METAL-RICH RR LYRAE VARIABLES .1. THE EVOLUTIONARY SCENARIO

This paper presents evolutionary computations that investigate the the oretical predictions concerning metal-rich RR Lyrae pulsators found bo th in the Galactic field and in the Galactic bulge. The main aim of th is investigation is to provide a homogeneous evolutionary context for further analyses concerning the pulsational properties of these evolvi ng structures. In this connection, a suitable set of stellar models ch aracterized by two different metal contents, namely, Z = 0.01 and Z = 0.02, were followed through both H- and He-burning phases. This evolut ionary scenario covers the theoretical expectations for pulsating He-b urning structures with ages ranging from 20 to less than 1 Gyr. For ea ch given assumption about the star metallicity, we find that ''old'' H e-burning pulsators with ages larger than 2 Gyr have a common behavior , with Z = 0.01 pulsators slightly more luminous and more massive than Z = 0.02 pulsators. However, as soon as the metallicity increases abo ve the solar value, the luminosity of horizontal-branch (HB) stars inc reases again. This effect is a direct consequence of the expected simu ltaneous increase of both original He and metals. The occurrence of '' young'' RR Lyrae pulsators is discussed, and we show that this peculia r group of variable stars can be produced if the mass loss pushes He-b urning stars originating from more massive progenitors into the instab ility region, where electron degeneracy plays a minor role. In this ca se the luminosity of the pulsators could be substantially reduced with respect to the case of ''old'' variables; therefore, the theoretical expectations should also provide for this group of variable stars shor ter pulsation periods. Hydrogen-burning isochrones for the quoted meta llicity values are also presented and discussed. Finally, the occurren ce of a new ''gravonuclear instability'' in some HB models during the ignition of He shell burning is discussed. We have found that the appe arance of this phenomenon is due to the opacity ''bump'' connected wit h iron. At the same time, we have also outlined the role that this fea ture could play on observable stars.