A new analysis of the red giant branch 'tip' distance scale and the value of the Hubble constant

Abstract

The theoretical evaluations of the red giant branch tip (TRGB) luminosity presented previously by Salaris &38; Cassisi are extended to higher metallicities, and compared with analogous independent results recently published. The present sets of stellar models agree quite well in the determination of the TRGB brightness. Relations between TRGB bolometric and l (Cousins) magnitude and zero-age horizontal branch (ZAHB) V magnitude with respect to the metallicity are provided by adopting empirical, semi-empirical and theoretical evaluations of bolometric corrections, after a careful calibration of the zero-point of the bolometric correction scales. The comparison between our ZAHB and TRGB distance scales for galactic globular clusters presented in Paper I is now supplemented with a comparison with the Hipparcos distance scale set by local subdwarfs with accurate parallax determinations. The overall agreement between ZAHB and Hipparcos distances is quite good. The TRGB distances for globular clusters are compatible with the ZAHB distances in the limit of the small sample of red giants observed. The ZAHB and TRGB distances to resolved galaxies are in good agreement, whereas the comparison between TRGB and Cepheid distances, computed by using the calibration suggested earlier by Madore &38; Freedman, reveals a systematic discrepancy of the order of 0.12 mag. The TRGB distances are systematically longer in comparison with the Cepheid ones. This result supports the case for a revision of the zero-point of the Cepheid distance scale, as already suggested by other authors on the basis of Hipparcos parallaxes. We do not find any clear correlation of the difference between TRGB and Cepheid distances with metal content. The application of our TRGB distance scale to NGC 3379 provides a distance to the Leo I group that is about 8 per cent higher than the one obtained recently by Sakai et al. adopting the TRGB-metallicity calibration by Lee, Freedman &38; Madore. Our distance to the Leo I group, coupled with recent independent determinations of the Coma cluster-Leo I distance, obtained differentially by means of secondary distance indicators, provides a determination of H0 at the Coma cluster in the range H0 = 60 ± 11 km s−1 Mpc−1. For choices of Ω in agreement with the observations (0.3 ≤ Ω ≤ 1) and cosmological constant equal to zero, our derived H0 value is compatible with the most recent determinations of galactic globular cluster ages, thus removing the long-standing conflict between the Hubble age and the age of the oldest stars in the Galaxy.