A new calibration of the RGB Tip as a Standard Candle
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We have obtained an accurate estimate of the absolute I magnitude of the Tip of the Red Giant Branch (M_I(TRGB)) for the globular cluster Omega Cen, based {\em (a)} on the largest photometric database ever assembled for a globular, by Pancino et al. (2000), and {\em (b)} on a direct distance estimate for \omen, 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 \mtip -- [Fe/H] relation, at [Fe/H] ~ -1.7.
6 pages, 2 .ps figures, latex, hrd_pasp.sty macro included. To appear in "Observed HR diagrams and stellar evolution: the interplay between observational constraints and theory", (Coimbra, June, 2001) ASP Conf. Ser., 2001, T. Lejeune and J. Fernandes, eds
6 pages, 2 .ps figures, latex, hrd_pasp.sty macro included. To appear in "Observed HR diagrams and stellar evolution: the interplay between observational constraints and theory", (Coimbra, June, 2001) ASP Conf. Ser., 2001, T. Lejeune and J. Fernandes, eds