Probing the Galactic halo with RR Lyrae stars -- IV. On the Oosterhoff dichotomy of RR Lyrae stars

TL;DR

This study investigates the Oosterhoff dichotomy among RR Lyrae stars in the Milky Way halo and satellite galaxies, revealing its dependence on location, metallicity, and galaxy environment, and suggesting a complex origin.

Contribution

It provides a comprehensive analysis of the Oosterhoff dichotomy in the Galactic halo and satellite galaxies using 7D data, highlighting its variability and complex origins.

Findings

Oo I stars are more metal-rich with radial orbits.

Oo II stars are more metal-poor with mildly radial orbits.

The Oosterhoff dichotomy varies with position and galaxy environment.

Abstract

We use 3653 (2661 RRab, 992 RRc) RR Lyrae stars (RRLs) with 7D (3D position, 3D velocity, and metallicity) information selected from SDSS, LAMOST, and Gaia EDR3, and divide the sample into two Oosterhoff groups (Oo I and Oo II) according to their amplitude-period behaviour in the Bailey Diagram. We present a comparative study of these two groups based on chemistry, kinematics, and dynamics. We find that Oo I RRLs are relatively more metal rich, with predominately radially dominated orbits and large eccentricities, while Oo II RRLs are relatively more metal poor, and have mildly radially dominated orbits. The Oosterhoff dichotomy of the Milky Way's halo is more apparent for the inner-halo region than for the outer-halo region. Additionally, we also search for this phenomenon in the halos of the two largest satellite galaxies, the Large and Small Magellanic clouds (LMC, SMC), and compare over different bins in metallicity. We find that the Oosterhoff dichotomy is not immutable, and varies based on position in the Galaxy and from galaxy-to-galaxy. We conclude that the Oosterhoff dichotomy is the result of a combination of stellar and galactic evolution, and that it is much more complex than the dichotomy originally identified in Galactic globular clusters.