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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.


  1. 1

    Meylan, G. & Heggie, D. C. Internal dynamics of globular clusters. Annu. Rev. Astron. Astrophys. 8, 1–143 (1997)

    Article  Google Scholar 

  2. 2

    Shara, M. M., Saffer, R. A. & Livio, M. The first direct measurement of the mass of a blue straggler in the core of a globular cluster: BSS19 in 47 Tucanae. Astrophys. J. 489, L59–L63 (1997)

    ADS  CAS  Article  Google Scholar 

  3. 3

    Hills, J. G. & Day, C. A. Stellar collisions in globular clusters. Astrophys. J. 17, 87–93 (1976)

    Google Scholar 

  4. 4

    Sills, A., Adams, T., Davies, M. B. & Bate, M. R. High-resolution simulations of stellar collisions between equal-mass main-sequence stars in globular clusters. Mon. Not. R. Astron. Soc. 332, 49–54 (2002)

    ADS  Article  Google Scholar 

  5. 5

    McCrea, W. H. Extended main-sequence of some stellar clusters. Mon. Not. R. Astron. Soc. 128, 147–155 (1964)

    ADS  Article  Google Scholar 

  6. 6

    Sollima, A., Lanzoni, B., Beccari, G., Ferraro, F. R. & Fusi Pecci, F. The correlation between blue straggler and binary fractions in the core of Galactic globular clusters. Astron. Astrophys. 481, 701–704 (2008)

    ADS  Article  Google Scholar 

  7. 7

    Knigge, C., Leigh, N. & Sills, A. A binary origin for ‘blue stragglers’ in globular clusters. Nature 457, 288–290 (2009)

    ADS  CAS  Article  Google Scholar 

  8. 8

    Marín-Franch, A. et al. The ACS survey of Galactic globular clusters. VII. Relative ages. Astrophys. J. 694, 1498–1516 (2009)

    ADS  Article  Google Scholar 

  9. 9

    Dotter, A., Sarajedini, A. & Yang, S.-C. Globular clusters in the outer Galactic halo: AM-1 and Palomar 14. Astron. J. 136, 1407–1414 (2008)

    ADS  Article  Google Scholar 

  10. 10

    Ferraro, F. R. et al. Blue stragglers in the galactic globular clusters M3: evidence for two populations. Astron. J. 106, 2324–2334 (1993)

    ADS  Article  Google Scholar 

  11. 11

    Ferraro, F. R. et al. the pure noncollisional blue straggler population in the giant stellar system ω Centauri. Astrophys. J. 638, 433–439 (2006)

    ADS  CAS  Article  Google Scholar 

  12. 12

    Mapelli, M. et al. The contribution of primordial binaries to the blue straggler population in 47 Tucanae. Astrophys. J. 605, L29–L32 (2004)

    ADS  Article  Google Scholar 

  13. 13

    Mapelli, M. et al. The radial distribution of blue straggler stars and the nature of their progenitors. Mon. Not. R. Astron. Soc. 373, 361–368 (2006)

    ADS  Article  Google Scholar 

  14. 14

    Davies, M. B., Piotto, G. & de Angeli, F. Blue straggler production in globular clusters. Mon. Not. R. Astron. Soc. 349, 129–134 (2004)

    ADS  Article  Google Scholar 

  15. 15

    Geller, A. M. & Mathieu, R. D. A mass transfer origin for blue stragglers in NGC188 as revealed by half-solar-mass companions. Nature 478, 356–359 (2011)

    ADS  CAS  Article  Google Scholar 

  16. 16

    Binney, J. & Tremaine, S. Galactic Dynamics (Princeton Univ. Press, 1987)

    MATH  Google Scholar 

  17. 17

    Lanzoni, B. et al. The blue straggler population of the globular cluster M5. Astrophys. J. 663, 267–276 (2007)

    ADS  Article  Google Scholar 

  18. 18

    Mapelli, M. et al. Blue straggler stars in dwarf spheroidal galaxies. II. Sculptor and Fornax. Mon. Not. R. Astron. Soc. 396, 1771–1782 (2009)

    ADS  Article  Google Scholar 

  19. 19

    Monelli, M. et al. The ACS LCID Project. VII. The blue stragglers population in the isolated dSph galaxies Cetus and Tucana. Astrophys. J. 744, 157–170 (2012)

    ADS  Article  Google Scholar 

  20. 20

    Ferraro, F. R. et al. Two distinct sequences of blue straggler stars in the globular cluster. Nature 462, 1028–1031 (2009)

    ADS  CAS  Article  Google Scholar 

  21. 21

    Trager, S. C., Djorgovski, S. & King, I. R. in Structure and Dynamics of Globular Clusters (eds Djorgovski, S. G. & Meylan, G. ) (Astronomical Society of the Pacific Conference Series Vol. 50) 347–355 (Astronomical Society of the Pacific, 1993)

    Google Scholar 

  22. 22

    Heggie, D. C. & Giersz, M. Monte Carlo simulations of star clusters. V. The globular cluster M4. Mon. Not. R. Astron. Soc. 389, 1858–1870 (2008)

    ADS  Article  Google Scholar 

  23. 23

    Ferraro, F. R. et al. The puzzling dynamical status of the core of the globular cluster NGC 6752. Astrophys. J. 595, 179–186 (2003)

    ADS  Article  Google Scholar 

  24. 24

    Lanzoni, B. et al. The surface density profile of NGC 6388: a good candidate for harboring an intermediate-mass black hole. Astrophys. J. 668, L139–L142 (2007)

    ADS  CAS  Article  Google Scholar 

  25. 25

    Renzini, A. & Buzzoni, A. in Spectral Evolution of Galaxies (eds Chiosi, C. & Renzini, A. ) 195–231 (Reidel, 1986)

    Book  Google Scholar 

  26. 26

    Bekki, K. & Freeman, K. C. Formation of ω Centauri from an ancient nucleated dwarf galaxy in the young Galactic disc. Mon. Not. R. Astron. Soc. 346, L11–L15 (2003)

    ADS  Article  Google Scholar 

  27. 27

    Djorgovski, S. in Structure and Dynamics of Globular Clusters (eds Djorgovski, S. G. & Meylan, G. ) (Astronomical Society of the Pacific Conference Series Vol. 50) 373–382 (Astronomical Society of the Pacific, 1993)

    Google Scholar 

  28. 28

    King, I. R. The structure of star clusters. III. Some simple dynamical models. Astron. J. 71, 64–75 (1966)

    ADS  Article  Google Scholar 

  29. 29

    Ferraro, F. R. et al. The giant, horizontal, and asymptotic branches of Galactic globular clusters. I. The catalog, photometric observables, and features. Astron. J. 118, 1738–1758 (1999)

    ADS  Article  Google Scholar 

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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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The authors declare no competing financial interests.

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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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