HIPPARCOS Subdwarfs and Globular Clusters: Towards Reliable Absolute Ages

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Scientific paper

The distance and age of the key globular cluster M92 are determined with Hipparcos field subdwarfs. [Fe/H] values for subdwarfs we remeasured with the Coravel spectrometer. The distance modulus of M92 is obtained by fitting the cluster sequence to subdwarfs of a similar metallicity, and the age by comparison of the turnoff magnitude with up-to-date evolution models. Particular attention is given to systematic and selection biases, and to the effect of binaries. Field subdwarfs evolved past the turnoff stage are also considered. Our best estimate of the distance modulus of M92 is mu_0=14.61 +/- 0.08 mag, using 17 Hipparcos subdwarfs with [Fe/H] < -1.8. The age obtained is 14 +/- 1.2 Gyr, implying an age of at least 14 Gyr for the Universe. The value of the small colour shift needed in the models to fit the M92 sequence is confirmed with an independent set of intermediate-metallicity Hipparcos subdwarfs.

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