DYNAMO GENERATION OF MAGNETIC-FIELDS IN WHITE-DWARFS

Our earlier analysis of aw dynamos in white dwarfs, aimed specifically at explaining the time-varying magnetic field in the DBV white dwarf GD 358 discovered by Winget et al., is improved and extended to a broa der range of white dwarf types. Our nonlinear, local dynamo equations, based on those of Robinson and Durney, are modified to account for ne w evidence concerning the Sun's alpha omega dynamo. We calculate dynam o magnetic fields for both He-envelope (DB) and H-envelope (DA) white dwarfs, for a range of values of mass, luminosity, rotation rate, and amount of differential rotation, and for convective envelopes computed with different formulations of convection theory. The results strengt hen the case for dynamo generation of the magnetic field observed in G D 358. The results also show that dynamo generation of magnetic fields could occur in many DB and DA white dwarfs, including other pulsating DBV stars (besides GD 358) and DAV stars for which the magnetic field s may be detectable by asteroseismological techniques.