Cyanogen strengths and ultraviolet excesses of evolved stars in 17 globular clusters from DDO photometry.
We have observed 145 evolved stars in 17 globular clusters with the intermediate-band DDO photometric system which measures ultraviolet excesses and CN band strengths. The results, summarized in IVa, may be succinctly stated as follows: 1.The CN strength index of giant stars within a given globular cluster can vary more than it does in controlled Population I samples. 2.The DDO colors provide a natural ranking of the globular clusters, a ranking we believe to be by [Fe/H]; however, some inconsistencies in the original calibration relating (3842), the ultraviolet excess, with [Fe/H] have been uncovered now that the sample includes metal-rich globular clusters. The only exceptions to the general ranking are the peculiar objects Pal 5, Draco, and Ursa Minor. Table 5 provides estimates of [Fe/H] based on the DDO photometric ranking. 3.Sufficient similarities exist to suggest that M22 may share many of the w Cen anomalies; in particular, the giants of both clusters show a wide range of ultraviolet excesses and CN strengths. 4.Metal-rich stars from w Cen and M22 behave similarly to 47 Tuc stars in a diagram correlating observed CN strength and ultraviolet excess on the DDO system. 5.We suggest that ROA stars 234, 430, 3091, and 3898 of w Cen are good candidates for a very metal-poor component, perhaps representative of an early stage of star formation, within the cluster. 6.Both the failure of the C-M diagrams of metal-rich globulars to superpose with those of old disk objects and the difference in CN index behavior between Population II and Population I clusters leads us to suspect that our understanding of the chemical transition from halo to disk population is incomplete. 7.A systematic survey of the spectra of globular cluster stars is urgently needed; we believe that even low-resolution spectra of statistically significant samples within a single cluster may prove to be useful. Brief discussions of these observations are presented in IV. An appendix provides B, V photoelectric data for 37 giant stars in w Cen and 24 in M22. From the available photoelectric data we find that the M22 giant-branch slope implies a metallicity higher than that found either from AS of the RR Lyrae stars or broad-band color determinations for giants on the Washington system; hence we conclude that M22 very probably has an anomalous C-M diagram. Subject headings: clusters: globular - photometry - stars: abundances - stars: evolution - stars: late-type - stars: weak-line