Isochrones, Ages, Curves of Evolutionary Deviation, and the Composite C-M Diagram for Old Galactic Clusters
Abstract
Isochrones in the }J-R diagram are computed in the range 4.5 »= Mb~r »= 2 by using models due to Aizenman, Demarque, and Miller, and to then. The curves apply for chemical compositior~s of X = 0.67, Z = 0.03 for the first set of models, and X = 0.708, Z = 0.020 for the second. The phase of hydrogen exhaustion on the evolving main sequence, and the shallow departure from the zero-age main sequence of tracks with 9)~/9J? o ~ 1.3 cause adjacent isochrones to intersect one another near Mb~I ~ +3 Observational data shown suggest that the predicted crossover effect may occur. New curves of evolutionary deviation are calculated from the isochrones. Distance moduli for M67 and NGC 188, derived with these curves, are used to assign absolute luminosities to the cluster stars. Comparison of the cluster sequences, converted to MbOl and log Te, with the isochrones, with account taken of the chemical composition, gives ages of (5.5 ± 0.5) X 1O~ years for M67 and (8-10) X 1O~ years for NGC 188. A composite C-M diagram for six galactic clusters shows several new features, notable of which is the funneling of all giant branches into a small region of the H-R diagram which may be defined by the Hayashi fully convective track
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 1969
- DOI:
- 10.1086/150229
- Bibcode:
- 1969ApJ...158..685S