On the photometric metallicity of Oosterhoff-type I and II RRab stars

TL;DR

This study compares photometric metallicity estimates of RRab stars from V- and I-band light curves across different Oosterhoff types, revealing systematic differences and dependencies on star classification.

Contribution

It identifies and quantifies the discrepancies between two widely used metallicity formulae and highlights their dependence on Oosterhoff classification.

Findings

V-band formula yields higher metallicities by ~0.05 dex.

Oosterhoff type influences metallicity estimates, with Oo I stars appearing more metal-rich.

Differences are partly due to sample composition dominated by Oo I variables.

Abstract

We are revising the consistency of photometric metallicity formulae widely used for fundamental-mode RR Lyrae (RRab) variables, based on their V- and I-band light curves, published by Jurcsik and Kov\'acs (1996) and Smolec (2005), respectively. 293 RRab variables belonging to 10 globular clusters, all simultaneously containing stars of both Oosterhoff types, are selected for this purpose. We find that on average, the V-band formula results in higher estimated metallicities by about 0.05 dex than the I-band formula. Moreover, we detect a dependency on the Oosterhoff class of the variables for both formulae, as well. Using the V-band formula, Oo I stars are 0.05-0.10 dex more metal rich than Oo II stars of the same cluster. Although with less significance, but the I-band results indicate a reversed trend. Therefore, we surmise that the average difference we have found between the V- and I-band formulae is the consequence of the total sample being dominated by Oo I variables.