Accretion-Powered Stellar Winds III: Spin Equilibrium Solutions

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 ${\dot{M}}_{\mathrm{w}}/{\dot{M}}_{\mathrm{a}}$ and a dimensionless magnetization parameter, $\Psi \equiv B_{\ast }^2{R}_{\ast }^2({\dot{M}}_{\mathrm{a}}{v}_{\mathrm{esc}}{)}^{-1}$, where ${\dot{M}}_{\mathrm{w}}$ is the stellar wind mass outflow rate, ${\dot{M}}_{\mathrm{a}}$ the accretion rate, $B_{\ast }$ the stellar surface magnetic field strength, $R_{\ast }$ the stellar radius, and $v_{\mathrm{esc}}$ the surface escape speed. For parameters typical of accreting pre-main-sequence stars, this explains spin rates of $\sim 10$% of breakup speed for ${\dot{M}}_{\mathrm{w}}/{\dot{M}}_{\mathrm{a}}\sim 0.1$. 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 $\dot M_{\rm w} / \dot M_{\rm a} \la 0.6$.

Accepted for publication in ApJ

References in corpus (5)

• Which jet launching mechanism(s) in TTauri stars?
• Probing T Tauri Accretion and Outflow with 1 Micron Spectroscopy
• Accretion-Powered Stellar Winds II: Numerical Solutions for Stellar Wind Torques
• Accretion Disks around Young Stars: Lifetimes, Disk Locking and Variability
• Modeling T Tauri Winds from He I 10830 Profiles

Cited by in corpus (5)

• Turbulence-driven Polar Winds from T Tauri Stars Energized by Magnetospheric Accretion
• Magnetocentrifugally Driven Flows from Young Stars and Disks. VI. Accretion with a Multipole Stellar Field
• Redshifted Absorption at He I 10830 as a Probe of the Accretion Geometry of T Tauri Stars
• The non-dipolar magnetic fields of accreting T Tauri stars
• On the Effect of Magnetic Spots on Stellar Winds and Angular Momentum Loss