A mass transfer origin for blue stragglers in NGC 188 as revealed by half-solar-mass companions

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In open star clusters, where all members formed at about the same time, blue straggler stars are typically observed to be brighter and bluer than hydrogen-burning main-sequence stars, and therefore should already have evolved into giant stars and stellar remnants. Correlations between blue straggler frequency and cluster binary star fraction1, core mass2 and radial position3 suggest that mass transfer or mergers in binary stars dominates the production of blue stragglers in open clusters. Analytic models4,5, detailed observations6 and sophisticated N-body simulations7, however, argue in favour of stellar collisions. Here we report that the blue stragglers in long-period binaries in the old8 (7 × 109-year) open cluster NGC 188 have companions with masses of about half a solar mass, with a surprisingly narrow mass distribution. This conclusively rules out a collisional origin, as the collision hypothesis predicts a companion mass distribution with significantly higher masses. Mergers in hierarchical triple stars9 are marginally permitted by the data, but the observations do not favour this hypothesis. The data are highly consistent with a mass transfer origin for the long-period blue straggler binaries in NGC 188, in which the companions would be white dwarfs of about half a solar mass.

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Figure 1: Companion mass distribution for the 12 blue straggler binaries in NGC 188 with periods of order 1,000 d.
Figure 2: Cumulative distribution of mass functions of the NGC 188 blue straggler binaries with periods of order 1,000 d.
Figure 3: Distributions of binary orbital elements for the blue stragglers in the NGC 188 N -body model.


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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. Thanks to J. Hurley, A. Sills, N. Leigh, R. Taam and H. Perets for their comments and suggestions. Both A.M.G. and R.D.M. were visiting astronomers at Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy under cooperative agreement with the National Science Foundation. The WIYN Observatory is a joint facility of the University of Wisconsin–Madison, Indiana University, Yale University and the US National Optical Astronomy Observatories. This work was funded by the US National Science Foundation grant AST-0908082 to the University of Wisconsin–Madison, the Wisconsin Space Grant Consortium and the Lindheimer Fellowship at Northwestern University.

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Both A.M.G. and R.D.M. obtained spectra of the NGC 188 blue stragglers using the WIYN telescope. A.M.G. analysed these data and constructed the N-body model of the cluster. A.M.G. and R.D.M. contributed equally to the writing of the manuscript.

Correspondence to Aaron M. Geller.

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Geller, A., Mathieu, R. A mass transfer origin for blue stragglers in NGC 188 as revealed by half-solar-mass companions. Nature 478, 356–359 (2011) doi:10.1038/nature10512

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