ESTIMATION OF STELLAR METAL ABUNDANCE .1. CALIBRATION OF THE CA-II K-INDEX

We have calibrated a method for estimating stellar metal abundance, parametrized as [Fe/H], based on medium-resolution (1 Å) optical spectra. The equivalent width of the Ca II K line (3933 Å) as a function of [Fe/H] and broadband B - V color, as predicted from spectral synthesis and model atmosphere calculations, is compared with observations of 239 stars with abundances available from the literature. Corrections to predicted abundances from the models are derived from a residual fit to 80 standard stars with high-resolution spectroscopic metallicity determinations. Metallicities based on the Ca II K line strength for dwarfs and giants in the color range 0.33≤(B - V)0≤0.85 exhibit a 1σ scatter of approximately 0.15 dex over the abundance range - 4.5≤[Fe/H]≤ - 1.0. Improved estimates of [Fe/H] are reported for 108 stars (from the lists of Bidelman and MacConnell and Sandage and Fouts) where only modest-accuracy photometric abundances were previously available. Reliable metal abundances for five nearby Galactic globular clusters are derived from observations of member stars (typically 4-9 stars per cluster). We obtain: M4: [Fe/H] = - 1.31 (- 1.36, - 1.28); M92: [Fe/H] = - 2 12 (- 2.31, - 1.97); NGC 3201: [Fe/H] = - 1.55 (- 1.64, - 1.40); NGC 6397: [Fe/H] = - 2.10 (- 2.38, - 1.80); NGC 6752: [Fe/H] = - 1.22 (- 1.28, - 1.11). Radial velocities, accurate to ≤10 km s-1, are reported for all 239 stars we observe.