title: Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars.
abstract: A knowledge of stellar ages is crucial for our understanding of many astrophysical phenomena, and yet ages can be difficult to determine. As they become older, stars lose mass and angular momentum, resulting in an observed slowdown in surface rotation. The technique of 'gyrochronology' uses the rotation period of a star to calculate its age. However, stars of known age must be used for calibration, and, until recently, the approach was untested for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for stars in an open cluster of intermediate age (NGC 6819; 2.5 Gyr old), and for old field stars whose ages have been determined with asteroseismology. The data for the cluster agree with previous period-age relations, but these relations fail to describe the asteroseismic sample. Here we report stellar evolutionary modelling, and confirm the presence of unexpectedly rapid rotation in stars that are more evolved than the Sun. We demonstrate that models that incorporate dramatically weakened magnetic braking for old stars can--unlike existing models--reproduce both the asteroseismic and the cluster data. Our findings might suggest a fundamental change in the nature of ageing stellar dynamos, with the Sun being close to the critical transition to much weaker magnetized winds. This weakened braking limits the diagnostic power of gyrochronology for those stars that are more than halfway through their main-sequence lifetimes.
authors.affiliation: Carnegie Observatories, 813 Santa Barbara Street, Pasadena, California 91101, USA.|Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA.|Kavli Institute for Theoretical Physics, University of California, Santa Barbara, California 93106-4030, USA.
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: van Saders
authors.affiliation: Laboratoire AIM, Alternative Energies and Atomic Energy Commission (CEA)/Physical Sciences Division (DSM)-Centre National d'Études Spatiales (CNRS)-Université Paris Diderot-Institute of Research into the Fundamental Laws of the Universe (IRFU)/Service d'Astrophysique (SAp), Centre de Saclay, 91191 Gif-sur-Yvette Cedex, France.
authors.forename: Tugdual
authors.initials: T
authors.lastname: Ceillier
authors.affiliation: Space Science Institute, 4750 Walnut Street Suite 205, Boulder, Colorado 80301, USA.
authors.forename: Travis S
authors.initials: TS
authors.lastname: Metcalfe
authors.affiliation: Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus, Denmark.
authors.forename: Victor Silva
authors.initials: VS
authors.lastname: Aguirre
authors.affiliation: Kavli Institute for Theoretical Physics, University of California, Santa Barbara, California 93106-4030, USA.|Ohio State University, Department of Astronomy, 140 West 18th Avenue, Columbus, Ohio 43210, USA.
authors.forename: Marc H
authors.initials: MH
authors.lastname: Pinsonneault
authors.affiliation: Kavli Institute for Theoretical Physics, University of California, Santa Barbara, California 93106-4030, USA.|Laboratoire AIM, Alternative Energies and Atomic Energy Commission (CEA)/Physical Sciences Division (DSM)-Centre National d'Études Spatiales (CNRS)-Université Paris Diderot-Institute of Research into the Fundamental Laws of the Universe (IRFU)/Service d'Astrophysique (SAp), Centre de Saclay, 91191 Gif-sur-Yvette Cedex, France.
authors.forename: Rafael A
authors.initials: RA
authors.lastname: García
authors.affiliation: Kavli Institute for Theoretical Physics, University of California, Santa Barbara, California 93106-4030, USA.|Space Science Institute, 4750 Walnut Street Suite 205, Boulder, Colorado 80301, USA.
authors.forename: Savita
authors.initials: S
authors.lastname: Mathur
authors.affiliation: Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus, Denmark.|School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK.
authors.forename: Guy R
authors.initials: GR
authors.lastname: Davies