OLD STELLAR POPULATIONS .4. EMPIRICAL FITTING FUNCTIONS FOR FEATURES IN THE SPECTRA OF G-STARS AND K-STARS

Empirical fitting functions are presented that model the behavior of 1 1 strong atomic and molecular features in the spectra of Galactic G an d K stars. The functions express the strength of spectral features as a function of V - K, surface gravity, and metallicity. The gravity cal ibration rests on stars in Galactic globular and open clusters, for wh ich gravities have been derived by fitting to stellar evolutionary iso chrones. The range of application of the fitting functions is set by t he calibrating stars. There is good metallicity coverage from [Fe/H] = -0.75 to +0.50 dex, and cluster ages span 3.15 x 10(9) yr. However, t he range in age is large only near solar metallicity, and the gravity partial derivatives may therefore be uncertain at metallicities much h igher or lower than this value. In addition, the fitting functions bui ld in whatever abundance ratio trajectories of elements relative to ir on are present in local Galactic stars; these might differ from the el ement ratios present in external galaxies. In general, the behavior of the fitting functions matches expectations as to how these spectral f eatures should behave as a function of basic stellar atmospheric param eters. The exception is CN (4150 angstrom), for which we find strong e vidence for rapid onset of giant-branch mixing of CNO products to the stellar surface, setting in at V - K = 2.10. This confirms other recen t studies of this index. The paper also presents new Lick line strengt h data on 112 field dwarfs and 92 cluster stars, plus colors, metallic ities, and surface gravities for all Lick G and K stars published prev iously. The fitting functions are inverted to produce new output gravi ty and metallicity estimates for these stars and new average metallici ties for all clusters, with errors of +/- 0.15 dex in [Fe/H] and +/-0. 23 dex in log g per star.