Exploring the evolution of stellar rotation using Galactic kinematics

TL;DR

This study uses galactic kinematics as an age proxy to investigate the rotational evolution of low-mass stars, revealing that traditional models do not apply to stars older than 1 Gyr and highlighting differences in rotation rates among spectral types.

Contribution

It demonstrates the potential of kinematic ages for calibrating gyrochronology and uncovers mass-dependent rotational behaviors in older stars, challenging existing models.

Findings

Gyrochronology models calibrated on young clusters do not apply beyond 1 Gyr.

Late-K dwarfs rotate faster or at the same rate as early-K dwarfs at old ages.

Older GKM stars tend to stay magnetically active longer than more massive stars.

Abstract

The rotational evolution of cool dwarfs is poorly constrained after around 1-2 Gyr due to a lack of precise ages and rotation periods for old main-sequence stars. In this work we use velocity dispersion as an age proxy to reveal the temperature-dependent rotational evolution of low-mass Kepler dwarfs, and demonstrate that kinematic ages could be a useful tool for calibrating gyrochronology in the future. We find that a linear gyrochronology model, calibrated to fit the period-Teff relationship of the Praesepe cluster, does not apply to stars older than around 1 Gyr. Although late-K dwarfs spin more slowly than early-K dwarfs when they are young, at old ages we find that late-K dwarfs rotate at the same rate or faster than early-K dwarfs of the same age. This result agrees qualitatively with semi-empirical models that vary the rate of surface-to-core angular momentum transport as a function of time and mass. It also aligns with recent observations of stars in the NGC 6811 cluster, which indicate that the surface rotation rates of K dwarfs go through an epoch of inhibited evolution. We find that the oldest Kepler stars with measured rotation periods are late-K and early-M dwarfs, indicating that these stars maintain spotted surfaces and stay magnetically active longer than more massive stars. Finally, based on their kinematics, we confirm that many rapidly rotating GKM dwarfs are likely to be synchronized binaries.