A step toward the calibration of the RGB Tip as a Standard Candle

The absolute I magnitude of the Tip of the Red Giant Branch M_I(TRGB) 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_I(TRGB) for the globular cluster Omega Centauri, based (a) on the largest photometric database ever assembled for a globular, by Pancino et al.(2000), and (b) on a direct distance estimate for Omega Cen, recently obtained by Thompson et al.(2001) from a detached eclipsing binary. The derived value M_I(TRGB) =-4.04\pm 0.12 provides, at present, the most accurate empirical zero-point for the calibration of the M_I(TRGB) - [Fe/H] relation, at [Fe/H] ~ -1.7. We also derived a new empirical M_I(TRGB) - [Fe/H] relation, based on the large IR dataset of red giants in Galactic Globular Clusters recently presented by Ferraro et al.(2000). This database (extending up to [Fe/H]=-0.2) covers a more appropriate metallicity range, for extragalactic applications, than previous empirical calibrations (limited to [Fe/H]<= -0.7). The proposed relation is in excellent agreement with the newly determined zero-point.