Angular Momentum Loss and Stellar spin-down in Magnetic Massive Stars

TL;DR

This study investigates how magnetic fields influence angular momentum loss and spin-down in hot stars with stellar winds, revealing that dipole magnetic fields cause slower spin-down compared to monopole models, with implications for stellar evolution.

Contribution

It provides a detailed 2-D MHD simulation analysis of magnetic hot star winds, comparing dipole and monopole field effects on angular momentum loss and stellar spin-down.

Findings

Angular momentum loss follows Weber-Davis scaling on average.

Dipole fields have smaller Alfvén radii than monopole fields.

Typical spin-down times are around 1 million years.

Abstract

We examine the angular momentum loss and associated rotational spin-down for magnetic hot stars with a line-driven stellar wind and a rotation-aligned dipole magnetic field. Our analysis here is based on our previous 2-D numerical MHD simulation study that examines the interplay among wind, field, and rotation as a function of two dimensionless parameters, W(=Vrot/Vorb) and 'wind magnetic confinement', $\eta_\ast$ defined below. We compare and contrast the 2-D, time variable angular momentum loss of this dipole model of a hot-star wind with the classical 1-D steady-state analysis by Weber and Davis (WD), who used an idealized monopole field to model the angular momentum loss in the solar wind. Despite the differences, we find that the total angular momentum loss averaged over both solid angle and time follows closely the general WD scaling $\dot {J} \sim \dot {M} \Omega R_A^2$. The key distinction is that for a dipole field Alfv\`en radius $R_A$ is significantly smaller than for the monopole field WD used in their analyses. This leads to a slower stellar spin-down for the dipole field with typical spin-down times of order 1 Myr for several known magnetic massive stars.