On the age difference between the oldest Population I and the extreme Population II stars

The age difference between the oldest Population I stars and the extreme Population II stars is investigated by comparison of observational H-R diagrams with theoretical results using the common input physics and computer program for both Populations. Chemical compositions adopted for Population I and II stars are,X=0.7 andZ=0.02, andX=2×10^–4, respectively. Evidence collected indicates that the open cluster NGC 188 and the globular cluster M92 are the representative samples of the oldest Population I and extreme Population II stars, respectively. Comparison between the observed H-R diagrams and theoretical isochrones in terms of the luminosity of the subgiant region and the turnoff point for NGC 188 and M92, respectively, then suggests that there is a significant age difference between Population I and II objects. The uncertainty of the oxygen abundance in the extreme Population II stars and its effect on their age determination is briefly discussed.     

The luminosity function for the globular cluster M15

We obtained the luminosity function from the Main-Sequence turnoff region to the subgiants for M15 (V=17.0–19.6), using plates taken by the Kitt Peak 4 m reflector. The measured region isr=2.5–8.0 zone. We extrapolated the star counts to the cluster center and to the tidal limit, using King's (1966) photometry, Kinget al.'s (1968) counts, Peterson and King's (1975) result, and Simoda and Iben's (1970) theoretical results.The comparison of the observed function with the theoretical ones by Simoda and Iben (1970) indicates that the observation is consistent with the theory forX0.9±0.1, thus marginally inconsistent with the Big Bang cosmology value (X=0.76). The comparison with Bertelliet al. (1970) theoretical ones gives contradictory results. Possible causes of this problem are the improper extrapolation of star counts to the cluster center and/or the inaccuracies of the theoretical luminosity functions so far published, and/or the impropriety of the canonical structure and evolution theory of Population II stars. In the last two cases, the ages and the helium abundances of globular clusters may vary.We also obtained the luminosity function for blue horizontal branch stars fromV=17.0 to 19.6.Kitt Peak National Observatory, U.S.A., which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

On the number ratio of horizontal branch stars to red giant stars in globular clusters

The number ratios of horizontal branch stars to red giant stars were obtained for globular clusters and Draco dwarf galaxy and the helium abundance was estimated using model results without semiconvection zone (SCZ) and with fully developed one. The analysis was confined to the four clusters (M4, M5, M13, and 47 Tuc) and the Draco galaxy, for which fairly precise star counts had been carried out. The effect of the difference in radial distribution between horizontal and red giant branch stars were taken into account, if necessary. The statistically significant difference inR exists among these objects. The cause may be the difference in the helium abundance and/or in the development of the SCZ. In the case of the fully developed SCZ, the helium abundance for M5 and Draco is appreciably smaller than the value given by the big-bang cosmology. It may be taken as an evidence against the full development of the SCZ for the horizontal branch stars in these objects.