METAL-RICH RR LYRAE VARIABLES .2. THE PULSATIONAL SCENARIO

We present a theoretical investigation on the pulsational behavior of metal-rich RR Lyrae variables over the range of evolutionary parameter s suitable for stars with metallicities Z = 0.006, 0.01, and 0.02. Wit h the addition of similar results for metal-poor pulsators, we discuss the theoretical pulsational scenario covering the metallicity range Z = 0.0001-0.02. By connecting pulsational constraints to evolutionary prescriptions for He-burning stars, we discuss the observed behavior o f the RR Lyrae population in the Galactic field. We find that the dist ribution of field ab-type RR Lyrae stars in the period-metallicity pla ne can be easily understood within the framework of the present theore tical scenario, suggesting that the Oosterhoff dichotomy also affects field variables. Theoretical predictions concerning the amplitude-peri od diagram are discussed and compared with observational data. We find a general agreement for metal-poor ([Fe/H] < -1.4) RR Lyrae stars, wh ereas more metal-rich variables show amplitudes smaller than those pre dicted for pulsators originated from old, low-mass evolving stars. Alt ernatively, the agreement between theory and observations would ; requ ire that a substantial fraction of metal-rich RR Lyrae variables in th e Galactic field were younger than approximate to 2 Gyr. The compariso n between the pulsational behavior of RR Lyrae either in the Galactic held or in the Galactic bulge discloses the evidence that, at least as far as RR Lyrae variables are concerned, the metal-rich components of the bulge and of the field population appear quite similar. We finall y suggest that the peak at log P similar or equal to -0.55 in the peri od frequency distribution of first-overtone RR Lyrae stars in the Larg e Magellanic Cloud, considered as possible evidence for second-overton e pulsators, could be more simply taken as evidence of a metal-rich st ellar population.