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A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes

Nature volume 465, pages 194–196 (2010)Cite this article

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Abstract

NGC 6791 is a well studied open cluster1 that it is so close to us that can be imaged down to very faint luminosities2. The main-sequence turn-off age (∼8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (∼6 Gyr) are very different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars3,4,5. At lower temperatures, white dwarfs are expected to crystallize and phase separation of the main constituents of the core of a typical white dwarf (12C and 16O) is expected to occur6,7. This sequence of events is expected to introduce long delays in the cooling times8,9, but has not hitherto been proven. Here we report that, as theoretically anticipated5,6, physical separation processes occur in the cores of white dwarfs, resolving the age discrepancy for NGC 6791.

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Figure 1: Colour–magnitude diagrams of the white dwarfs in NGC 6791.
Figure 2: White dwarf luminosity function of NGC 6791.

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Acknowledgements

This research was partially supported by MCINN, AGENCIA, the Generalitat de Catalunya, STFC and CONICET. L.G.A. also acknowledges a PIV grant from the AGAUR of the Generalitat de Catalunya. We are indebted to L. Bedin and co-authors for providing the observational colour–magnitude diagram of Fig. 1.

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Authors and Affiliations

  1. Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain ,

    Enrique García-Berro, Santiago Torres, Leandro G. Althaus, Isabel Renedo & Pablo Lorén-Aguilar

  2. Institut d’Estudis Espacials de Catalunya, Ed. Nexus-201, c/Gran Capità 2-4, 08034 Barcelona, Spain ,

    Enrique García-Berro, Santiago Torres, Isabel Renedo, Pablo Lorén-Aguilar & Jordi Isern

  3. Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina ,

    Leandro G. Althaus & Alejandro H. Córsico

  4. Instituto de Astrofísica de La Plata (CCT La Plata), CONICET. 1900 La Plata, Argentina

    Leandro G. Althaus & Alejandro H. Córsico

  5. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio, CONICET, Av. de España 1512 (Sur) CC 49, 5400 San Juan, Argentina ,

    René D. Rohrmann

  6. Astrophysics Research Institute, Liverpool John Moores University, 12 Quays House, Birkenhead CH41 1LD, UK ,

    Maurizio Salaris

  7. Institut de Ciències de l’Espai (CSIC), Facultat de Ciències, Campus UAB, Torre C5-parell, 2a planta, 08193 Bellaterra, Spain ,

    Jordi Isern

Authors
  1. Enrique García-Berro
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  2. Santiago Torres
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  3. Leandro G. Althaus
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  4. Isabel Renedo
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  5. Pablo Lorén-Aguilar
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  6. Alejandro H. Córsico
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  7. René D. Rohrmann
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  8. Maurizio Salaris
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  9. Jordi Isern
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Contributions

E.G.-B. and L.G.A. conceived the study. E.G.-B. wrote the paper. J.I., P.L.-A. and E.G.-B. computed the theoretical expressions for the time delays introduced by the different separation processes. L.G.A., I.R., A.H.C. and E.G.-B. computed the cooling sequences. R.D.R. provided the appropriate colours. S.T., E.G.-B. and J.I. did the Monte Carlo simulations. M.S. computed the main-sequence evolutionary ages and errors. All authors discussed the results and made substantial contributions to the manuscript.

Corresponding author

Correspondence to Enrique García-Berro.

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The authors declare no competing financial interests.

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García-Berro, E., Torres, S., Althaus, L. et al. A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes. Nature 465, 194–196 (2010). https://doi.org/10.1038/nature09045

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