The Oosterhoff period groups and multiple populations in globular clusters

  • Jang, Sohee (Center for Galaxy Evolution Research and Department of Astronomy, Yonsei University) ;
  • Lee, Young-Wook (Center for Galaxy Evolution Research and Department of Astronomy, Yonsei University) ;
  • Joo, Seok-Joo (Center for Galaxy Evolution Research and Department of Astronomy, Yonsei University) ;
  • Na, Chongsam (Center for Galaxy Evolution Research and Department of Astronomy, Yonsei University)
  • 발행 : 2014.04.10

초록

The presence of multiple populations is now well-established in most globular clusters (GCs) in the Milky Way. In light of this progress, here we suggest a new model explaining 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 models, while matching the observed color-magnitude diagrams, the difference in is naturally reproduced as the instability strip is occupied by different subpopulations with increasing metallicity. The instability strip in the metal-poor group II clusters is populated by second generation stars (G2) with enhanced helium and CNO abundances, while the RR Lyraes in the metal-rich group I clusters are mostly produced by first generation stars (G1) without these enhancements. This population shift within the instability strip can create the observed period-shift between the two groups, since both helium and CNO abundances play a role in increasing the period of RR Lyrae variables. The presence of more metal-rich Oosterhoff group III clusters having RR Lyraes with longest can also be reproduced, if more helium-rich third generation stars (G3) are present in these GCs.

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