Multiple populations in globular clusters and the origin of the Oosterhoff period groups

The presence of multiple populations is now well established in most globular clusters in the Milky Way. In light of this, here we propose a new model to explain the origin of the Sandage period-shift and the difference in mean period of type ab RR Lyrae variables between the two Oosterhoff groups. In our model, the instability strip in the metal-poor group II clusters, such as M15, is populated by second-generation stars (G2) with enhanced helium and CNO abundances, while the RR Lyraes in the relatively metal-rich group I clusters such as M3 are produced mostly by first-generation stars (G1) without these enhancements. This population shift within the instability strip with metallicity can create the observed period-shift between the two groups, as both helium and CNO abundances play a role in increasing the period of RR Lyraes. The presence of more metal-rich clusters having Oosterhoff-intermediate characteristics, such as NGC 1851, as well as of most metal-rich clusters having RR Lyraes with the longest periods (group III) can also be reproduced, as more helium-rich third and later generations of stars (G3) penetrate into the instability stripwith a further increase inmetallicity. Therefore, although there are systems in which the suggested population shift cannot be a viable explanation, for the most general cases our models predict that the RR Lyraes are produced mostly by G1, G2 and G3 for the Oosterhoff groups I, II and III, respectively.