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

TL;DR

This paper provides a theoretical calibration of RR Lyrae star relations in the Str"omgren photometric system, demonstrating how pseudo-color terms improve the precision of period-luminosity and period-color relations for distance and reddening measurements.

Contribution

It introduces analytical PLpsC and PCpsC relations incorporating pseudo-color to reduce scatter caused by evolutionary effects in the Str"omgren system.

Findings

Pure period-luminosity relations are scattered due to evolutionary effects.

Pseudo-color terms significantly tighten the relations.

Application suggests some RR Lyrae stars are overluminous compared to the average.

Abstract

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