A Step toward the Calibration of the Red Giant Branch Tip as a Standard Candle

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Abstract
The absolute I magnitude of the tip of the red giant branch (M) 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 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 = -4.04 ± 0.12 provides, at present, the most accurate empirical zero point for the calibration of the M- relation at ~ -1.7. We also derive a new empirical M-[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.
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