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Stellar mergers as the origin of the blue main-sequence band in young star clusters

Nature Astronomy volume 6, pages 480–487 (2022)Cite this article

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Abstract

Recent high-quality Hubble Space Telescope photometry shows that the main-sequence stars of young star clusters form two discrete components in the colour–magnitude diagram. On the basis of their distribution in the colour–magnitude diagram, we show that stars of the blue main-sequence component can be understood as slow rotators originating from stellar mergers. We derive the masses of the blue main-sequence stars, and find that they follow a nearly flat mass function, which supports their unusual formation path. Our results imply that the cluster stars gain their mass in two different ways: by disk accretion leading to rapid rotation, contributing to the red main sequence, or by binary merger leading to slow rotation, populating the blue main sequence. We also derive the approximate merger time of the individual stars of the blue main-sequence component, and find a strong early peak in the merger rate, with a lower-level merger activity prevailing for tens of millions of years. This supports recent binary-formation models, and explains new velocity dispersion measurements for members of young star clusters. Our findings shed new light on the origin of the bimodal mass, spin and magnetic-field distributions of main-sequence stars.

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Fig. 1: CMD of NGC 1755 and the isochrone fits.
Fig. 2: Mass function of the red and the blue MS stars in NGC 1755.
Fig. 3: Binary evolution models at 30 Myr in the CMD.
Fig. 4: Stellar merger history in NGC 1755.

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Data availability

The observational data in this work can be found at https://doi.org/10.5281/zenodo.5770868. The MESA inlist files used to compute the single- and binary-star models in this work can be downloaded at: https://doi.org/10.5281/zenodo.5233209.

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Acknowledgements

The research leading to these results has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement 772225: MULTIPLES). C.W. acknowledges funding from a CSC scholarship. This work has received funding from the ERC under the European Union’s Horizon 2020 research innovation programme (grant agreement ERC-StG 2016, 716082 ‘GALFOR’, principal investigator A.M., http://progetti.dfa.unipd.it/GALFOR). A.M. acknowledges support from MIUR through the FARE project R164RM93XW SEMPLICE (principal investigator A.M.) and the PRIN programme 2017Z2HSMF (principal investigator Bedin). H.S. and J.B. acknowledge support from the FWO Odysseus programme under project G0F8H6N. N.C. gratefully acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG)—CA 2551/1-1. D.J.L. acknowledges support from the Spanish Government Ministerio de Ciencia, Innovación y Universidades through grants PGC-2018-091 3741-B-C22 and from the Canarian Agency for Research, Innovation and Information Society (ACIISI) of the Canary Islands Government, and the European Regional Development Fund (ERDF), under grant ProID2017010115. P.M. acknowledges support from FWO junior postdoctoral fellowship 12ZY520N. S.E.d.M. acknowledges funding by the Netherlands Organization for Scientific Research (NWO) as part of the Vidi research programme BinWaves with project number 639.042.728.

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Authors and Affiliations

  1. Argelander-Institut für Astronomie, Universität Bonn, Bonn, Germany

    Chen Wang, Norbert Langer, Abel Schootemeijer, Ben Hastings & Xiao-Tian Xu

  2. Max Planck Institute for Astrophysics, Garching, Germany

    Chen Wang & Selma E. de Mink

  3. Max-Planck-Institut für Radioastronomie, Bonn, Germany

    Norbert Langer, Ben Hastings & Xiao-Tian Xu

  4. Dipartimento di Fisica e Astronomia ‘Galileo Galilei’, Università di Padova, Padua, Italy

    Antonino Milone

  5. Institute of Astronomy, KU Leuven, Leuven, Belgium

    Julia Bodensteiner, Hugues Sana, Pablo Marchant & A. de Koter

  6. European Southern Observatory, Garching bei München, Germany

    Julia Bodensteiner

  7. Leibniz-Institut für Astrophysik Potsdam (AIP), Potsdam, Germany

    Norberto Castro

  8. Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain

    D. J. Lennon

  9. Departamento Astrofísica, Universidad de La Laguna, La Laguna, Tenerife, Spain

    D. J. Lennon

  10. Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, Amsterdam, The Netherlands

    A. de Koter & Selma E. de Mink

  11. Center for Astrophysics, Harvard–Smithsonian, Cambridge, MA, USA

    Selma E. de Mink

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Contributions

C.W., A.S., B.H. and X.-T.X. performed the stellar evolution calculations, based on earlier work by P.M. and on his advice. N.L. carried out the analysis and interpretation of the results, together with C.W., A.S., B.H. and S.E.d.M. A.M., J.B., H.S., N.C., D.J.L. and A.d.K. provided an interpretation of the related observations. All authors reviewed the manuscript.

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Correspondence to Chen Wang.

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Supplementary Sections A–E, Tables 1 and 2 and Figs. 1–20.

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Wang, C., Langer, N., Schootemeijer, A. et al. Stellar mergers as the origin of the blue main-sequence band in young star clusters. Nat Astron 6, 480–487 (2022). https://doi.org/10.1038/s41550-021-01597-5

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