The interpretation of the two-color and color-magnitude diagrams of M15 and M92

From the theoretical relation between U - B, B - Vand Teff, g for Population II stars and the observed color-magnitude diagrams, we determine the Teff and L values for the red giants and horizontal-branch stars in M15 and M92. Several possibilities for the stellar masses, the metal abundances, and distance moduli are considered. The "empirical" (luminosity, )-diagrams are compared with theoretical evolutionary tracks. The comparison suggests that Z(M15) < Z(M92), the age t(M15) > t(M92), and the masses M(M15) < M(M92), while possibly Y(M15) > Y(M92). The average values found for both clusters are log Z = -4 + 0.3, Y = 0.25 + 0.03, t10 = 1.36 t 0.05, MIM0 = 0.85 i 0.05. Subject headings: abundances, stellar - globular clusters - luminosities - RR Lyrae stars