A binary star fraction of 76 per cent and unusual orbit parameters for the blue stragglers of NGC 188


Blue straggler stars lie on or near the main sequences of star clusters (all members of which formed around the same time), but typically are more luminous than the turn-off stars and therefore long ago should have evolved off the main sequence to become giants and white dwarfs. They are thought to derive from normal main-sequence stars that have undergone a recent increase in mass. Statistical evidence indicates that in globular star clusters the blue stragglers probably form from binary stars1. The specific formation processes, such as mass transfer, mergers or stellar collisions during dynamical encounters of binary stars, remain unresolved. Here we report that 16 of the 21 blue stragglers (76 per cent) in the old (7-Gyr; ref. 2) open cluster NGC 188 are currently in binary systems, a frequency three times that found among normal solar-type main-sequence stars. These blue straggler binaries have a remarkable period–eccentricity distribution, with all but three having orbital periods of 1,000 days. Moreover, these stars are rotating faster than normal main-sequence stars of the same surface temperatures. These data show that most, and possibly all, blue stragglers derive from multiple-star systems, and indicate that the several formation processes operate simultaneously. We suggest that rapid rotation of blue stragglers may place upper limits on their ages.

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Figure 1: Colour–brightness diagram for members of NGC 188.
Figure 2: NGC 188 orbital eccentricity/log period ( e –log  P ) distributions.
Figure 3: Distribution of projected rotation velocity in the colour–brightness diagrams of NGC 188 and NGC 6819.


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We thank the staff of the WIYN Observatory and the many graduate and undergraduate students who have assisted in observing NGC 188. The WIYN Observatory is a joint facility of the University of Wisconsin–Madison, Indiana University, Yale University and the US National Optical Astronomy Observatories. We thank D. Fabrycky, J. Hurley, N. Leigh, H. Perets and A. Sills for their comments. This work was funded by the US National Science Foundation grant AST-0406615 and the Wisconsin Space Grant Consortium.

Author Contributions R.D.M. and A.M.G. contributed equally to this work.

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Correspondence to Robert D. Mathieu.

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Mathieu, R., Geller, A. A binary star fraction of 76 per cent and unusual orbit parameters for the blue stragglers of NGC 188. Nature 462, 1032–1035 (2009). https://doi.org/10.1038/nature08568

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