title: A spin-down clock for cool stars from observations of a 2.5-billion-year-old cluster.
abstract: The ages of the most common stars--low-mass (cool) stars like the Sun, and smaller--are difficult to derive because traditional dating methods use stellar properties that either change little as the stars age or are hard to measure. The rotation rates of all cool stars decrease substantially with time as the stars steadily lose their angular momenta. If properly calibrated, rotation therefore can act as a reliable determinant of their ages based on the method of gyrochronology. To calibrate gyrochronology, the relationship between rotation period and age must be determined for cool stars of different masses, which is best accomplished with rotation period measurements for stars in clusters with well-known ages. Hitherto, such measurements have been possible only in clusters with ages of less than about one billion years, and gyrochronology ages for older stars have been inferred from model predictions. Here we report rotation period measurements for 30 cool stars in the 2.5-billion-year-old cluster NGC 6819. The periods reveal a well-defined relationship between rotation period and stellar mass at the cluster age, suggesting that ages with a precision of order 10 per cent can be derived for large numbers of cool Galactic field stars.
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA.
authors.forename: Søren
authors.initials: S
authors.lastname: Meibom
authors.affiliation: 1] Leibniz Institute for Astrophysics, An der Sternwarte 16, 14482 Potsdam, Germany [2] Space Science Institute, 4750 Walnut Street #205, Boulder, Colorado 80301, USA.
authors.forename: Sydney A
authors.initials: SA
authors.lastname: Barnes
authors.affiliation: Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
authors.forename: Imants
authors.initials: I
authors.lastname: Platais
authors.affiliation: Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
authors.forename: Ronald L
authors.initials: RL
authors.lastname: Gilliland
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA.
authors.forename: David W
authors.initials: DW
authors.lastname: Latham
authors.affiliation: Department of Astronomy, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: Robert D
authors.initials: RD
authors.lastname: Mathieu