A NEW ANALYSIS OF THE RED GIANT BRANCH TIP DISTANCE SCALE AND THE VALUE OF THE HUBBLE CONSTANT

The theoretical evaluations of the red giant branch tip (TRGB) luminos ity presented previously by Salaris & Cassisi are extended to higher m etallicities, 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 bolometr ic and I (Cousins) magnitude and zero-age horizontal branch (ZAHB) V m agnitude with respect to the metallicity are provided by adopting empi rical, semi-empirical and theoretical evaluations of bolometric correc tions, 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 supp lemented with a comparison with the Hipparcos distance scale set by lo cal subdwarfs with accurate parallax determinations. The overall agree ment between ZAHB and Hipparcos distances is quite good. The TRGB dist ances for globular clusters are compatible with the ZAHB distances in the limit of the small sample of red giants observed. The ZAHB and TRG B distances to resolved galaxies are in good agreement, whereas the co mparison between TRGB and Cepheid distances, computed by using the cal ibration suggested earlier by Madore & Freedman, reveals a systematic discrepancy of the order of 0.12 mag. The TRGB distances are systemati cally longer in comparison with the Cepheid ones. This result supports the case for a revision of the zero-point of the Cepheid distance sca le, as already suggested by other authors on the basis of Hipparcos pa rallaxes. We do not find any clear correlation of the difference betwe en TRGB and Cepheid distances with metal content. The application of o ur TRGB distance scale to NGC 3379 provides a distance to the Leo I gr oup that is about 8 per cent higher than the one obtained recently by Sakai et al, adopting the TRGB-metallicity calibration by Lee, Freedma n & Madore, Our distance to the Leo I group, coupled with recent indep endent determinations of the Coma cluster-Leo I distance, obtained dif ferentially by means of secondary distance indicators, provides a dete rmination of H-0 at the Coma cluster in the range H-0 = 60 +/- 11 km s (-1) Mpc(-1). For choices of Omega in agreement with the observations (0.3 less than or equal to Omega less than or equal to 1) and cosmolog ical constant equal to zero, our derived H-0 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 ag e of the oldest stars in the Galaxy.