The potential for luminosity functions (LFs) of post-turnoff stars to constrain basic cluster parameters such as age, metallicity, and helium abundance is examined in this dissertation. A review of the published LFs for the globular cluster (GC) M92 suggests that the morphology of the transition from the main sequence to the red giant branch (RGB) is sensitive to these parameters. In particular, a small bump in this region may provide an important age discriminant for GCs. A significant deficiency in the number of stars over a 2 mag interval, just below the turnoff, remains unexplained.
A method of interpolating isochrones and LFs accurately from evolutionary sequences, from the lower main sequence to the RGB tip, is discussed. The interpolation scheme is based on primary interpolation points which are identified by the behaviour of the derivative [special characters omitted] along an evolutionary sequence.
New BV CCD observations, calibrated with Landolt and Graham standard stars, for the old open cluster NGC 2243 and for the bright stars in the GCs NGC 288 and NGC 7099 are presented. The colour magnitude diagram (CMD) of NGC 2243 contains a strong binary star component. Comparisons with the fiducial sequences of the GC 47 Tuc (Hesser et al. 1987) indicate that the two clusters have similar abundances, while comparisons with the new oxygen-enhanced isochrones (Bergbusch & VandenBerg 1992) suggest that NGC 2243 has an age of 4-5 Gyr, and a metallicity [special characters omitted]. The morphology of both the CMD and the LF through the turnoff region cannot be attributed to the merging of the binary and single star sequences, but convective overshooting works in the correct sense to account for the differences between the isochrones and the CMD.
For NGC 288 and NGC 7099, excellent overall consistency among the Zero-Age Horizontal Branch, isochrone, and LF fits is obtained for cluster ages of 14-16 Gyr. The manifestation of the transition bump in NGC 288's LF provides a particularly strong constraint on the age, since this feature becomes more prominent as the metallicity increases. R-method helium abundance estimates give Y ≈ 0.23 for NGC 288 and Y ≈ 0.31 for NGC 7099. The 2nd parameter problem is discussed in light of these results. The RGB bump, present in canonical LFs, is only weakly identified in the cumulative LF (CLF) of NGC 288, and may not be present at all in NGC 7099's CLF. However, the brightest RGB stars in both clusters are found within ≈ 0.2 mag of the RGB tip predicted by the oxygen-enhanced models. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/9575 |
| Date | 03 July 2018 |
| Creators | Bergbusch, Peter Anthony |
| Contributors | VandenBerg, Don A, |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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