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C. Palma, S. R. Majewski, M. H. Siegel, R. J. Patterson (UVa Astronomy)
We have conducted a >10 degrees2 survey centered on the Ursa Minor dSph using Washington M and T2 filters and the stellar gravity-sensitive DDO51 filter. Using the Majewski et al.\ (2000) technique, we map the radial distribution of candidate UMin RGB stars that: (1) lie in the expected giant region in the M - T2, M - DDO51 plane, and (2) lie within a narrow RGB region defined in the CMD using a proper motion-selected sample. We also utilize the very obvious blue HB population to explore the extended spatial distribution of UMin stars.
We have constructed radial profiles for UMin using both BHB stars and RGB stars. Both give virtually identical results, which indicates that our RGB selection process is as efficient in eliminating contaminating field dwarf and giant stars as is the BHB selection process even though there are far fewer potential contaminants in the BHB color-magnitude range than there are in the RGB color-magnitude range. A King profile of 60' tidal radius fits the Umin core, but beyond ~60' the density appears to follow a shallow power law decay. Our King tidal radius is consistent with the 51' value of Irwin & Hatzidimitriou (1995), but differs with the 34' value of Kleyna et al.\ (1998); however, the latter survey only extended to a radius of ~35'.
We identify a significant population of candidate Umin RGB stars (~20% of 800 identified) and candidate UMin BHB stars (~15% of 450 identified) that lie outside the tidal radius of UMin and to the 2.5 degree radial limit of our survey. The distribution of candidate extratidal stars (which includes a number of the best-detected UMin RGB candidates) appears azimuthally isotropic. Our data suggest that either UMin presently has a prodigious mass loss rate, or it has an extensive halo population.
We acknowledge support for this research from NSF CAREER Award grant AST 97-02521, the David and Lucile Packard Foundation, and the Research Corporation.
The author(s) of this abstract have provided an email address for comments about the abstract: cp4v@virginia.edu