重力の星の安定性への役割

オリジナルソース

タイトル: Gravity's role in taming the Tayler instability in red giant cores

概要: The stability of toroidal magnetic fields within the interior of stars remains a significant unresolved issue in contemporary astrophysics. In this study, we combine a nonlocal linear analysis with 3D direct numerical simulations to examine the instability of toroidal fields within nonrotating, stably stratified stellar interiors in spherical geometry. Both analyses start from an equilibrium solution derived from balancing the Lorentz force with an anisotropic component of the fluid pressure, which is unstable to the (nonaxisymmetric) Tayler instability, and account for the combined effects of gravity and thermal diffusion. The numerical simulations incorporate finite magnetic resistivity and fluid viscosity while reaching a regime of highly stable stratification that has never been explored before. The linear analysis, which is global in the radial direction, shows that gravity significantly reduces the growth rate of the instability and uncovers the importance of unstable modes with low radial wavenumbers operating at low latitudes. The simulations trace the entire evolution of the instability from the linear to the nonlinear phase and strongly corroborate the findings of the linear analysis. Our results reveal that in highly stratified stellar interiors, the newly configured magnetic fields remain unstable only on the thermal diffusion timescale. Combining the linear analysis results with stellar evolution models of low-mass stars, we find that the limiting toroidal field strength for Tayler instability in red giant cores decreases with the stellar evolution. The predicted field strengths align with the ones expected from recent asteroseismic observations, suggesting that the observed fields may be remnants of a Tayler instability during the transition from the main sequence to the giant phase.