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To my family
I hereby declare that the work in this thesis is that of the candidate alone, except where indicated in the text, and as described below.
Chapter 2 is a modified version of the paper: ``Variable stars in and around the SMC cluster NGC 330'', Sebo K.M. and Wood P.R., 1994, AJ, 108, 932.
Chapter 3 is a modified version of the paper: ``Variable stars in Magellanic Cloud clusters II: NGC 1850'', Sebo K.M. and Wood P.R., 1995, ApJ, 449, 164.
Chapter 5 is a modified version of the paper: ``On the pulsation mode of Mira variables: Evidence from the LMC'', Wood P.R. and Sebo K.M., 1996, MNRAS, accepted.
Dr. Peter Wood contributed to the text of the above papers.
The Bump Cepheid models described in Section 3.8 were also computed by Dr Wood, as were the theoretical AGB models and LPV pulsation models in Sections 5.5 and 5.6.
Much of the early CCD data was obtained by Dr Wood. Almost all the data taken since 1991 was obtained by the candidate. The reduction of all CCD data, and the data analysis was undertaken by the candidate. The data analysis for Chapter 5 was shared with Dr Wood.
Kim Michael Sebo
August 1996
The production of this thesis has been made easier through the efforts of many people, to whom I am indebted.
Thanks to Hugo Giordano for dragging me away from the mountain occasionally for recreational purposes, and for reminding me of what happens out there in the real world. Simon Chan, the self-appointed social organiser, for the fantastic Friday night dinners, ``midnight'' coffees, and many other social events. Dave Fullagar, my office-mate for a long period, for being one of the few people with compatible tastes in office decor and office organisation strategy. Ben Stappers, for many enlightening discussions, some of them even related to astronomy, and Ron Hola, for making every day seem like a Friday, and adding a unique aspect to life on the mountain. I am grateful also to the numerous other people who have helped me out in various ways in recent times. A special thanks to Claudia McCain, for her companionship and encouragement, and for enriching my life in so many ways.
My gratitude also to the staff of MSO Computer Section, for their maintenance of the excellent computing facilities, and their prowess on the tennis court; and the technical staff at Siding Spring Observatory, who always managed to sort out the many glitches one encounters while observing.
I thank my supervisor, Peter Wood for his patience, persistence and guidance throughout my time here, and Mike Bessell for numerous valuable discussions.
Finally, I wish to acknowledge the financial support of an ANU scholarship and the Joan Duffield Scholarship.
This thesis reports the results of the analysis of a large body of CCD photometry of three Magellanic Cloud fields containing young clusters (NGC 330, NGC 1850 and NGC 2058-65). Long time series of VI photometry have been used to identify Cepheids and Long Period Variables (LPVs) in the clusters and the surrounding fields. A variety of other variables have also been discovered during the course of this analysis.
The Cepheids discovered are employed to investigate the Cepheid Mass discrepancy, namely, the unresolved differences between evolution and pulsation masses derived for Cepheid variables. It is shown that Cepheid masses derived from pulsation theory are inconsistent with masses computed using modern evolutionary models, unless convective core overshoot is incorporated into these models during the main-sequence phase of stellar evolution. For the low mass Cepheids studied in the Small Magellanic Cloud, low to moderate overshoot is sufficient to bring agreement between the evolution and pulsation masses, but for the more massive Large Magellanic Cloud Cepheids, a large degree of overshoot is required.
Analysis of periods and JK photometry of a sample of Large Magellanic Cloud LPVs has provided a new handle on the determination of the mode of pulsation of these objects. The LPVs appear to lie on two parallel, but separate, sequences which may be interpreted as being due to two distinct pulsation modes. Theoretical analysis suggests that the large-amplitude LPVs (Mira variables) of the LMC are pulsating in the fundamental mode. This method avoids the difficult requirement of determining the (ill-defined) stellar radius for the star. However, the sample of LPVs available for this study was relatively small so that the results are subject to some uncertainty. If the method can be applied to a larger sample of LPVs, the question of the modes of pulsation of these objects should be resolved unambiguously.