2010MNRAS.405.1796H

A new moving group comprising at least four blue horizontal branch (BHB) stars is identified at (l, b) = (65°, 48°). The horizontal branch at g₀= 18.9mag implies a distance of 50kpc from the Sun. The heliocentric radial velocity is <V_r> = -157±4 km/s, corresponding to V_gsr= -10 km/s; the dispersion in line-of-sight velocity is consistent with the instrumental errors for these stars. The mean metallicity of the moving group is [Fe/H] ∼ -2.4, which is significantly more metal poor than the stellar spheroid. We estimate that the BHB stars in the outer halo have a mean metallicity of [Fe/H] = -2.0, with a wide scatter and a distribution that does not change much as a function of distance from the Sun. We explore the systematics of Sloan Digital Sky Survey DR7 surface gravity metallicity determinations for faint BHB stars, and present a technique for estimating the significance of clumps discovered in multidimensional data. This Hercules Corona Borealis stream cannot be distinguished in density, and highlights the need to collect many more spectra of Galactic stars to unravel the merger history of the Galaxy.