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

Enrique García-Berro (), Santiago Torres, Leandro G. Althaus, Isabel Renedo, Pablo Lorén-Aguilar, Alejandro H. Córsico, René D. Rohrmann, Maurizio Salaris and Jordi Isern
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Enrique García-Berro: Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain
Santiago Torres: Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain
Leandro G. Althaus: Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain
Isabel Renedo: Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain
Pablo Lorén-Aguilar: Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain
Alejandro H. Córsico: Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina
René D. Rohrmann: Instituto de Ciencias Astronómicas, de la Tierra y del Espacio, CONICET, Av. de España 1512 (Sur) CC 49, 5400 San Juan, Argentina
Maurizio Salaris: Astrophysics Research Institute, Liverpool John Moores University, 12 Quays House, Birkenhead CH41 1LD, UK
Jordi Isern: Institut d’Estudis Espacials de Catalunya, Ed. Nexus-201, c/Gran Capità 2-4, 08034 Barcelona, Spain

Nature, 2010, vol. 465, issue 7295, 194-196

Abstract: The 'white dwarf' clock reset White dwarfs are the most common endpoint of stellar evolution, so they convey important information about the structure and evolution of a galaxy — its age for instance. NGC 6791 is a metal-rich open cluster that is so close to us that it can be imaged down to the very faint luminosities of white dwarfs. Its age determined from turn-off ages of its main sequence stars is estimated at ∼8 billion years, but its 'white dwarf luminosity' age, reflecting termination of the white dwarf cooling sequence, is ∼6 billion years. This apparent inconsistency casts doubts on the reliability of white dwarfs as galactic chronometers. One possible explanation is that as white dwarfs cool, 22Ne produced as helium burns sinks into the star's interior, causing crystallization and phase separation of 12C and 16O, which delays cooling. García-Berro et al. use numerical modelling of the entire white dwarf evolution process to show that physical separation does occur in the core of NGC 6791. This confirms 8 billion years as the cluster's age, and restores the reputation of white dwarfs as reliable chronometers.

Date: 2010
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DOI: 10.1038/nature09045

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