Star formation history of the Milky Way halo traced by the Oosterhoff dichotomy among globular clusters

TL;DR

This study models the Oosterhoff dichotomy in globular clusters to trace the Milky Way halo's star formation history, revealing different formation timelines for inner and outer halo clusters consistent with dual origin theories.

Contribution

It extends previous models to all metallicity regimes, demonstrating that the Oosterhoff dichotomy can be explained by distinct star formation histories in the inner and outer halo GCs.

Findings

Inner halo GCs formed earlier with shorter timescales.

Outer halo GCs had delayed and more extended formation periods.

Oosterhoff groups result from population shifts during star formation episodes.

Abstract

In our recent investigation of the Oosterhoff dichotomy in the multiple population paradigm (Jang et al. 2014), we have suggested that the RR Lyrae variables in the Oosterhoff groups I, II, and III globular clusters (GCs) are produced mostly by the first, second, and third generation stars (G1, G2, and G3), respectively. Here we show, for the first time, that the observed dichotomies in the inner and outer halo GCs can be naturally reproduced when these models are extended to all metallicity regimes, while maintaining reasonable agreements in the horizontal-branch type versus [Fe/H] correlations. In order to achieve this, however, specific star formation histories are required for the inner and outer halos. In the inner halo GCs, the star formation commenced and ceased earlier with relatively short formation timescale between the subpopulations (~0.5 Gyr), while in the outer halo, the formation of G1 was delayed by ~0.8 Gyr with more extended timescale between G1 and G2 (~1.4 Gyr). This is consistent with the dual origin of the Milky Way halo. Despite the difference in detail, our models show that the Oosterhoff period groups observed in both outer and inner halo GCs are all manifestations of the "population-shift" effect within the instability strip, for which the origin can be traced back to the two or three discrete episodes of star formation in GCs.