Physical properties of double-mode RR Lyrae stars based on pulsation and evolution models (Istvan Dekany, DAA-PUC)

I present a new method for determining the fundamental physical parameters of double-mode RR Lyrae (RRd) stars from their observed periods and the requirement of consistency between pulsational and evolutionary constraints. With the combination of the above two different kinds of models, the periods of an object determine a sequence of solutions for its mass, luminosity, effective temperature, and metallicity, parametrized by the time elapsed from the zero age horizontal branch state. In possession of additional simple observables, even accurate color indices, the time parameter can be further confined, thus reducing the ranges of uncertainty in all other parameters. Following this approach, I determine the physical parameters of altogether 20 RRd stars in the Galactic Field and the Large Magellanic Cloud. The stars were selected to cover wide ranges of periods and period ratios, implying diverse stellar parameters, uniformly covering most of the parameter space populated by double-mode RR Lyrae stars. The calculated theoretical metallicity and color index values show a general consistency with previous observations. The derived solution sequences yield various important relations between the physical parameters (mass, radius, luminosity, effective temperature, metallicity) of the stars which are, in the case of some parameter combinations, nearly independent of the age. I demonstrate that these relations are only marginally altered by the effects related to convection, the non-linearity of the pulsation, and the possible chemical composition variations due to the enhancement of alpha-elements. I show that the cosmic distance scale stemmed from the results is consistent with the generally accepted one.