On large-scale dynamos with stable stratification and the application to stellar radiative zones

Abstract

Our understanding of large-scale magnetic fields in stellar radiative zones remains fragmented and incomplete. Such magnetic fields, which must be produced by some form of dynamo mechanism, are thought to dominate angular-momentum transport, making them crucial to stellar evolution. A major difficulty is the effect of stable stratification, which generally suppresses dynamo action. We explore the effects of stable stratification on mean-field dynamo theory with a particular focus on a non-helical large-scale dynamo (LSD) mechanism known as the magnetic shear-current effect. We find that the mechanism is robust to increasing stable stratification as long as the original requirements for its operation are met: a source of shear and non-helical magnetic fluctuations (e.g. from a small-scale dynamo). Both are plausibly sourced in the presence of differential rotation. Our idealized direct numerical simulations, supported by mean-field theory, demonstrate the generation of near equipartition large-scale toroidal fields. Additionally, a scan over magnetic Reynolds number shows no change in the growth or saturation of the LSD, providing good numerical evidence of a dynamo mechanism resilient to catastrophic quenching, which has been an issue for helical dynamos. These properties -- the absence of catastrophic quenching and robustness to stable stratification -- make the mechanism a plausible candidate for generating in-situ large-scale magnetic fields in stellar radiative zones.

References in corpus (17)

• Slowing the Spins of Stellar Cores
• On magnetic instabilities and dynamo action in stellar radiation zones
• White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Linewidths: Kepler Observations of 27 Pulsating DA White Dwarfs Through K2 Campaign 8
• Numerical demonstration of fluctuation dynamo at low magnetic Prandtl numbers
• Generation of Magnetic Field by Combined Action of Turbulence and Shear
• Population synthesis of isolated Neutron Stars with magneto--rotational evolution
• On Self-Sustained Dynamo Cycles in Accretion Discs
• Constraining the efficiency of angular momentum transport with asteroseismology of red giants: the effect of stellar mass
• Asteroseismology of evolved stars to constrain the internal transport of angular momentum II. Test of a revised prescription for transport by the Tayler instability
• The spins of compact objects born from helium stars in binary systems
• Strong mean field dynamos require supercritical helicity fluxes
• Turbulent transport in radiative zones of stars
• Shear mixing in stellar radiative zones I. Effect of thermal diffusion and chemical stratification
• Statistical simulation of the magnetorotational dynamo
• Numerical study of large-scale vorticity generation in shear-flow turbulence
• Asteroseismology of evolved stars to constrain the internal transport of angular momentum. III. Using the rotation rates of intermediate-mass stars to test the Fuller-formalism
• Horizontal shear instabilities at low Prandtl number

Cited by in corpus (6)

• Magnetism in High-Mass Stars
• Numerical simulations of the Tayler-Spruit dynamo in proto-magnetars
• Tayler Instability Revisited
• Vorticity and magnetic dynamo from subsonic expansion waves
• Vorticity and magnetic dynamo from subsonic expansion waves II: Dependence on magnetic Prandtl number, forcing scale, cooling time
• Effect of flow shear on the onset of dynamos