Illustration of halo BHB stars revealed in the T_eff–log g diagram. Left: Background in gray scale showing the stellar density (in logarithmic scale) of our sample stars with S/N > 50. The black dots represent the stars with |b| > 15° and [Fe/H] < –1.2 dex. There are two sequences separated in log g: the upper sequence with lower log g values is composed of halo BHB stars, and the lower sequence with larger log g values is presumed to comprise blue straggler stars. The solid lines in red are MIST evolutionary tracks of He-burning stars with [Fe/H] = –1.5, V/V_c = 0.4, and masses of 0.65, 0.60, and 0.58 M_⊙, respectively, from upper to lower. The dashed lines in red show similar tracks but for [Fe/H] = –2.0. Right: [Fe/H] distribution of our sample stars with S/N > 50 and |b| > 15°. Only the metal-poor side of [Fe/H] < –0.5 dex is shown. The vertical dashed line indicates a constant [Fe/H] of –1.2 dex, the sample below which mostly comprises the metal-poor halo BHB stars.