On the Masses, Luminosities, and Compositions of Horizontal-Branch Stars

For RR Lyrae stars in globular clusters, three different relations between mass and luminosity can be obtained from a combination of available observations and theoretical models. By requiring compatibility among these relations, we infer the masses and luminosities of these stars, as functions of the composition parameters assumed for the models. This requirement also restricts the composition parameters. For the globular cluster M3, the helium abundance Y cannot be less than 0.15 and the abundance of heavier elements Z is probably not greater than 3 X 10- . Mass loss during -branch evolution is neither required nor excluded. The ratio of horizontal-branch luminosity to that at the mainsequence turnoff is a sensitive function of Z; values of this function are given. The problem of determining Y in RR Lyrae stellar envelopes from the blue edge of the instability region is reexamined with the help of linear nonadiabatic models of pulsating stars. We obtain F = 0.35 + 0.10 for M3. The theoretical uncertainties, and the consequent uncertainties in the inferred values of , L, F, and Z are emphasized.