THE RR LYRAE STARS: NEW PERSPECTIVES

Abstract

We demonstrate that the Oosterhoff II (Oo?II) RR Lyrae ab variables are hotter by ~270?K, at the same period, than Oo?I variables. Or, at the same (B ? V)0 value the Oo?II variables have larger radii than Oo?I variables. This accounts for the reason Oo?II variables are brighter (0.12-0.20?mag) than Oo?I variables. The dependence of the light amplitude of RR Lyrae variables on temperature is independent of Oo type. This makes it possible to derive an accurate set of equations to relate intrinsic (B ? V)0 color indices to light amplitudes, which in turn can be used to determine the interstellar reddening (E (B ? V)). With just a few variables (~5), it is possible to determine the E (B ? V) to an accuracy of <0.01?mag in the absence of systematic photometric errors. We discuss the errors introduced in color excess determinations by including the Blazhko stars in a solution. A comparison of color excess values of 23 globular clusters and two regions of the Large Magellanic Cloud (LMC), determined with the aid of our newly developed equations, are found to compare favorably (~0.01?mag) with color excess values found in the literature. Four new Oo?III variables, some found in metal-poor clusters, are discussed. An analysis of the galactic-field variables indicates the majority are Oo?I and Oo?II variables, but a few short-period (log P?< ?0.36) metal-strong variables, so far not found in galactic globular clusters are evidently ~0.30?mag fainter than Oo?I variables. Oo?III variables may also be present in the field. We conclude that the RR Lyrae ab variables are primarily restricted to four sequences or groups. If we assume that the Oo?I variables' mean absolute magnitude is Mv?= 0.61, the mean absolute magnitudes of the other three sequences are: short-period variables Mv?~ 0.89?mag, Oo?II Mv?~ 0.43?mag, and Oo?III Mv?~ 0.29?mag. The Oo?I fundamental RR Lyrae ab red edge (FRE) and fundamental blue edge (FBE) occur at approximately the following temperatures: FRE T?~ 6180?K and FBE T?~ 6750?K. There is a strong dependence of Mv on [Fe/H] as we proceed from the short-period variables to the Oo?I variables and to the Oo?II variables, but there seems to be little or no dependence of Mv on [Fe/H] for stars within a group, at least for the Oo?I and Oo?II groups. The Oo?II variables exhibit a weak period luminosity relation in V in many globular clusters unlike the Oo?II-like variables in Oo?I clusters which do not exhibit a P-L relation. The properties of some intermediate LMC clusters are discussed.