Globular cluster giant branches and the metallicity scale
Using the data base of Frogel, Persson, and Cohen and our earlier work, we derive characteristic parameters that describe the systematic behavior of the giants in each of 33 globular clusters. The globular cluster giant branches form a strictly homologous sequence in the H-R diagram, and their ordering correlates well with the metallicity scale published by Zinn. Nevertheless, when the giant branch parameters are used to define a metallicity ranking scheme, it appears that the Zinn abundance scale systematically underestimates the metallicity of clusters with exceptionally blue horizontal branches. The observed luminosities of the brightest giant in each cluster agree with the theoretical core helium flash luminosity; the small dispersion of these luminosities implies that at a given metallicity the intrinsic scatter in the absolute magnitude of horizontal-branch stars is less than 0.1 mag. The mean CO index increases as metallicity increases; analysis of the residuals from the mean relationships both of CO and of horizontal-branch type against metallicity imply that the CO abundance is unrelated to the second parameter problem. All metal-rich clusters with an adequate sample of stars show a spread in CO within each cluster comparable to that seen in 47 Tuc. Metal-poor clusters which exhibit a large star to star spread in CO are NGC 362, 2808, and 6656. The integrated light measurements of (V - K)_0 and CO published by Aaronson and colleagues correlate with the cluster parameters determined from measurements of individual stars in a manner that can be predicted from stellar evolutionary calculations for old populations. The integrated light of such old systems, at least from V to K, is therefore well understood.
Additional Information© American Astronomical Society • Provided by the NASA Astrophysics Data System. Received 1983 February 28; accepted 1983 May 24. CTIO is operated by AURA, Inc. under National Science Foundation contract AST 78-27879. J. A. F. wishes to thank Marshall Cohen and the Caltech Astronomy Department for their hospitality while on sabbatical leave during 1982 when this paper was prepared. Particular thanks go to Jeff Pier for use of his plotting routine and to Mike Lesser for instruction in use of the VAX 11/780 computer.
Published - 1983ApJ___275__773F.pdf