Estimation of stellar metal abundance. II. A recalibration of the Ca II K technique, and the autocorrelation function method

We have recalibrated a method for the estimation of stellar metal abundance, parameterized as [Fe/H], based on medium-resolution (1-2 Å) optical spectra (the majority of which cover the wavelength range 3700-4500 Å). 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 spectrum synthesis and model atmosphere calculations, is compared with observations of 551 stars with high-resolution abundances available from the literature (a sevenfold increase in the number of calibration stars that were previously available). A second method, based on the Fourier autocorrelation function technique first described by Ratnatunga & Freeman, is used to provide an independent estimate of [Fe/H], as calibrated by comparison with 405 standard-star abundances. Metallicities based on a combination of the two techniques for dwarfs and giants in the color range 0.30 ≤ (B-V)₀ ≤ 1.2 exhibit an external 1 σ scatter of approximately 0.10-0.20 dex over the abundance range -4.0 ≤ [Fe/H] ≤ 0.5. Particular attention has been given to the determination of abundance estimates at the metal-rich end of the calibration, where our previous attempt suffered from a considerable zero-point offset. Radial velocities, accurate to approximately 10 km s^-1, are reported for all 551 calibration stars.