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On the age difference between the oldest Population I and the extreme Population II stars
Authors:Hideyuki Saio  Yukio Shibata  Mahiro Simoda
Institution:(1) Astronomical Institute, Faculty of Science, Tôhoku University, Sendai, Japan;(2) Research Institute for Scientific Measurements, Tôhoku University, Sendai, Japan;(3) Dept. of Astronomy and Earth Science, Tokyo Gakugei University, Koganei, Tokyo, Japan
Abstract: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.
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