The weak-G-band effect in M13, M92, NGC 6397, and M15.

A spectroscopic and photometric survey of 122 giants in Ml 3, M92, N0C 6397, and Ml 5 has revealed (or in some cases confirmed) the existence of 27 weak-0-band stars. No CH or CN stars have been discovered. Our data show: (i) 0-band weakness is not accompanied by abnormalities in heavy element abundances (Ca, Fe). (ii) The effect is dominant on the asymptotic giant branch (A0B); 67% of our A0B sample have very weak 0 bands. (iii) There is a range in 0-band strength below the luminosity at which the A0B and the giant branch merge. The 0-band weakening, however, is not as extreme as on the A0B. (iv) No range in 0-band strength is apparent within 0.8 mag of the tip of the giant branch. Our results strengthen the belief that the weak- 0-band phenomenon results from the mixing of CNO processed material to the surface, and not from primordial abundance variations. While standard stellar evolution computations do not predict the observed effects, it appears that only minor modifications, of the type considered by Dearborn et al. (mass loss, thermal instability), are necessary on the lower giant branch. The degree of 0-band weakening on the A0B, however, probably requires a radical departure. Mixing at the helium core flash is an attractive possibility, but is not clearly supported by the available spectroscopic observations of blue-horizontalbranch stars. A closer theoretical examination of post-horizontal-branch evolution could prove worthwhile. Finally, we suggest that the apparent lack of weak-0-band stars at the tip of the giant branch results from substantial mass loss at, or following, the helium flash. Subject headings: clusters: globular - stars: abundances - stars: evolution