Magnetic Field Evolution in Accreting White Dwarfs

Abstract

We discuss the evolution of the magnetic field of an accreting white dwarf. We first show that the timescale for ohmic decay in the liquid interior is 8 to 12 billion years for a dipole field, and 4 to 6 billion years for a quadrupole field. We then compare the timescales for ohmic diffusion and accretion at different depths in the star, and for a simplified field structure and spherical accretion, calculate the time-dependent evolution of the global magnetic field at different accretion rates. In this paper, we neglect mass loss by classical nova explosions and assume the white dwarf mass increases with time. In this case, the field structure in the outer layers of the white dwarf is significantly modified for accretion rates above the critical rate (1-5) x 10^(-10) solar masses per year. We consider the implications of our results for observed systems. We propose that accretion-induced magnetic field changes are the missing evolutionary link between AM Her systems and intermediate polars. The shorter ohmic decay time for accreting white dwarfs provides a partial explanation of the lack of accreting systems with 10^9 G fields. In rapidly accreting systems such as supersoft X-ray sources, amplification of internal fields by compression may be important for Type Ia supernova ignition and explosion. Finally, spreading matter in the polar cap may induce complexity in the surface magnetic field, and explain why the more strongly accreting pole in AM Her systems has a weaker field. We conclude with speculations about the field evolution when classical nova explosions cause the white dwarf mass to decrease with time.