All Figures

$\begin{figure} \par\includegraphics[width=8.6cm,clip]{h4497f1.eps} \end{figure}$ Figure 1: Colour of the main sequence at an absolute magnitude M_V=6. The upper panel is for the Johnson BV photometry, the lower panel is for the Strömgren by photometry. Symbols represent individual unevolved subdwarfs (that is, stars with M_V>5.5) with parallax error $\Delta \pi /\pi <0.12$ . The lines are the model predictions by Straniero et al. (1997). Filled symbols are the field stars considered in this paper; open squares are additional unevolved subdwarfs, used as a sanity check (see text).
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In the text

$\begin{figure} \par\includegraphics[width=13cm,clip]{h4497f2.eps} \end{figure}$ Figure 2: Main Sequence Fitting distances to the program clusters (NGC 6397: upper panels; NGC 6752: middle panels; 47 Tuc: lower panels). For each cluster fits obtained both from Johnson B-V (left) and Strömgren b-y (right) colours are shown. The parameters adopted in these fits are listed in Tables 2 and 3. Data for individual bona fide single subdwarfs are shown with their error bars. Solid and dashed lines in the upper left panel are the cluster mean loci from Kaluzny (1997) and Alcaino et al. (1997), respectively. Filled symbols are the subdwarfs used to estimate the distance moduli (upper portion of Table 2); open squares are additional unevolved subdwarfs, used as a sanity check (see text).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{h4497f3.eps} \end{figure}$ Figure 3: Comparison between our absolute ages for the program clusters and the relative age determined by Rosenberg et al. (1999) with the horizontal method.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{h4497f4.eps} \end{figure}$ Figure 4: Values of the matter density parameter $\Omega _{\rm M}$ as a function of the Hubble constant H₀ in a flat Universe, obtained from a 10 000 random extraction of the values of the age of the Universe, assumed to be 0.3 Gyr older than the age of the oldest globular clusters ( $13.4\pm 1.4$ Gyr), and of H₀ determined by Spergel et al. (2003) from WMAP experiment and by the HST Key-Project (Freedman et al. 2001): $71\pm 4$ km s^-1 Mpc^-1. Different gray areas correspond to 67, 95 and 99% levels of confidence.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{h4497f5.eps} \end{figure}$ Figure 5: Age as a function of redshift z for standard $\Lambda$ CDM Universe models. The thick solid line represents the curve obtained for the best choice of parameters given by Spergel et al. (2003); the thin solid lines are obtained summing 1- $\sigma$ variations of the parameters (assumed to be independent to each other). Dashed lines are the 1- $\sigma$ lower limits to the age of the oldest (NGC 6397 and NGC 6752) and youngest globular clusters (47 Tuc).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{h4497f3.eps} \end{figure}$	Figure 3: Comparison between our absolute ages for the program clusters and the relative age determined by Rosenberg et al. (1999) with the horizontal method.
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