WFPC2 Observations of the Ursa Minor Dwarf Spheroidal Galaxy

We present our analysis of archival Hubble Space Telescope Wide FieldPlanetary Camera 2 (WFPC2) observations in F555W (~V) and F814W (~I) of thecentral region of the Ursa Minor dwarf spheroidal galaxy. The V versus V-Icolor-magnitude diagram features a sparsely populated blue horizontal branch, asteep thin red giant branch, and a narrow subgiant branch. The main sequencereaches ~2 magnitudes below the main-sequence turnoff (V_TO ~ 23.27 +- 0.11mag) of the median stellar population. We compare the fiducial sequence of UrsaMinor with the fiducial sequence of the Galactic globular cluster M92 (NGC6341). The excellent match between Ursa Minor and M92 confirms that the medianstellar population of the UMi dSph galaxy is metal poor ([Fe/H]_UMi ~[Fe/H]_M92 ~ -2.2 dex) and ancient (age_UMi ~ age_M92 ~ 14 Gyr). The B-Vreddening and the absorption in V are estimated to be E(B-V) = 0.03 +- 0.01 magand A_V = 0.09 +- 0.03 mag. A new estimate of the distance modulus of UrsaMinor, (m-M)_0 = 19.18 +- 0.12 mag, has been derived based on fiducial-sequencefitting with M92 and the adoption of the apparent V distance modulus for M92 of(m-M)_V = 14.67 +- 0.08 mag (Pont et al. 1998, A&A, 329, 87). These HSTobservations indicate that Ursa Minor has had a very simple star formationhistory consisting mainly of a single major burst of star formation about 14Gyr ago which lasted <=2 Gyr. If the ancient Galactic globular clusters, likeM92, formed concurrently with the early formation of the Milky Way galaxyitself, then the Ursa Minor dwarf spheroidal is probably as old as the MilkyWay.