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