Rotational evolution of slow-rotators sequence stars

TL;DR

This paper models the rotational evolution of slow-rotator stars, revealing how core-envelope coupling and wind braking influence their spin-down, and provides gyro-chronology tools for age estimation based on rotation periods.

Contribution

It introduces a coupled rotational evolution model with a new wind braking law, fitting data with MCMC, and derives period isochrones for stellar age inference.

Findings

Core-envelope coupling time-scale scales as M^{-7.28} in the 0.85-1.10 M_sun range.

Solid-body rotation is achieved after 1-2 Gyr, affecting period-age relations.

Empirical wind loss depends on convective turnover time and stellar moment of inertia.

Abstract

The observed mass-age-rotation relationship in open clusters shows the progressive development of a slow-rotators sequence. The observed clustering on this sequence suggests that it corresponds to some equilibrium or asymptotic condition that still lacks a complete theoretical interpretation, crucial to our understanding of the stellar angular momentum evolution. We couple a rotational evolution model, which takes into account internal differential rotation, with classical and new proposals for the wind braking law, and fit models to the data using a MCMC method. The description of the evolution of the slow-rotators sequence requires taking into account the transfer of angular momentum from the radiative core to the convective envelope; we find that, in the mass range 0.85-1.10 $M_{\odot}$, the core-envelope coupling time-scale for stars in the slow-rotators sequence scales as $M^{-7.28}$. Quasi-solid body rotation is achieved only after 1-2 Gyr, depending on stellar mass, which implies that observing small deviations from the Skumanich law ($P \propto \sqrt{t}$) would require period data of older open clusters than available to date. The observed evolution in the 0.1-2.5 Gyr age range and in the 0.85-1.10 $M_{\odot}$ mass range is best reproduced by assuming an empirical mass dependence of the wind angular momentum loss proportional to the convective turnover time-scale and to the stellar moment of inertia. Period isochrones based on our MCMC fit provide a tool for inferring stellar ages of solar-like main-sequence stars from their mass and rotation period largely independent from the wind braking model adopted. These effectively represent gyro-chronology relationships that take into account the physics of the two-zone model for the stellar angular momentum evolution.