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Paper: The Oosterhoff Dichotomy in Multiple Population Paradigm
Volume: 529, RR Lyrae/Cepheid 2019: Frontiers of Classical Pulsators
Page: 169
Authors: Lee, Y.; Jang, S.
Abstract: One of the long-standing problems in modern astronomy is the Oosterhoff (1939) dichotomy, which shows that the Milky Way globular clusters (GCs) can be divided into two groups according to the mean period of type ab RR Lyrae variables (〈Pab〉) in GCs. The Oosterhoff group II GCs are generally metal-poor ([Fe/H] < -1.5) and have longer 〈Pab〉 of ∼0.65 day, while the Oosterhoff group I GCs are relatively metal-rich with shorter 〈Pab〉 of ∼0.55 day. In addition to these two traditional Oosterhoff period groups, a few most metal-rich ([Fe/H] > -0.7) GCs showing the longest 〈Pab〉 of ∼0.70 day are often called the Oosterhoff group III GCs. Since the period of RR Lyrae stars depends on surface temperature, luminosity, and mass of a star, it was first suggested that the temperature difference induced by the "hysteresis effect" is responsible for the Oosterhoff dichotomy (van Albada & Baker 1973). Sandage (1981) then found the period-shift at a fixed temperature, and therefore the luminosity difference was suggested to be the origin of the dichotomy. In the meantime, Castellani (1983) and Renzini (1983) realized that the Oosterhoff dichotomy has something to do with non-monotonic behavior of horizontal-branch (HB) morphology with decreasing metallicity. Using synthetic HB models, Lee, Demarque, and Zinn (1990) showed that the luminosity difference required to explain the Sandage period-shift effect can be explained by the post-zero-age-horizontal-branch (ZAHB) evolution from the blue HB. While this effect can play an important role in the Oosterhoff dichotomy (see Lee & Zinn 1990), it was not clear whether this effect alone could reproduce the observed two discrete distribution in 〈Pab〉. More recently, Yoon & Lee (2002) further suggested that metal-poor Oosterhoff group II GCs in the outer halo were accreted from satellite galaxies. While most of these effects and mechanisms are probably contributing factors in our understanding of the Oosterhoff dichotomy, we still need a more natural solution for a complete understanding of the phenomenon. The presence of multiple stellar populations in GCs shed new light on this long-standing problem.
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