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Dynamical age differences among coeval star clusters as revealed by blue stragglers



Globular star clusters that formed at the same cosmic time may have evolved rather differently from the dynamical point of view (because that evolution depends on the internal environment) through a variety of processes that tend progressively to segregate stars more massive than the average towards the cluster centre1. Therefore clusters with the same chronological age may have reached quite different stages of their dynamical history (that is, they may have different ‘dynamical ages’). Blue straggler stars have masses greater2 than those at the turn-off point on the main sequence and therefore must be the result of either a collision3,4 or a mass-transfer event5,6,7. Because they are among the most massive and luminous objects in old clusters, they can be used as test particles with which to probe dynamical evolution. Here we report that globular clusters can be grouped into a few distinct families on the basis of the radial distribution of blue stragglers. This grouping corresponds well to an effective ranking of the dynamical stage reached by stellar systems, thereby permitting a direct measure of the cluster dynamical age purely from observed properties.

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Figure 1: The radial distribution of BSSs in three dynamically young stellar systems (family I).
Figure 2: The radial distribution of BSSs in systems of intermediate dynamical ages (family II).
Figure 3: The radial distribution of BSSs in dynamically old clusters (family III).
Figure 4: A first calibration of the clock.


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The authors dedicate this paper to the memory of co-author Bob Rood, a pioneer in the theory of the evolution of low mass stars and a friend who shared our enthusiasm for the BSS topic, who passed away on 2 November 2011. This research is part of the project COSMIC-LAB funded by the European Research Council (under contract ERC-2010-AdG-267675). G.B. acknowledges the European Community’s Seventh Framework Programme under grant agreement no. 229517. F.R.F. acknowledges support from the ESO Visiting Scientist Programme. This research is based on data acquired with the NASA/ESA HST, under programmes GO-11975, GO-10524, GO-8709, GO-6607 and GO-5903 at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. The research is also based on data collected at the ESO telescopes under programmes 62.L-0354, 64.L-0439, 59.A-002(A), 69.D-0582(A), 079.D-0220(A) and 079.D-0782(A), and made use of the ESO/ST-ECF Science Archive facility, which is a joint collaboration of the European Southern Observatory and the Space Telescope – European Coordinating Facility.

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F.R.F. designed the study and coordinated the activity. E.D., G.B., R.C., B.L., N.S. and A.M. analysed the data. M.P. and P.M. developed N-body simulations. F.R.F. and B.L. wrote the paper. E.V., A.S., S.S., M.M. and R.T.R. critically contributed to discussion and presentation of paper. All authors contributed to discussion of the results and commented on the manuscript.

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Correspondence to F. R. Ferraro.

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Ferraro, F., Lanzoni, B., Dalessandro, E. et al. Dynamical age differences among coeval star clusters as revealed by blue stragglers. Nature 492, 393–395 (2012).

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