We present a B,V color-magnitude diagram ( CMD ) of the Milky Way dwarf satellite Ursa Major II ( UMa II ) , spanning the magnitude range from V \sim 15 to V \sim 23.5 mag and extending over a 18 \times 18 arcmin ^ { 2 } area centered on the galaxy .
Our photometry goes down to about 2 magnitudes below the galaxy ’ s main sequence turn-off , that we detected at V \sim 21.5 mag .
We have discovered a bona-fide RR Lyrae variable star in UMa II , which we use to estimate a conservative dereddened distance modulus for the galaxy of ( m - M ) _ { 0 } = 17.70 \pm 0.04 \pm 0.12 mag , where the first error accounts for the uncertainties of the calibrated photometry , and the second reflects our lack of information on the metallicity of the star .
The corresponding distance to UMa II is 34.7 ^ { +0.6 } _ { -0.7 } ( ^ { +2.0 } _ { -1.9 } ) kpc .
Our photometry shows evidence of a spread in the galaxy subgiant branch , compatible with a spread in metal abundance in the range between Z =0.0001 and Z =0.001 .
Based on our estimate of the distance , a comparison of the fiducial lines of the Galactic globular clusters ( GCs ) M68 and M5 ( [ Fe/H ] = -2.27 \pm 0.04 dex and -1.33 \pm 0.02 dex , respectively ) , with the position on the CMD of spectroscopically confirmed galaxy members , may suggest the existence of stellar populations of different metal abundance/age in the central region of UMa II .