Globular Clusters are the fossils of the remote Galaxy formation epoch, thus they can provide meaningful constraints to a few fundamental quantities (like for example the primordial helium abundance, and the distance scale, hence the age scale of the Universe) of primary cosmological impact. The OAB projects in this field are shortly described below.
The measure of age differences between galactic globulars is a
powerful tool to reconstruct the earliest phases of the formation of
the Milky Way. We have performed a specific test to estimate the age
difference between two very important clusters, NGC 288 and NGC 362,
that have very similar chemical composition but very different
Horizontal Branch morphology. We applied a refined version of the so
called bridge test (Stetson, Vandenberg & Bolte 1996)
that makes use of a cluster with bimodal HB morphology, in
the present case NGC 1851, to match the otherwise incompatible HBs of
NGC 288 and NGC 362. Adopting the above technique on a very
homogeneous database we provided a very robust estimate of
Gyr, independent of distance and
reddening and supported by recent detailed abundance analysis. It is
interesting to note that though NGC 362 formed a couple of Gyr later
than NGC 288, both clusters have 
elements abundances enhanced
by the same amount with respect to the Sun. This indicates that the
two clusters were born in former galactic sub-units that had an
independent star formation history and chemical evolution.
The data-base presented in Ferraro et al. (1999) 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 is being carried out in collaboration with Limongi (OAR), Straniero (OATe), and Chieffi (CNR, Roma).
The problem of a better determination of the age of GGCs is strictly connected with a better determination of their distance. Once distances are known, ages follow from the absolute magnitude of the TO which is the "stellar clock" for dating the clusters. Different standard candles can be used in order to derive the distance to GGCs. Here we list some of the method adopted by different groups at the OAB:
(I) The white dwarf cooling sequence
Involved people at OAB: Bragaglia, Ferraro.
The principal source of uncertainty of the absolute age of GGCs
is mainly due to the large uncertainty (
mag, for
the pre-Hipparcos era) in
measuring the distance modulus of these objects. OAB researchers are
collaborating to an HST project (a total of 
orbits have been
allocated) which uses a new standard candle, the White Dwarf cooling
sequence, in order to determine the distance to a sample of GGCs. A
first result has been published in Renzini et al. (1996):
for the very fist time the distance to
NGC 6752 was measured with an uncertainty of only 0.1
mag. This result showed that there is the possibility to derive the
age of GGCs, hence a significant lower limit to the age of the
Universe, with an accuracy better than 1 Gyr.
We are now extending this study to other nearby GGCs: recently the procedure
was applied to 47 Tuc and a distance modulus of
and an
age of 
Gyr were derived (Zoccali et al. 2001, ApJ, in press).
(II) The main sequence fitting technique
Involved people at OAB: Bragaglia, Clementini, Fusi Pecci.
One of the simplest and most robust technique for deriving distances to GCs is the Main Sequence Fitting technique: the GC Main Sequence is compared to a suitable "template" formed by metal-poor subdwarfs in the solar neighborhood, whose distances are accurately measured via trigonometric parallaxes.
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 errorbars on
ages (
4 Gyr). With the release
of the 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, 1998; Pont et al. 1998; Gratton et al. 1997).
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.
(a) In collaboration with Carretta and Gratton (OAPd), 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.
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.
(b) Besides parallaxes, a number of different ingredients and assumptions enter
into the MSF technique, which all contribute to its present accuracy
(0.12 mag). The major contribution arises from possible systematic errors
(at about 0.1 and 0.02 mag level, respectively) in the reddening and
metallicity scales adopted for the field subdwarfs and the GC stars, with
errorbars of 0.07 mag from each source.
An ESO Large Programme (PI Gratton) is trying to cut the uncertainty affecting
the MSF distances down to 
mag (i.e., an uncertainty dominated by the
parallax error) and the corresponding errors in the GC ages to 1 Gyr, by
addressing these effects. In 2000, we obtained high resolution spectra
(R
40000, with UVES at VLT2, 12 nights in total) of a large number of
stars at the main sequence turn-off and at the base of the sub-giant branch of
NGC6752 and NGC6397, and another 6 nights have been allocated to extend the
spectroscopic analysis to NGC104, and to the two very metal poor clusters NGC
6809 and NGC 7099.
The abundance analysis of the NGC6752 and NGC6397 stellar spectra has
demonstrated that in both clusters the [Fe/H]'s obtained for the TO-stars
agree perfectly (within a few per cents) with that obtained for stars at the
base of the RGB (Gratton et al. 2001, A&A, in press, and ESO Press Release
03/01), thus removing one of the possible major sources of uncertainty claimed
to affect the MSF distances.
This part of the project is conducted in collaboration with Bonifacio, Centurion and Molaro (OATs), Carretta, Claudi, Desidera, Gratton and Lucatello (OAPd), Castellani (Univ. Pisa), Chieffi (CNR, Roma), D'Antona (OAR), Francois and Pasquini (ESO), Grundhal (DAO), Sneden (Univ. Texas), Spite (Meudon), and Straniero (OATe).
(III) The theoretical ZAHB level
Involved people at OAB: Ferraro, Fusi Pecci.
The large database published in Ferraro et al. (1999) has allowed us 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 clusters in the low-metallicity domain. Also other features in the CMD of GGCs, like the RGB-bump and the AGB-bump can be safely used as standard candles. This part of the research has been carried out in collaboration with Limongi (OAR), Straniero (OATe), and Chieffi (CNR, Roma).
High quality V and I HST data have been obtained, analyzed, and will
soon be 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 to accurately date this GGC and
set a lower limit to the age of the Galaxy formation epoch. This part of the
research has been carried out in collaboration with Rood
(Univ. Virginia), Buonanno and Andreuzzi (OAR).