New branch of the Tayler-Spruit dynamo in stellar radiative zones

TL;DR

This paper introduces a new understanding of the Tayler-Spruit dynamo mechanism in stellar radiative zones, revealing bistable magnetic field configurations and their implications for angular momentum transport and asteroseismic observations.

Contribution

The study uncovers two bistable dynamo branches driven by different instabilities and provides new scaling laws and constraints for magnetic field effects in stellar models.

Findings

Discovery of two bistable dynamo branches with different magnetic field locations.

The polar branch driven by Tayler instability can operate under strong stratification.

New scaling laws for magnetic fields and shear thresholds.

Abstract

The recent asteroseismic observations constitute a great challenge for rotating stellar evolution models, which predict too fast internal rotation rates when only hydrodynamic processes are included. This suggests the absence of one or several unidentified angular momentum transport processes in these models. Transport by large-scale and strong magnetic fields in the radiative zone is a promising candidate to explain the observations. While these fields may have a fossil origin, a dynamo driven by the Tayler instability in a shear flow constitute a primary mechanism to form the necessary magnetic fields. Despite recent numerical studies, this mechanism remains poorly known. Motivated by this, we investigate the Tayler-Spruit dynamo through a new set of three-dimensional direct numerical simulations. We model the radiative zone as a Boussinesq stably stratified fluid whose differential rotation is maintained by a volumetric body force. We report for the first time the existence of two bistable dynamo branches, which mainly differ by the magnetic field location (near the equator and the polar axis). While the equatorial branch is driven by the magnetorotational instability, we mainly investigate the newly identified polar branch, which is driven by the Tayler instability. We show that this branch can still operate and transport angular momentum efficiently in a very strong stratification regime. We extract new scaling laws for the different magnetic field components, transports processes, and the minimum shear to trigger the Tayler instability-driven dynamo. Finally, we roughly constrain the signature of the generated magnetic fields on asteroseismic modes propagating in main-sequence and evolved stars. Thus, our results fosters new studies using stellar evolution models including our prescriptions and the search of asteroseismic signals impacted by large-scale azimuthal magnetic fields.