Globular Clusters are the fossils of the remote Galaxy formation epoch, thus they can provide meaningful constrains to a few fundamental quantities (like for example the primordial helium abundance, the distance scale and, hence, the age scale) of primary cosmological impact. The OAB projects in this field are shortly described in what follows.
Involved people at OAB: Bragaglia, Clementini, Fusi Pecci.
The simplest and most robust technique for deriving distances to GCs is the Main Sequence Fitting technique: the comparison of the GC Main Sequence to a suitable "template" formed by the metal-poor subdwarfs in the solar neighborhood, whose distances are accurately measured via trigonometric parallaxes (Sandage 1970, ApJ, 492, 110). Once distances are known, ages follow from the absolute magnitude of the turn-off point (TO) which is the "stellar clock" for dating the clusters.
Extensively applied in the eighties, the method heavily suffered from the
lack of metal-poor subdwarfs with accurate trigonometric parallaxes, implying
derived distance moduli accurate to 
mag, and large error bars on
ages (15
4 Gyr, VandenBerg et al 1986, ARA&A, 34, 461). With the release
of Hipparcos parallax catalogue, in June 1997, it became possible to build
accurate subdwarf template sequences, with metallicities bracketing the CG
ones. Moreover, Hipparcos major observational result was that parallaxes for
the local subdwarfs are systematically smaller (by about 0.2 mag) than
ground-based measurements. This directly translated into a "stretching'' of
the globular cluster distances, and, in turn, in a 2-3 Gyrs decrease of their
ages (see Reid 1997, AJ, 114, 161; 1998, AJ, 115, 204; Pont et al. 1998, A&A,
329, 87; Gratton et al. 1997,
ApJ, 491, 749). The Hipparcos based MSF method definitely favored the
long distance scale, and the derivation of younger ages for the
Galactic GCs comfortably smaller than the age of the Universe.
In collaboration with
Carretta and Gratton (OAP), the study of the distances and ages
of the Galactic GCs via MSF based on Hipparcos trigonometric parallaxes
has been continued.
The MSF distances of a sample of Galactic GCs have been redetermined
using an
enlarged sample of subdwarfs which includes 95
of the metal-poor subdwarfs
in the full Hipparcos catalogue and whose metallicities have been determined
from abundance analysis of high resolution spectroscopic data purposely
acquired (Clementini et al. 1999, MNRAS, 302, 22). A
careful and comprehensive analysis of the corrections and statistical biases
which hamper the MSF distance derivations has been performed and the
residual total uncertainty still affecting the Hipparcos based MSF technique
has been estimated (
mag, to compare with the 
0.25 mag of the
pre-Hipparcos analyses).
An estimate of the lower limit for the age of the Universe has been derived
from the absolute age of the Galactic GCs of 12.9 Gyr, with a residual
uncertainty of 
2.9 Gyr [Carretta et al. (2000, ApJ, 533, 215)].
However, there is still a 0.2-0.3 mag difference between the long
distance scale derived from the MSF and the Cepheids, and the short
scale, mainly based on statistical parallaxes for RR Lyrae with some support
from the Baade Wesselink method.
Error bars are still large enough that a final choice between the two scales
cannot be made.
Besides parallaxes, a number of different ingredients and assumptions enter
into the MSF technique, which all contribute to the final result (i.e. the
derived distance moduli), and to its present accuracy (
0.12 mag). The
major contribution to the present MSF uncertainty arises from possible
systematic errors (at about 0.1 and 0.02 mag level, respectively) in the
reddening and metallicity scale adopted for the field subdwarfs and the GC
stars, with errorbars of 
0.07 mag from each source.
In order to cut the 0.12 mag residual uncertainty still affecting the MSF
distances down to 
mag (again an uncertainty dominated by the
parallax error) and the corresponding errors in the GC ages to 
1 Gyr,
devoted observations have been proposed for high
resolution spectroscopy of globular cluster stars with VLT-UVES
[in collaboration with Bonifacio and
Molaro (OATS), Carretta, Claudi, Desidera and Gratton (OAPD), Castellani
(Univ. Pisa), Chieffi (IAS-CNR), D'Antona (OAR), Francois and Pasquini
(ESO), Grundhal (DAO), Sneden (Univ. of Texas), Spite and Spite (Meudon), and
Straniero (OA Teramo)]. A total of 12 nights of observing time have been
obtained. The first 6 nights allotment is in June 2000 and will be devoted to
observations of NGC 6752 and NGC 6397.
Involved people at OAB: Ferraro, Fusi Pecci, Messineo.
The large database published in Ferraro et al. (1999, AJ, 118, 1738) has allowed to determine new homogeneous distance moduli for 61 GGCs by adopting the zero-age HB as standard candle. The results indicate that the new distance moduli are in agreement within 0.07 mag with the distance obtained by Carretta et al. (2000) based on Hipparcos, for cluster in the low-metallicity domain. Also that other features in the CMD of GGCs as the RGB-bump and the AGB-bump can be safely used as standard candle. [This research has been carried out in collaboration with M. Limongi (OAR), O. Straniero (OATe), A. Chieffi (CNR, Roma)].
High quality V and I
HST data have been obtained, analyzed and will soon published
for M92 one of the most metal poor GGCs and maybe the oldest one.
The high photometric accuracy obtained (error 
mag at the MS Turn Off)
will allow us to accurately date this GGCs and set a lower limit to the age of
the Galaxy formation epoch.
[This research has been carried out in collaboration with R.T. Rood (Univ. of
Virginia), R. Buonanno (OAR)].
Involved people at OAB: Ferraro, Fusi Pecci, Messineo.
The data-base presented in Ferraro et al. (1999, AJ, 118, 1738) will allow to quantitatively study the population ratios in GGCs to derive a direct estimate of the primordial Helium abundance using the R-method. This will represent a direct check of the Big-Bang explosive genesis of the Universe. [This research has been carried out in collaboration with M. Limongi (OAR) O. Straniero (OATe), A. Chieffi (CNR, Roma)].