title: A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes.
abstract: NGC 6791 is a well studied open cluster that it is so close to us that can be imaged down to very faint luminosities. The main-sequence turn-off age ( approximately 8 Gyr) and the age derived from the termination of the white dwarf cooling sequence ( approximately 6 Gyr) are very different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, (22)Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallize and phase separation of the main constituents of the core of a typical white dwarf ((12)C and (16)O) is expected to occur. This sequence of events is expected to introduce long delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, resolving the age discrepancy for NGC 6791.
authors.affiliation: Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain. garcia@fa.upc.edu
authors.forename: Enrique
authors.initials: E
authors.lastname: García-Berro
authors.forename: Santiago
authors.initials: S
authors.lastname: Torres
authors.forename: Leandro G
authors.initials: LG
authors.lastname: Althaus
authors.forename: Isabel
authors.initials: I
authors.lastname: Renedo
authors.forename: Pablo
authors.initials: P
authors.lastname: Lorén-Aguilar
authors.forename: Alejandro H
authors.initials: AH
authors.lastname: Córsico
authors.forename: René D
authors.initials: RD
authors.lastname: Rohrmann
authors.forename: Maurizio
authors.initials: M
authors.lastname: Salaris
authors.forename: Jordi
authors.initials: J
authors.lastname: Isern