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Studies of stellar populations : star clusters in M31, the galaxy and the Magellanic clouds


Rabin, Douglas (1981) Studies of stellar populations : star clusters in M31, the galaxy and the Magellanic clouds. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/962q-x238.


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This study focusses on medium resolution spectra of the integrated light of star clusters in our own Galaxy and in three nearby systems: M31 and the two Magellanic Clouds. Since only integrated properties will normally be observable for clusters in more distant galaxies, or for the galaxies themselves, it is important to develop quantitative techniques which relate features of the integrated light to physical characteristics such as age and chemical composition. Here, digital SIT spectra are analyzed both internally, as multivariate data sets, and externally, by comparison with evolutionary models of star clusters and with detailed observations of nearby clusters.

For 22 clusters in M31, direct comparison of the equivalent widths of various metallic features in the wavelength region [...]3800-5300 shows that they determine a one-dimensional sequence in cluster metallicity. However, a robust principal components analysis reveals that the Balmer lines ([...], [...], [...]) behave distinctively, both in the Galaxy and in M31. For galactic globular clusters, the independence of the hydrogen lines shows up as a poor correlation between the strength of the Balmer lines in the integrated spectrum and the observed distribution of stars along the horizontal branch. Synthesis models are used to evaluate possible explanations, none wholly satisfactory. Compared with spectra of standard stars, the M31 spectra provide a clue which may bear on the problem in both galaxies: all the Balmer lines are contaminated by metallic features, and this is advanced as the cause of the enhanced [...] strength in strong lined M31 clusters. In some cases the enhancement is seen directly.

It is speculatively suggested that the M31 clusters exhibit in integrated light a mild form of the "hyperstrong line" phenomenon ascribed to some stars and to the nuclei of some elliptical galaxies.

Balmer lines are also central to the interpretation of the spectra of 17 "red" clusters in the Magellanic Clouds, but now as a manifestation of stars at the turnoff in the HR diagram and hence as direct indicators of age. In these spectra, the strength of the Ca II K line is well correlated with the strength of the G band, but the relation between K and <H>, the median Balmer line strength, scatters widely. The K-<H> diagram is interpreted with the guidance of a (metallicity) sequence of galactic globular clusters and an age sequence of model clusters. It is shown that the K-<H> diagram maintains sensitivity to turnoffs which are impracticable to reach by conventional methods, and thus that this technique resolves some outstanding questions of relative age, and to some extent metallicity, raised by previous analyses of color-magnitude diagrams.

It is found that the strength of the spectrum break at CaK correlates poorly with the strength of metallic features but correlates well with a break in the slope of the spectrum near the G band. The cause of this additional degree of freedom is not known.

Finally, I trace the steps in modifying theoretical isochrones and in combining them with empirical and theoretical stellar data to produce integrated colors, integrated spectra, and theoretical color-magnitude diagrams. The model colors are compared with observations. Also, apart from the details of stellar evolution, it is shown that strict limits on the allowed colors of composite models may be derived solely from properties of the stellar synthesis library.

A brief analysis of the color-magnitude diagrams provides no support for an age-abundance correlation among globular clusters. However, the models strongly exclude ages less than 1.3 x10[...] y and thus conflict with ages implied by standard Friedman models having [...] (particularly if the universe is dense, [...] 0.2).

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Astronomy
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Searle, Leonard T.
Group:Astronomy Department
Thesis Committee:
  • Unknown, Unknown
Defense Date:23 September 1980
Record Number:CaltechETD:etd-09102008-084100
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3432
Deposited By: Imported from ETD-db
Deposited On:18 Sep 2008
Last Modified:16 Apr 2021 22:16

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