view Abstract Citations (97) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Time-Constant Loci and Theoretical Luminosity Functions for Metal-Deficient Clusters Simoda, Mahiro ; Iben, Icko, Jr. Abstract We present a more detailed account of our calculations conceining the hydrogen-burning phases of halo Population II stars. Evolutionary model results are given for (Z; X) = (1(h3; 0.65, 0.90, and 1.0) and ( X 1(h , 10 0.65,0.80, and 1.0). Cox-Stewart opacity tables have been employed. We show that, apart from the well-known de endence on helium content, evolutionary tracks exhibit a significant dependence on Z, mainly throug the abundances of the CNO elements, particularly the abundance of 0. The CNO elements contribute to this dependence both through their role in the generation of nuclear energy and through their role in opacity. In view of the dominant importance of the CNO elements in effecting differences in the H-R diagram between metal-deficient clusters, we stress the need for more reliable CNO abundance determinations and suggest that the CNO abundance should be regarded as a primary distinguishing parameter in cluster classification. We resent time-constant loci and theoretical luminosity functions for all composition choices. On deman ng a best fit in the H-R diagram between cluster loci and theoretical time-constant loci, we find ages (t9> 25) that are considerably larger than those (t9 1() 18) obtained by (a) fitting the cluster main sequence to Groombridge 1830 or (b) using the observed magnitude separation between RR Lyrae and cluster-turnoff stars in conjunction with theoretical-observational estimates of RR Lyrae magnitudes and theoretical-observational estimates of the initial helium content (V 0.3). We suggest that the discrepancy may be substantially reduced by choosing an effective 1/H that increases with decreasing surface temperature or by choosing a larger envelope opacity than that which we have adopted. We suggest further that if all clusters are of roughiy the same age and helium content, then the turnoff luminosity for clusters with Z T5 X 1(h may be brighter by about 0.25 mag than that for clusters with Z the effective temperature at turnoff may be higher, but the slope of the subgiant branch may be essentially the same. We discuss evidence for and against the hypothesis of equal age and helium content for clusters. Publication: The Astrophysical Journal Supplement Series Pub Date: September 1970 DOI: 10.1086/190219 Bibcode: 1970ApJS...22...81S full text sources ADS | data products SIMBAD (16)