Abstract
While magnetic fields have long been considered significant for the evolution of magnetic non-degenerate stars and compact stars, it has become clear in recent years that, in fact, all stars are deeply affected by their effects. This is particularly true regarding their internal angular momentum distribution, but magnetic fields may also influence internal mixing processes and even the fate of the star. We propose a new framework for stellar evolution simulations in which the interplay between magnetic field, rotation, mass loss, and changes in the stellar density and temperature distributions are treated self-consistently. For average large-scale stellar magnetic fields that are symmetric to the axis of the rotation of the star, we derive 1D evolution equations for the toroidal and poloidal components from the mean-field magnetohydrodynamic equation by applying Alfvén’s theorem; and, hence, a conservative form of the angular momentum transfer due to the Lorentz force is formulated. We implement our formalism into a numerical stellar evolution code and simulate the magneto-rotational evolution of 1.5M⊙stars. The Lorentz force aided by the Ω effect imposes torsional Alfvén waves propagating through the magnetized medium, leading to near-rigid rotation within the Alfvén timescale. Our models, with different initial spins andB-fields, can reproduce the main observed properties of Ap/Bp stars. Calculations that are extended to the red-giant regime show a pronounced core-envelope coupling, which are capable of reproducing the core and surface rotation periods already determined by asteroseismic observations.
Highlights
Magnetic fields are visible for many different types of stars
We propose a new framework for stellar evolution simulations in which the interplay between magnetic field, rotation, mass loss, and changes in the stellar density and temperature distributions are treated self-consistently
We have not yet determined the functional form of the electromotive force induced by the α effect, namely, (α · B)/c. We keep this issue open and our results omit the α-effect. This is because α is a pseudo-tensor, which can be a function of the magnetic field strength, the rotation frequency and the local thermodynamic quantities as well, so that it will be complex in the general case
Summary
Magnetic fields are visible for many different types of stars. As the most evident example, the Sun exhibits magnetic activity in the form of spots, prominences, flares, and mass ejections (e.g., Solanki et al 2006). The Sun is considered to be prototypical of FGK-type main-sequence stars that are known to have convective envelopes and most of these cool stars are known to host magnetic fields (Landstreet 1992; Donati & Landstreet 2009) These magnetic fields are thought to have a “dynamo origin”, meaning that the field is continuously amplified through hydrodynamic induction and would otherwise decay within the Alfvén timescale. Strong internal fields can modify the stellar structure via magnetic pressure and tension and, more importantly, by affecting the adiabatic indices, which, in turn, modifies the efficiency of convective energy transport Such effects have been considered in Feiden & Chaboyer (2012, 2013, 2014) for low-mass star evolution, following prescriptions developed by Lydon & Sofia (1995).
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