D. Butler

Carbon and nitrogen abundances in giant stars of the metal-poor globular cluster M92

Zinn in 1973 and 1977 and Norris and Zinn in 1977 showed that in M92 and several other metal-poor globular clusters the G bands (mostly due to CH) in the spectra of asymptotic giant branch (AGB) stars are systematically weaker than those found in the less highly evolved subgiant branch (SGB) stars. If carbon is depleted in the atmospheres of evolved stars because material at the base of the envelope, processed through the CN cycle, has been mixed with the material above, then the atmospheric nitrogen abundance should be correspondingly increased. In this paper we test the hypothesis that C and N abundances in M92 giants are negatively correlated as the evolutionary state becomes more advanced. We find that this simple hypothesis is not adequate to describe the complex behavior of C and N in the cluster giants.

Metal abundances of RR Lyrae variables in selected galactic star fields. II. The Lick astrograph fields near the North Galactic Pole

Revised periods, light curves, and metal abundances (Preston ..delta..ss and corresponding (Fe/H) values) are given for nearly 40 RR Lyrae variables of the Lick North Galactic Pole survey. The most distant stars lie at a height z above the galactic plane of 20--25 kpc. The data give no evidence for composition gradients in the RR Lyraes apart from the composition difference between the variables in the halo ( =7.7, =-1.5) and the disk ( =5.0, =-1.0). We show from a statistical argument that the sample of stars is too small to detect an abundance gradient over the 20-kpc interval in z unless the space density of RR Lyraes in Oosterhoff group I falls off about four times faster than the RR Lyraes of group II. Difficulties in inferring the halo abundance gradient for common subdwarfs, based on studies of halo RR Lyraes, are cited. It is not clear whether the paucity of RR Lyraes with (Fe/H)< or approx. =-2.2 is related to a corresponding lack of extremely metal-poor subdwarfs or due to an effect of stellar evolution.

Carbon and Nitrogen Abundances in Giant Stars of the Metal Poor Globular Cluster M92 — A Preliminary Report

Observational data on the C12/C13 and O/H-abundance ratios in red giants with near solar metal abundance have recently been put forward by Lambert, his associates and students. In summarizing these results, Dearborn, Eggleton, and Schramm (1976) concluded that in the atmospheres of pre-He-core flash red giants there is evidence for substantial depletion of C12 and enhancement of C13 presumably as a result of deep mixing of envelope material into layers that have undergone CNO-cycle processing. Moreover, values of the ratio C12/C13 in the range 10–20 seem to be achieved by stars with luminosities too low, and evolutionary states too early, for compatibility with convectively mixed standard stellar models. It is true, however, as recognized by Dearborn, et al., that red giants of the general star field have checkered backgrounds: giants in the same part of the HR diagram may have quite different masses and ages, even for a fixed initial metal-abundance. Moreover, comparison of a given star with theoretical models may be rendered somewhat uncertain by errors in parrallax or other criteria of absolute magnitude.