The uvbyHbeta Metallicity Calibration for G and K Dwarfs

Abstract

The metallicity dependence of the primary indices of the uvby photometric system for cooler dwarfs (Teff ~ 6500 K to 5000K) is investigated. The data base for the analysis is composed of the overlap between a composite catalog of selected, high-dispersion spectroscopic abundances for 1801 stars on the metallicity scale of Valenti and Fischer (2005) and a merged catalog of high-precision uvbyHbeta photometry for over 35,000 stars. While [Fe/H] for F dwarfs is best estimated from m_1, with a modest dependence on c_1 as expected, for hotter G dwarfs the pattern reverses and c_1 becomes the dominant index. For cooler G dwarfs and K stars, the c_1 dominance continues, but a discontinuity appears such that stars between (b-y) = 0.50 and 0.58 with [Fe/H] >= +0.25 have m_1 and c_1 indices that classify them as subgiants, confirming an earlier result based upon a much smaller sample. The reversal in the sensitivity to m_1 and c_1 is traced, in part, to the metallicity sensitivity of the (b-y) index. Moreover, (b-y) grows larger in a non-linear fashion for stars above solar metallicity, leading to an overestimate of the reddening for super-metal-rich stars from some standard intrinsic color relations. Based upon successful tests using indices from synthetic spectra and the empirical trends among the observations, metallicity calibrations tied to Hbeta rather than (b-y) have been derived for [Fe/H] >= -1.0, generating dispersions among the residuals ranging from 0.061 dex to 0.085 dex over the entire temperature range of interest. The new calibrations have the added advantage of being significantly less sensitive to errors in reddening than previous calibrations.