Magnetic Field Amplification via Protostellar Disc Dynamos

TL;DR

This paper uses numerical simulations to show how an $ ext{α} ext{Ω}$-dynamo in protostellar discs can generate magnetic fields capable of launching observed MHD outflows, with specific growth rates and mode excitations.

Contribution

It demonstrates the magnetic field growth mechanism via an $ ext{α} ext{Ω}$-dynamo in protostellar discs and links it to observed outflow properties, highlighting the dynamo's role in magnetic field generation.

Findings

Magnetic fields grow exponentially with rate proportional to $ ext{Ω}_K imes ext{sqrt}( ext{α}_d)$.

Dynamo excites dipole and octupole modes, suppresses quadrupole modes.

Weak seed fields can grow to launch winds with mass fluxes of about $10^{-9} M_{ ext{sun}}/yr$.

Abstract

We numerically investigate the generation of a magnetic field in a protostellar disc via an $\alpha \Omega$-dynamo and the resulting magnetohydrodynamic (MHD) driven outflows. We find that for small values of the dimensionless dynamo parameter $\alpha_d$ the poloidal field grows exponentially at a rate $\sigma \propto \Omega_K \sqrt{\alpha_d}$, before saturating to a value $\propto \sqrt{\alpha_d}$. The dynamo excites dipole and octupole modes, but quadrupole modes are suppressed, because of the symmetries of the seed field. Initial seed fields too weak to launch MHD outflows are found to grow sufficiently to launch winds with observationally relevant mass fluxes of order $10^{-9} M_{\odot}/\rm{yr}$ for T Tauri stars. This suggests $\alpha \Omega$-dynamos may be responsible for generating magnetic fields strong enough to launch observed outflows.