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 1,2 because traditional dating methods use stellar properties that either change little as the stars age 3,4 or are hard to measure 5-8 . 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 2,9-11 . 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 12-16 , and gyrochronology ages for older stars have been inferred from model predictions 2,7,11,17 . Here we report rotation period measurements for 30 cool stars in the 2.5-billion-year-old cluster NGC 6819. The periods reveal a welldefined 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.Prior observations in star clusters with ages ≲300million years (Myr) have shown that cool stars begin their main-sequence phase with a dispersion in their rotation periods, P, spanning two orders of magnitude [12][13][14]18 (0.1-10 d). However, this dispersion diminishes rapidly with cluster age, t, as they lose angular momentum through magnetically channelled winds 19 , causing their periods to increase and converge to a well-defined relationship with stellar mass, M, by the age (600 Myr) of the Hyades cluster 15,20 . These observations suggest that cool main-sequence stars older than the Hyades probably occupy a single surface, P = P(t, M), in P-t-M space, which can be defined by measurements of their periods and photometric colours (a proxy for stellar mass) in a series of age-ranked clusters (Fig. 1). Measuring stellar rotation periods in clusters older than the Hyades will confirm or deny the existence of such a surface, and, if it exists, define its shape and thickness.Models of cool-star rotational evolution also describe a convergence of rotation with age 2,11,17,21 . However, they differ in their predictions of the location and shape of the P-t-M surface and, beyond the common assumption that the rotation period of the Sun is typical for stars of its mass and age, were until 2011 16 unconstrained at ages greater than 600 Myr.

Stellar rotation, binarity, and lithium in the open cluster IC 4756

“…The rotation periods of stars in open clusters with well-determined ages will not only confirm the gyro-ages put forward in previous years by Barnes (2007Barnes ( , 2010), but will also help to quantitatively establish the angular-momentum transport mechanism as a function of stellar evolution (Barnes & Kim 2010;Gallet & Bouvier 2013;Meibom et al 2013Meibom et al , 2015. The magnitude and sign of the surface differential rotation pattern can even specify the role of the stellar magnetic field because differential rotation constrains the large-scale magnetic surface configurations, and will tell us about the underlying dynamo process (e.g., Küker et al 2011;Käpylä et al 2013;Hathaway et al 2013;Rüdiger et al 2014).…”

Improved angular momentum evolution model for solar-like stars

“…Indeed, the recent results of Meibom et al (2015) on NGC 6819, a 2.5 Gyr open cluster, yield a tight mass-rotation period relationship over the mass range 0.85−1.3 M at this age. In this cluster, solar-mass stars have rotational periods narrowly distributed within P 1 M = 17−19d; while 0.85 M stars have periods in the range P 0.85 M = 21−24d.…”

Asteroseismology of red giants: From analysing light curves to estimating ages

“…With the advent of the Kepler (Borucki et al 2010) and CoRoT (Baglin et al 2006) space missions, time series measuring stellar variability of very-high quality have become widely available. Analysis of these time series can deliver precise estimates of stellar ages Lebreton et al 2014;Meibom et al 2015;Metcalfe et al 2015;Miglio et al 2013b;Silva Aguirre et al 2015) -a quantity critical for reconstructing the history of the Milky Way. With the re-purposed Kepler mission K2 (Howell et al 2014) currently capturing solar-like oscillations Stello et al 2015) in a number of different galactic directions, and the future missions of TESS (Ricker et al 2014) and PLATO (Rauer et al 2014) adding to this, ages for many thousands of stars in many different galactic distances and directions present an exciting possibility.…”