The RR Lyrae Period-Luminosity-(Pseudo-)Color and Period-Color-(Pseudo-)Color Relations in the Str'omgren Photometric System: Theoretical Calibration

摘要

We present a theoretical calibration of the RR Lyrae period-luminosity-colorand period-color-color relations in the multiband uvby Stroemgren photometricsystem. Our theoretical work is based on calculations of synthetic horizontalbranches (HBs) for four different metallicities, fully taking into accountevolutionary effects for a wide range in metallicities and HB morphologies.While our results show that "pure" period-luminosity and period-color relationsdo not exist in the Stroemgren system, which is due to the large scatter thatis brought about by evolutionary effects when the uvby bandpasses are used,they also reveal that such scatter can be almost completely taken into accountby incorporating Stroemgren pseudo-color [C_0 = (u-v)_0 - (v-b)_0] terms intothose equations, thus leading to tight period-luminosity-{\em pseudo}-color(PLpsC) and period-color-{\em pseudo}-color (PCpsC) relations. We provide thelatter in the form of analytical fits, so that they can be applied with highprecision even in the case of field stars. In view of the very smallsensitivity of C_0 to interstellar reddening, our PLpsC and PCpsC relationsshould be especially useful for the derivation of high-precision distance andreddening values. In this sense, we carry out a first application of ourrelations to field RR Lyrae stars, finding evidence that the stars RR Lyr, SUDra, and SS Leo -- but not SV Hya -- are somewhat overluminous (by amountsranging from ~0.05 to 0.20 mag in y, and thus V) with respect to the averagefor other RR Lyrae stars of similar metallicity.