A spin-down clock for cool stars from observations of a 2.5-billion-year-old cluster

TL;DR

This paper presents rotation period measurements for cool stars in a 2.5-billion-year-old cluster, enabling more accurate age estimates for such stars through gyrochronology.

Contribution

It provides the first calibration of stellar rotation periods at an age beyond one billion years, improving age determination methods for cool stars.

Findings

Established a clear rotation-mass relationship at 2.5 billion years

Demonstrated gyrochronology can estimate stellar ages with ~10% precision

Extended calibration of stellar ages to older stars than previously possible

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.