A step toward the calibration of the red giant branch tip as a standard candle

The absolute I magnitude of the tip of the red giant branch (M-I(TRGD)) 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 (1) on the largest photometric database ever assembled for a globular cluster, by Pancino and colleagues, and (2) on a direct distance estimate for omega Cen, recently obtained by Thompson and colleagues from a detached eclipsing binary. The derived value M-I(TRGB) = -4.04 +/- 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] similar to -1.7. We also derive a new empirical M-I(TRGB)-[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] less than or equal to -0.7). The proposed relation is in excellent agreement with the newly determined zero point.