Ages of Globular Clusters fromHIPPARCOSParallaxes of Local Subdwarfs

Abstract

We report here initial but strongly conclusive results for absolute ages of Galactic globular clusters (GGCs). This study is based on high-precision trigonometric parallaxes from the HIPPARCOS satellite coupled with accurate metal abundances ([Fe/H], [O/Fe], and [α/Fe]) from high-resolution spectroscopy for a sample of about thirty subdwarfs. Systematic effects due to star selection (Lutz-Kelker corrections to parallaxes) and the possible presence of undetected binaries in the sample of bona fide single stars are examined, and appropriate corrections are estimated. They are found to be small for our sample. The new data allow us to reliably define the absolute location of the main sequence (MS) as a function of metallicity. These results are then used to derive distances and ages for a carefully selected sample of nine globular clusters having metallicities determined from high-dispersion spectra of individual giants according to a procedure totally consistent with that used for the field subdwarfs. Very precise and homogeneous reddening values have also been independently determined for these clusters. Random errors for our distance moduli are ±0.08 mag, and systematic errors are likely of the same order of magnitude. These very accurate distances allow us to derive ages with internal errors of ~12% (±1.5 Gyr). The main results are: 1. HIPPARCOS parallaxes are smaller than corresponding ground-based measurements, leading, in turn, to longer distance moduli (~0.2 mag) and younger ages (~2.8 Gyr). 2. The distance to NGC 6752 derived from our MS fitting is consistent with that determined using the white dwarf cooling sequence. 3. The relation between the zero-age HB (ZAHB) absolute magnitude and metallicity for the nine program clusters is This relation is fairly consistent with some of the most recent theoretical models. Within quoted errors, the slope is in agreement with that given by the Baade-Wesselink (BW) analysis of RR Lyrae stars by Fernley and Clementini et al., while it is somewhat shallower than the relation given by Sandage. The zero-point is 0.2 to 0.3 mag brighter than that obtained with BW, while it agrees fairly well with that given by Sandage. A comparison with alternative relationships is briefly discussed. 4. The corresponding LMC distance modulus is (m - M)0 = 18.60 ± 0.07, in good agreement with the recent values of 18.70 ± 0.10 and 18.54 ± 0.2 derived by Feast & Catchpole and van Leeuwen et al., respectively, from HIPPARCOS parallaxes of Galactic Cepheid and Mira variables. 5. The age of the bona fide old globular clusters (Oosterhoff II and BHB), based on the absolute magnitude of the turnoff (a theoretically robust indicator) is where the error bar is the 95% confidence range. The rms scatter of individual ages around the mean value is ~10%, in agreement with expectations from observational errors alone (that is, we do not find it necessary to introduce a real age scatter among these clusters). A reliable study of the relative ages requires the use of age indicators better suited to this purpose and data for a larger sample of GGCs. 6. Allowing for a minimum delay of 0.5 Gyr from the birth of the universe until the formation of globular clusters, our age estimate is compatible with an Einstein-de Sitter model if H0 ≤ 64 km s-1 Mpc-1, or H0 ≤ 83 km s-1 Mpc-1 in a flat universe with Ωm = 0.2. Since these upper limits are well within the confidence range of most determinations of H0, we conclude that the present age of globular clusters does not conflict with standard inflationary models of the universe.