Publication: Old stellar populations .4. Empirical fitting functions for features in the Spectra of G and K stars
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1993-05
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University Chicago Press
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Empirical fitting functions are presented that model the behavior of 11 strong atomic and molecular features in the spectra of Galactic G and K stars. The functions express the strength of spectral features as a function of V - K, surface gravity, and metallicity. The gravity calibration rests on stars in Galactic globular and open clusters, for which gravities have been derived by fitting to stellar evolutionary isochrones. The range of application of the fitting functions is set by the 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, the range in age is large only near solar metallicity, and the gravity partial derivatives may therefore be uncertain at metallicities much higher or lower than this value. In addition, the fitting functions build in whatever abundance ratio trajectories of elements relative to iron are present in local Galactic stars; these might differ from the element ratios present in external galaxies. In general, the behavior of the fitting functions matches expectations as to how these spectral features should behave as a function of basic stellar atmospheric parameters. The exception is CN (4150 Å), for which we find strong evidence for rapid onset of giant-branch mixing of CNO products to the stellar surface, setting in at V - K = 2.10. This confirms other recent studies of this index. The paper also presents new Lick line strength data on 112 field dwarfs and 92 cluster stars, plus colors, metallicities, and surface gravities for all Lick G and K stars published previously. The fitting functions are inverted to produce new output gravity and metallicity estimates for these stars and new average metallicities for all clusters, with errors of ±0.15 dex in [Fe/H] and ±0.23 dex in log g per star.
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© 1993. The American Astronomical Society.
We would like to thank Eileen Friel for her help on earlier stages of this work, Jay Frogel for unpublished V- K photometry, John Laird for providing bis G dwarf list in advance of publication, and Ron Marzke for assistance. Guy Worthey assisted in the gravity analysis in Appendix A, and a very careful referee made numerous suggestions that greatly improved the presentation. Gerri McLellan and Pat Shand ably prepared the manuscript and tables. J.G. would like to thank Lick Observatory for its hospitality. This work was partially supported by National Science Foundation grants AST 76-08258, 82-11551, and 87-02899 and by ASU Faculty Grants-in-Aid to D.B. J. G. was supported by a fellowship from the Fundación del Amo of the Universidad Complutense.
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