Transition of latitudinal differential rotation as a possible cause of weakened magnetic braking of solar-type stars

TL;DR

This paper explores how latitudinal differential rotation influences the spin-down of solar-type stars, proposing a model that explains observed rotation trends and the weakening of magnetic braking with stellar age.

Contribution

It introduces a new model linking latitudinal differential rotation transition to stellar spin evolution, accounting for observed rotation behaviors across different stellar ages and metallicities.

Findings

The model reproduces the current solar rotation rate.

It explains the average rotation trend of solar-type stars.

It accounts for the weakened magnetic braking in older stars.

Abstract

We investigate the role of latitudinal differential rotation (DR) in the spin evolution of solar-type stars. Recent asteroseismic observation detected the strong equator-fast DR in some solar-type stars. Numerical simulations show that the strong equator-fast DR is a typical feature of young fast-rotating stars and that this tendency is gradually reduced with stellar age. Incorporating these properties, we develop a model for the long-term evolution of stellar rotation. The magnetic braking is assumed to be regulated dominantly by the rotation rate in the low-latitude region. Therefore, in our model, stars with the equator-fast DR spin down more efficiently than those with the rigid-body rotation. We calculate the evolution of stellar rotation in ranges of stellar mass, $0.9 \, \mathrm{M}_{\odot} \le M \le 1.2\, \mathrm{M}_{\odot}$, and metallicity, $0.5\, \mathrm{Z}_{\odot} \le Z \le 2\, \mathrm{Z}_{\odot}$, where $\mathrm{M}_{\odot}$ and $\mathrm{Z}_{\odot}$ are the solar mass and metallicity, respectively. Our model, using the observed torque in the present solar wind, nicely explains both the current solar rotation and the average trend of the rotation of solar-type stars, including the dependence on metallicity. In addition, our model naturally reproduces the observed trend of the weakened magnetic braking in old slowly rotating solar-type stars because strong equator-fast DR becomes reduced. Our results indicate that LDR and its transition are essential factors that control the stellar spin down.