AGES OF GLOBULAR-CLUSTERS FROM HIPPARCOS PARALLAXES OF LOCAL SUBDWARFS

We report here initial but strongly conclusive results for absolute ag es of Galactic globular clusters (GGCs). This study is based on high-p recision trigonometric parallaxes from the HIPPARCOS satellite coupled with accurate metal abundances ([Fe/H], [O/Fe], and [alpha/Fe]) from high-resolution spectroscopy for a sample of about thirty subdwarfs. S ystematic effects due to star selection (Lutz-Kelker corrections to pa rallaxes) and the possible presence of undetected binaries in the samp le of bona fide single stars are examined, and appropriate corrections are estimated. They are found to be small for our sample. The new dat a allow us to reliably define the absolute location of the main sequen ce (MS) as a function of metallicity. These results are then used to d erive distances and ages for a carefully selected sample of nine globu lar clusters having metallicities determined from high-dispersion spec tra of individual giants according to a procedure totally consistent w ith that used for the field subdwarfs. Very precise and homogeneous re ddening values have also been independently determined for these clust ers. Random errors for our distance moduli are +/-0.08 mag, and system atic errors are likely of the same order of magnitude. These very accu rate distances allow us to derive ages with internal errors of similar to 12% (+/-1.5 Gyr). The main results are: 1. HIPPARCOS parallaxes ar e smaller than corresponding ground-based measurements, leading, in tu rn, to longer distance moduli (similar to 0.2 mag) and younger ages (s imilar to 2.8 Gyr). 2. The distance to NGC 6752 derived from our MS fi tting is consistent with that determined using the white dwarf cooling sequence. 3. The relation between the zero-age HB (ZAHB) absolute mag nitude and metallicity for the nine program clusters is M-v(ZAHB) = (0 .22 +/- 0.09)([Fe/H] + 1.5) + (0.49 +/- 0.04). This relation is fairly consistent with some of the most recent theoretical models. Within qu oted errors, the slope is in agreement with that given by the Baade-We sselink (BW) analysis of RR Lyrae stars by Fernley and Clementini et a l., 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 correspond ing LMC distance modulus is (m - M)(o) = 18.60 +/- 0.07, in good agree ment with the recent values of 18.70 +/- 0.10 and 18.54 +/- 0.2 derive d by Feast & Catchpole and van Leeuwen et al., respectively, from HIPP ARCOS 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 indicat or) is Age = 11.8(-2.5)(+2.1) Gyr, where the error bar is the 95% conf idence range. The rms scatter of individual ages around the mean value is similar to 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 a ges requires the use of age indicators better suited to this purpose a nd data for a larger sample of GGCs. 6. Allowing for a minimum delay o f 0.5 Gyr from the birth of the universe until the formation of globul ar clusters, our age estimate is compatible with an Einstein-de Sitter model if H-0 less than or equal to 64 km s(-1) Mpc(-1), or H-0 less t han or equal to 83 km s(-1) Mpc(-1) in a flat universe with Omega(m) = 0.2. Since these upper limits are well within the confidence range of most determinations of H-0, we conclude that the present age of globu lar clusters does not conflict with standard inflationary models of th e universe.