A [ITAL]V[/ITAL] and [ITAL]I[/ITAL] CCD Mosaic Survey of the Ursa Minor Dwarf Spheroidal Galaxy

We discuss a Johnson-Cousins V -and I-band CCD mosaic survey of the Ursa Minor dwarf spheroidal galaxy (dSph) to V D 22. We covered UMi with 27 overlapping CCD frames, each 10@ .5 ] 10@ .5 frame consisting of two 300 s exposures in each of V and I. We also observed several regions D3 from UMi to obtain an estimate of contamination by galaxies and Galactic stars. We report the rst H-R diagram of an entire dSph. Separation of dwarf stars from foreground stars by color allows a robust estimation of the structural parameters of UMi. We examine earlier evidence of two lumps in the UMi stellar distribution. We detect a statistically signicant asymmetry in the stellar distribution of UMi along the major axis. Structure in the stellar distribution of UMi might indicate a tidal origin for UMis high observed mass-to-light ratio. We demonstrate a technique for obtaining the absolute magnitude of UMi from horizontal-branch star counts compared with M92 ; this method of luminosity estimation is independent of the distance to UMi. We obtain where if M92 is M V \ [8.87 ^pMV , p MV \ 0.14 a perfect calibrator and is an upper bound on the error arising from di erences in stellar p MV [ 0.35 populations. Our value for is consistent with earlier measurements and has a smaller uncertainty.