DETECTION OF A GALACTIC COLOR GRADIENT FOR BLUE HORIZONTAL-BRANCH STARS OF THE HALO FIELD AND IMPLICATIONS FOR THE HALO AGE AND DENSITY DISTRIBUTIONS

From analysis of a large sample of field blue horizontal-branch stars we find that the mean unreddened color, w, in the color window -0.02 < (B-V)0 < + 0.18 increases outward in the Galaxy by approximately 0.025 on 2 kpc < R < 12 kpc. On the basis of several tests of the sample, we conclude that this result is due neither to errors in reddening estimates nor to contamination of the sample by stars of the young disk or by the population described by Lance (1988b).