Synthetic CMDs at the HB level from models with Z = 0.001 in the filters of interest of the Strömgren (top panels) and WFPC2 (middle and bottom panels) systems. The ZAHB locus for Y = 0.246 is represented as a black line. A uniform mass deviate ranging from 0.595 to 0.705 M⊙ with 2100 stars is used to represent a helium-normal population (Y = 0.246, light gray circles). Helium-rich populations (Y = 0.256,0.266) are created with 1000 stars with masses ranging from 0.595 to 0.645 M⊙ (dark gray circles). Stars identified as RHB stars with Y = 0.246 are plotted as red circles. BHB stars are plotted in cyan for Y = 0.246, blue for Y = 0.256, and green for Y = 0.266. In these simulations, photometric errors are included, following the photometric data, at the level of 0.01 mag in each bandpass. Panels g)–i) are zoom-ins of panels d)–f), respectively. Each panel is accompanied by histograms showing the distribution in color (top) and magnitude (right) of the different components.
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