Improved angular momentum evolution model for solar-like stars II. Exploring the mass dependence

TL;DR

This study presents improved angular momentum evolution models for low-mass stars, incorporating recent wind braking laws, and explores how stellar mass influences rotational behavior from pre-main sequence to old age.

Contribution

The paper introduces a new wind braking law and detailed models for different stellar masses, enhancing understanding of rotational evolution and internal dynamics of low-mass stars.

Findings

Models reproduce observed rotational distributions from 1 Myr to 10 Gyr.

Lower mass stars have longer core-envelope coupling timescales.

Magnetic topology changes may influence wind braking efficiency.

Abstract

We developed angular momentum evolution models for 0.5 and 0.8 $M_{\odot}$ stars. The parametric models include a new wind braking law based on recent numerical simulations of magnetised stellar winds, specific dynamo and mass-loss rate prescriptions, as well as core/envelope decoupling. We compare model predictions to the distributions of rotational periods measured for low mass stars belonging to star forming regions and young open clusters. Furthermore, we explore the mass dependence of model parameters by comparing these new models to the solar-mass models we developed earlier. Rotational evolution models are computed for slow, median, and fast rotators at each stellar mass. The models reproduce reasonably well the rotational behaviour of low-mass stars between 1 Myr and 8-10 Gyr, including pre-main sequence to zero-age main sequence spin up, prompt zero-age main sequence spin down, and early-main sequence convergence of the surface rotation rates. Fast rotators are found to have systematically shorter disk lifetimes than moderate and slow rotators, thus enabling dramatic pre-main sequence spin up. They also have shorter core-envelope coupling timescales, i.e., more uniform internal rotation. As to the mass dependence, lower mass stars require significantly longer core-envelope coupling timescale than solar-type ones, which results in strong differential rotation developing in the stellar interior on the early main sequence. Lower mass stars also require a weaker braking torque to account for their longer spin down timescale on the early main sequence, while they ultimately converge towards lower rotational velocities than solar-type stars on the longer term due to their reduced moment of inertia. We also find evidence that the mass-dependence of the wind braking efficiency may be related to a change of the magnetic topology in lower mass stars.