2001ApJ...556..635B

The absolute I magnitude of the tip of the red giant branch (M^TRGB_I) is one of the most promising standard candles actually used in astrophysics as a fundamental pillar for the cosmological distance scale. With the aim of improving the observational basis of its calibration, we have obtained an accurate estimate of the M^TRGB_I for the globular cluster ω Centauri, based (1) on the largest photometric database ever assembled for a globular cluster, by Pancino and colleagues, and (2) on a direct distance estimate for ω Cen, recently obtained by Thompson and colleagues from a detached eclipsing binary. The derived value M^TRGB_I=-4.04±0.12 provides, at present, the most accurate empirical zero point for the calibration of the M^TRGB_I-Fe/H relation at Fe/H~-1.7. We also derive a new empirical M^TRGB_I-[Fe/H] relation based on the large IR data set of red giants in Galactic globular clusters recently presented by Ferraro and colleagues. This database (extending up to [Fe/H]=-0.2) covers a more appropriate metallicity range for extragalactic applications than previous empirical calibrations (which were limited to [Fe/H]≤-0.7). The proposed relation is in excellent agreement with the newly determined zero point.