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Hubble Space Telescope (HST) fine guidance sensor observations were used toobtain parallaxes of eight metal-poor ([Fe/H] < -1.4) stars. The parallaxes ofthese stars determined by the revised Hipparcos reduction average 17% accuracy,in contrast to our new HST parallaxes which average 1% accuracy and have errorson the individual parallaxes ranging from 85 to 144 microarcsecond. Thisparallax data has been combined with HST ACS photometry in the F606W and F814Wfilters to obtain the absolute magnitudes of the stars with an accuracy of 0.02to 0.03 magnitudes. Six of these stars are on the main sequence (with -2.7 <[Fe/H] < -1.8), and suitable for testing metal-poor stellar evolution modelsand determining the distances to metal-poor globular clusters. Using theabundances obtained by O'Malley et al. (2017) we find that standard stellarmodels using the Vandenberg & Clem (2003) color transformation do a reasonablejob of matching five of the main sequence stars, with HD 54639 ([Fe/H] = -2.5)being anomalous in its location in the color-magnitude diagram. Stellar modelsand isochrones were generated using a Monte Carlo analysis to take into accountuncertainties in the models. Isochrones which fit the parallax stars were usedto determine the distances and ages of nine globular clusters (with -2.4 <=[Fe/H] <= -1.9$). Averaging together the age of all nine clusters, leads to anabsolute age of the oldest, most metal-poor globular clusters of 12.7+/- 1.0Gyr, where the quoted uncertainty takes into account the known uncertainties inthe stellar models and isochrones, along with the uncertainty in the distanceand reddening of the clusters.

Testing Metal Poor Stellar Models and Isochrones with HST Parallaxes of Metal Poor Stars