Magnetic field evolution in white dwarfs

Abstract

The authors have carried out detailed calculations of magnetic field decay in white dwarfs using an evolutionary sequence of 0.6 M_sun; carbon white dwarf models to solve the time-dependent MHD equation numerically. The models are radially symmetric and nonrotating. The magnetic field is purely poloidal. The central conductivity of the older models increases relatively quickly. As a result, the decay time scale also increases and in fact is always longer than the age of the star. In spite of this, a field consisting of an initially pure fundamental eigenmode decays appreciably (≡50% at the surface in 10<SUP>10</SUP>yr). This is because the evolving structure of the star changes the decay eigenmodes, so that the field becomes a mixture of higher order modes which decay more rapidly than the fundamental.