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 .
( 22 ) , and ( b ) on a direct distance estimate for \omega Centauri , recently obtained by Thompson et al .
( 29 ) 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 ] \sim - 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 .
( 11 ) .
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 ] \leq - 0.7 ) .
The proposed relation is in excellent agreement with the newly determined zero-point . Based on data taken at the European Southern Observatory , Chile , as part of the ESO observing programmes 62.L-0354 , 63.L-0349 and 65.L-0463 .