M3 HB stars compared to theoretical HB models with Z = 0.001 in the Strömgren y vs. (b − y) plane. The upper panel is a zoom-in around the ZAHB locus. PGPUC ZAHB loci are shown for helium abundances Y = 0.246 (red line), 0.256 (blue line, shown in the upper panel only, for clarity), and 0.266 (green line). C09 and BaSTI ZAHB loci for Y = 0.246 are also shown as magenta and black lines, respectively, with different distance moduli in order to match the PGPUC ZAHB locus with Y = 0.246. Theoretical HB evolutionary tracks for Y = 0.246 are shown as red dot-dashed lines for MHB = 0.582, 0.596, 0.609, 0.632, 0.652, 0.665, and 0.800 M⊙, which have been marked with circles at the ZAHB, MAHB, and 90AHB (colors depend on the HB mass, for clarity). An additional HB track for Y = 0.256 and MHB = 0.582 M⊙ is also shown in the top panel (blue dot-dashed lines with pink circles). Vertical dot-dashed lines indicate some Teff values at the ZAHB locus for Y = 0.246. The distance modulus was selected as described in Sect. 3.3 and Fig. 6. The data come from C09.
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