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The uvbyHβ Metallicity Calibration for G and K Dwarfs
The metallicity dependence of the primary indices of the uvbyphotometric system for cooler dwarfs (Te ~ 6500-5000 K) isinvestigated. The database for the analysis is composed of the overlapbetween a composite catalog of selected, high-dispersion spectroscopicabundances for 1801 stars on the metallicity scale of Valenti &Fischer and a merged catalog of high-precision uvbyHβ photometryfor over 35,000 stars. While [Fe/H] for F dwarfs is best estimated fromm1, with a modest dependence on c1 as expected,for hotter G dwarfs the pattern reverses, and c1 becomes thedominant index. For cooler G dwarfs and K stars, the c1dominance continues, but a discontinuity appears such that stars betweenb - y = 0.50 and 0.58 with [Fe/H] >= +0.25 have m1 andc1 indices that classify them as subgiants, confirming anearlier result based on a much smaller sample. The reversal in thesensitivity to m1 and c1 is traced, in part, tothe metallicity sensitivity of the b - y index. Moreover, b - y growslarger in a nonlinear fashion for stars above solar metallicity, leadingto an overestimate of the reddening for super-metal-rich stars from somestandard intrinsic color relations. Based on successful tests usingindices from synthetic spectra and the empirical trends among theobservations, metallicity calibrations tied to Hβ rather than b - yhave been derived for [Fe/H] >= -1.0, generating dispersions amongthe residuals ranging from 0.061 to 0.085 dex over the entiretemperature range of interest. The new calibrations have the addedadvantage of being significantly less sensitive to errors in reddeningthan previous calibrations.
| The chemical evolution of the solar neighborhood. I - A bias-free reduction technique and data sample
The possible ways of measuring the age-metallicity relation for thegalactic disk in the neighborhood of the sun are discussed. It is shownthat the use of a field star sample chosen on the basis of effectivetemperature introduces a bias which results in a monotonic increase inthe metal abundance of the disk with time. However, if theage-metallicity relation for the disk can be shown to satisfy certaincriteria, the bias introduced in such a sample can be neglected: thegalactic disk apparently satisfies the criteria. It is concluded that asample analyzed through the use of uvby and H(beta) photometry inconjunction with a self-consistent set of theoretical isochronesprovides the least biased, most accurate estimate of the age-metallicityrelation for the disk.
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