[ITAL]Hipparcos[/ITAL] Subdwarf Parallaxes: Metal-rich Clusters and the Thick Disk

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Abstract
We have used main-sequence fitting to calibrate the distances to the globular clusters NGC 6397, M5, NGC 288, M71, and 47 Tucanae, matching the cluster photometry against data for subdwarfs with precise Hipparcos parallax measurements and accurate abundance determinations. Both the cluster and subdwarf abundance scales are tied to high-resolution spectroscopic analyses. The distance moduli that we derive for the five clusters are 12.24, 14.52, 15.00, 13.19, and 13.59 mag, with uncertainties of ±0.15 mag. As in previous analyses by Reid and Gratton et al., these distances are higher than those derived in pre-Hipparcos investigations. The calibrated cluster color-magnitude diagrams provide fiducial sequences in the (MV, B - V)-plane, outlining the distribution expected for stars of a particular abundance. We have combined the photometric data for NGC 6397 ([Fe/H] = -1.82 dex), M5 (-1.10 dex), and 47 Tucanae (-0.70 dex) with the mean color-magnitude relation delineated by nearby FGK dwarfs to define a reference grid in the (MV, B - V)-plane, and we have matched this grid against data for stars drawn from the Lowell Proper Motion Survey, with both Hipparcos astrometry and abundance determinations by Carney et al. Limiting the comparison to nonbinaries, there are significantly fewer subluminous stars than expected given the spectroscopic metallicity distribution. Inverting the analysis, this implies a reduction by a factor of 3 in the proportion of stars contributing to the metal-poor ([Fe/H] < -0.4) tail of the Galactic disk. We discuss the implications of these results.