1. The Freedman et al. value is revised from 72 to 76 ± 8 km s-1 Mpc-1. These results are insensitive to Cepheid metallicity corrections. The Cepheids in the inner region of NGC 4258 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 Macri et al. leads to a true LMC modulus of 18.39 ± 0.05. © 2007 RAS.