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Two distinct sequences of blue straggler stars in the globular cluster M 30


Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxy’s history1. ‘Blue stragglers’ are stars massive enough2 that they should have evolved into white dwarfs long ago. Two possible mechanisms have been proposed for their formation: mass transfer between binary companions3 and stellar mergers resulting from direct collisions between two stars4. Recently the binary explanation was claimed to be dominant5. Here we report that there are two distinct parallel sequences of blue stragglers in M 30. This globular cluster is thought to have undergone ‘core collapse’, during which both the collision rate and the mass transfer activity in binary systems would have been enhanced6. We suggest that the two observed sequences are a consequence of cluster core collapse, with the bluer population arising from direct stellar collisions and the redder one arising from the evolution of close binaries that are probably still experiencing an active phase of mass transfer.

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Figure 1: The two blue straggler sequences of M 30.
Figure 2: The BSS radial distribution.
Figure 3: The star density profile of M 30.
Figure 4: Comparison with collisional and binary-evolution models.

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This research was supported by Progetti di Ricerca di Interesse Nazionale 2008, granted by the Istituto Nazionale di Astrofisica. We acknowledge the financial support of the Agenzia Spaziale Italiana and the Ministero dell’Istruzione, dell’Università e della Ricerca. F.R.F., B.L., E.D. and A.S. thank the Formation and Evolution of Globular Clusters programme and the Kavli Institute for Theoretical Physics in Santa Barbara (California, USA) for hospitality during their stay, when the motivations of this project were discussed and the work planned. We acknowledge support from the European Space Research and Technology Centre Faculty Visiting Scientist Programme. R.T.R. is partly supported by a Space Telescope Science Institute grant. This research has made use of the ESO/ST-ECF Science Archive facility, which is a joint collaboration of the European Southern Observatory (ESO) and the Space Telescope European Coordinating Facility (ST-ECF).

Author Contributions F.R.F. designed and coordinated the study. G.B., E.D. and S.B. analysed the data. A.S. and A.I.K. developed collisional models. B.L. and P.M. computed the surface density profile and performed comparisons with a single-mass King model. F.R.F. and B.L. wrote the paper. F.F.P., A.S. and R.T.R. critically contributed to discussions on, and the presentation of, the paper. All the authors contributed to discussions of the results and commented on the manuscript.

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Correspondence to F. R. Ferraro.

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Ferraro, F., Beccari, G., Dalessandro, E. et al. Two distinct sequences of blue straggler stars in the globular cluster M 30. Nature 462, 1028–1031 (2009).

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