Analysis of a Proper-Motion–Selected Sample of Stars in the Ursa Minor Dwarf Spheroidal Galaxy

We have studied the stellar population and internal structure of the core ofthe Ursa Minor dwarf spheroidal galaxy, using a sample of stars selected to bemembers based on their proper motions. In agreement with previous studies, wefind Ursa Minor to be dominated by an old, metal-poor stellar population. Asmall number of stars with high membership probabilities lie redward of the redgiant branch. The brightest (V <= 18) such stars are known to be Carbon stars,rather than metal-rich first-ascent giants. A number of stars with highmembership probabilities lie blueward of the red giant branch, and are moreluminous than the horizontal branch. We speculate that these arepost-horizontal branch stars. There may also be one or two stars in thepost-AGB phase. Spectroscopy of the candidate post-HB and post-AGB stars isrequired to determine their nature. We recover the internal substructure inUrsa Minor that has been noted by several authors in the last 15 years. Using avariety of two- and three-dimensional statistical tests, we conclude that thissubstructure is statistically significant at the 0.005 level. There is noevidence that the regions of density excess have stellar populations thatdiffer from the main body of Ursa Minor. The crossing time for a typicaldensity excess is only ~5 million years. They are therefore clearly not due tointermediate age star-forming bursts. We conclude that they are instead due totidal interactions between the Galaxy and Ursa Minor.Comment: LaTeX with AASTeX style file, 22 pages with 7 figures. Accepted for publication in The Astronomical Journal (Dec. 2001