# On the Oosterhoff dichotomy in the Galactic bulge: I. spatial distribution

## TL;DR

This study analyzes the spatial distribution and properties of Oosterhoff groups among RR Lyrae stars in the Galactic bulge, revealing a third group and differences in metallicity, mass, and location.

## Contribution

It identifies a potential third Oosterhoff group in the bulge and compares the properties and spatial distributions of the two main groups using extensive survey data.

## Key findings

- Oosterhoff II stars are more metal-poor, massive, and cooler.
- A third Oosterhoff group was identified among the variables.
- Spatial distribution varies between foreground, central, and background regions.

## Abstract

We present a study of the Oosterhoff (Oo) dichotomy in the Galactic bulge using 8\,141 fundamental mode RR~Lyrae stars. We used public photometric data from the Optical Gravitational Lensing Experiment (OGLE) and the Vista Variables in the V\'ia L\'actea survey (VVV). We carefully selected fundamental mode stars without modulation and without association with any globular cluster located toward the Galactic bulge. Subsequently, we identified and separated the Oosterhoff groups I and II on the basis of their period-amplitude distribution and using a relation fitted to the Oosterhoff I locus. Both Oosterhoff groups were then compared to observations of two bulge globular clusters and with models of stellar pulsation and evolution. We found that some of the variables classified as Oo\,II belong to a third Oo group. The Oosterhoff II variables are more metal-poor on average, more massive, and cooler than their Oosterhoff I counterparts. The analysis of their spatial distribution shows a systematic difference between \textit{foreground}, central and \textit{background} regions in the occurrence of the Oosterhoff II group. The difference between the Oo\,I and II groups is also seen in their distance distributions with respect to the Galactic bar, but neither group is associated with the bar.