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 in Salaris & Cassisi (1997) are extended to higher metallicities, and compared with analogous independent results recently published. The comparison between our ZAHB and TRGB distance scales for galactic globular clusters presented in Salaris & Cassisi (1997) 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 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 by Madore & Freedman (1991), 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. The application of our TRGB distance scale to NGC3379, provides a new distance to the Leo I group, which coupled with recent independent determinations of the distance Coma cluster-Leo I, provides a determination of H_0 at the Coma cluster in the range: H_0= 60+-11 Km s^{-1} Mpc^{-1}.