1998A&A...329...87P

A new determination of the distance and age of the old globular cluster M 92 (NGC 6341) is obtained from a consideration of a set of more than 500 subdwarf candidates with Hipparcos parallaxes. Precise [Fe/H] values are derived for most stars using the equivalent width of the Coravel cross-correlation function. We examine at some length the biases affecting the determination of the mean luminosity of a set of subdwarfs selected by metallicity and σ_π/π<lim criteria, by means of Monte Carlo simulations of the whole selection and analysis procedure. Effects other than the classic Lutz-Kelker bias are found to be significant, so that in most cases considered, the resulting bias acts as a slight shift in a direction opposite to the Lutz-Kelker correction. The effect of the presence of detected (and suspected) binaries is also examined and taken into account. Our best estimate of the distance of M 92 is (m-M)_V=14.67±0.08mag, from a fit of the cluster main sequence to the 17 subdwarfs in our set with [Fe/H]←1.8 and σ_π/π<15%. Bias and binarity corrections are included. The adoption of an alternative [Fe/H] scale causes only minor differences in this result. A more classic treatment of binaries (i.e., simply excluding the detected binaries from the sample) leads to (m-M)_V=14.74mag. The location of the evolved field subdwarfs (along the subgiant branch) provides a strong indication that M 92 and the most metal-poor subdwarfs are coeval. The M 92 C-M diagram is confronted with up-to-date stellar evolutionary models (recent opacities and nuclear reaction rates, non-ideal equation of state, α-element enhancement). The agreement between theory and observations is excellent, including the position of the horizontal branch, if the models are shifted by δ(B-V)= +0.012mag. An age of 14Gyr is derived from the luminosities of the cluster turnoff and subgiant branch stars. A similar colour shift is found from an examination of an independent set of subdwarfs having -1.8<[Fe/H]←1.1 and very accurate parallaxes (σ_π/π<5%). Because the inferred adjustment to the synthetic colours seems to be nearly constant over a fairly wide range in metallicity, we conclude that current theoretical models predict the systematic variation in the location of the lower main sequence as a function of [Fe/H] reasonably well. As discussed herein, a greater distance (and younger age) for M 92 would pose considerable difficulties for stellar structure theory. Thanks to more precise globular cluster (GC) distances, the dominant source of uncertainty in GC ages now becomes associated with the model stellar interiors and atmospheres. In particular, helium diffusion, which was not treated in the present models, is expected to lead to about a 10% age reduction, while the use of an alternative bolometric correction scale would imply higher ages by ∼1.5Gyr. If GCs formed about 1 Gyr after the Big Bang, then we obtain a minimum age for the Universe of at least 14 Gyr, confirming the necessity of rather low values of H₀ in the context of standard cosmological models.