Distances and Absolute Ages of Galactic Globular Clusters from Hipparcos Parallaxes of Local Subdwarfs

Abstract

High precision trigonometric parallaxes from the HIPPARCOS satellite and accurate metal abundances ([Fe/H], [O/Fe], and [alpha/Fe]) from high resolution spectroscopy for about 30 local subdwarfs have been used to derive distances and ages for a carefully selected sample of nine globular clusters. We find that HIPPARCOS parallaxes are smaller than the corresponding ground-based measurements leading, to a longer distance scale (~ 0.2 mag) and to ages ~ 2.8 Gyr younger. The relation between the zero age horizontal branch (ZAHB) absolute magnitude and metallicity for the nine programme clusters is : Mv(ZAHB) = (0.22 +/- 0.09)([Fe/H]+1.5) + (0.48 +/- 0.04) The corresponding Large Magellanic Cloud distance modulus is (m-M)o=18.61 +/- 0.07. The age of the bona fide old globular clusters (Oosterhoff II and Blue Horizontal Branch) based on the absolute magnitude of the turn-off is: Age = 11.8 (+2.1/-2.5) Gyr The present age of globular clusters does no longer conflict with standard inflationary models for the Universe.