Accretion-Powered Stellar Winds III: Spin Equilibrium Solutions

TL;DR

This paper develops a semi-analytic model for the spin equilibrium of pre-main-sequence stars, showing stellar winds can significantly regulate stellar rotation by removing angular momentum, with predictions matching observed slow rotation rates.

Contribution

It introduces a new formula linking stellar wind and accretion rates to the star's spin, providing a quantitative framework for understanding stellar angular momentum regulation.

Findings

Stellar winds can carry away more angular momentum than disk transfer.

The equilibrium spin rate depends on the ratio of wind to accretion mass flow and magnetic parameters.

Maximum wind-driven mass flow ratio is limited to about 0.6 by energy considerations.

Abstract

We compare the stellar wind torque calculated in a previous work (Paper II) to the spin-up and spin-down torques expected to arise from the magnetic interaction between a slowly rotating ($\sim 10$% of breakup) pre-main-sequence star and its accretion disk. This analysis demonstrates that stellar winds can carry off orders of magnitude more angular momentum than can be transferred to the disk, provided that the mass outflow rates are greater than the solar wind. Thus, the equilibrium spin state is simply characterized by a balance between the angular momentum deposited by accretion and that extracted by a stellar wind. We derive a semi-analytic formula for predicting the equilibrium spin rate as a function only of the ratio of $\dot M_{\rm w} / \dot M_{\rm a}$ and a dimensionless magnetization parameter, $\Psi \equiv B_*^2 R_*^2 (\dot M_{\rm a} v_{\rm esc})^{-1}$, where $\dot M_{\rm w}$ is the stellar wind mass outflow rate, $\dot M_{\rm a}$ the accretion rate, $B_*$ the stellar surface magnetic field strength, $R_*$ the stellar radius, and $v_{\rm esc}$ the surface escape speed. For parameters typical of accreting pre-main-sequence stars, this explains spin rates of $\sim 10$% of breakup speed for $\dot M_{\rm w} / \dot M_{\rm a} \sim 0.1$. Finally, the assumption that the stellar wind is driven by a fraction of the accretion power leads to an upper limit to the mass flow ratio of $\dot M_{\rm w} / \dot M_{\rm a} \la 0.6$.