EFFECTS OF MAGNETIC FIELDS ON MAIN SEQUENCE STARS

Abstract

A number of effects of low to medium strength (< 2000 gauss photospheric) magnetic fields on otherwise normal stars are proposed and examined. We consider magnetic perturbations to the standard stellar structure and evolutionary calculations in the core, the deep envelope, and the extreme outer envelope in intermediate to high mass stars. In the stellar core the gas pressure probably far exceeds the (B²/8π) magnetic field pressure term so that the only effect of such a field may come from its inhibiting convection in the core. We present isochrones of both convective and radiative core models of 2-5 M(⊙). In the deep envelope, we may expect to see mixing of partially nuclear processed material driven by rising and falling magnetic flux tubes. The effects of this mixing will be brought to the surface during the deep convection phase of the star's tenure as a red giant. We use this model to predict a signature for magnetic mixing based on the CNO isotope and abundance ratios. In the outer envelope the gas pressure is low enough that we might expect to see a perturbation of the stellar structure due to the magnetic field pressure itself. We calculate this perturbation under several physical models for intermediate and high mass stars and determine that sufficient magnetic field energy may be available in the outer envelope to expand a star by about 20% over its unperturbed radius. Finally we consider the evidence for the existence of non-magnetic neutron stars, concluding that while no non-magnetic neutron stars have ever been positively identified, we have no evidence that prevents the existence of at least as many non-magnetic as magnetic neutron stars.