AAS Meeting #193 - Austin, Texas, January 1999
Session 70. Star Formation and the ISM in Galaxies
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[70.05] Variable Stars in the Ursa Minor Dwarf Spheroidal Galaxy: A Story of Star Formation as told by the Horizontal Branch

P.L. Gay (Univ. of Texas, Austin)

Preliminary glass-plate surveys and small area ccd-observations of the Ursa Minor Dwarf Spheroidal Galaxy (dSph UMi - Agt 1968; Nemec 1988) suggest that this galaxy may have a small metallicity dispersion, and a globular cluster like RR Lyrae population. This raises the question: can the proto-galactic environment produce two morphologically different systems, globular clusters and dwarf spheroidal galaxies, with similar stellar populations? New photometry of dSph UMi, taken with the McDonald Observatory 30" Prime Focus Camera, has improved the period identification for 71 RR Lyrae stars and Anomalous Cepheids. We have compared the dSph UMi variable star population and several other stellar populations, namely: the classical Oosterhoff type I and II globular clusters (M3 and M15), a same metallicity globular cluster (M92), and a sample of Milky Way field RR Lyrae stars. We found that dSph UMi has an Oosterhoff type II period distribution, and a sufficiently low number of Anomalous Cepheids to explain by mass transfer. Assuming the variable star populations are tracers of the main population, we conclude that the stars in dSph UMi formed in a single star-formation event, and it is thus possible for dwarf spheroidal galaxies to have coeval stellar populations.

The author(s) of this abstract have provided an email address for comments about the abstract: pamela@astro.as.utexas.edu

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