We report here initial but still strongly conclusive results on 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 [ \alpha /Fe ] ) from high resolution spectroscopy for a sample of about thirty subdwarfs .
Systematic effects due to star selection ( Lutz-Kelker corrections to parallaxes ) and to 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 allowed 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 of our distance moduli are \pm 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 \sim 12 \% ( \pm 1.5 Gyr ) .

The main results are :

• HIPPARCOS parallaxes are smaller than the corresponding ground-based measurements leading , in turn , to longer distance moduli ( \sim 0.2 mag ) and younger ages ( \sim 2.8 Gyr ) .

• The distance to NGC6752 derived from our MS-fitting is consistent with that determined using the white dwarf cooling sequence .

• The relation between the zero age HB ( ZAHB ) absolute magnitude and metallicity for the nine programme clusters turns out to be :

M _ { V } ( ZAHB ) = ( 0.22 \pm 0.09 ) ( { [ Fe / H ] } +1.5 ) + ( 0.49 \pm 0.04 ) 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 Lyraes ( Fernley 1994 , Clementini et al .
1995 ) , while it is somewhat shallower than the relation given by Sandage ( 1993 ) .
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 .
Comparison with alternative relationships is briefly discussed .

• The corresponding LMC distance modulus is ( m - M ) _ { 0 } = 18.60 \pm 0.07 , in good agreement with the recent values of 18.70 \pm 0.10 and 18.54 \pm 0.2 derived by Feast and Catchpole ( 1997 ) and van Leeuwen et al .
( 1997 ) , respectively , from HIPPARCOS parallaxes of Galactic Cepheid and Mira variables .

• The age of the bona fide old globular clusters ( Oosterhoff II and BHB ) based on the absolute magnitude of the turn-off , a theoretically robust indicator , is :

{ Age } = { 11.8 ^ { +2.1 } _ { -2.5 } } { Gyr } ( where the error bar is the 95 % confidence range ) .
The r.m.s .
scatter of individual ages around the mean value is \sim 10 \% , in agreement with expectations from observational errors alone ( that is , we do not find necessary to introduce a real age scatter amongst these clusters ) .
A reliable study of the relative ages requires the use of age indicators better suited for this purpose and data for a larger sample of GGCs .

• Allowing for a minimum delay of 0.5 Gyr from the birth of the Universe before the formation of globular clusters , our age estimate is compatible with an Einstein-de Sitter model if H _ { 0 } \leq 64 km s ^ { -1 } Mpc ^ { -1 } , or H _ { 0 } \leq 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 globular clusters does not conflict with standard inflationary models for the Universe .