Magnetic dynamos powered by white dwarf superficial convection

When the ef fecti ve temperature of a cooling white dwarf T eff drops below the ionization limit, it develops a surface convection zone that may generate a magnetic field B through one of several dynamo mechanisms. We revisit this possibility systematically using detailed stellar evolution computations, as well as a simple analytical model that tracks the expansion of the convection zone. The magnetic field reaches a maximum of several kG (for a hydrogen atmosphere) shortly after a convection zone is established at a cooling time t = t conv . The field then declines as B ? T eff ? t -7 / 20 until the conv ectiv e env elope couples to the degenerate core at t = t coup . We compare the onset of convection t conv ? M 25 / 21 to the crystallization of the white dwarf's core t cryst ? M -5 / 3 and find that in the mass range 0 . 5 M _< M < 0 . 9 M _the order of events is t conv < t cryst < t coup . Specifically, surface dynamos are active for a period _t t cryst -t conv of about a Gyr (shorter for higher masses), before the convection zone is o v errun by a stronger magnetic field emanating from the crystallizing core. Our predicted magnetic fields are at the current detection limit, and we do not find any observed candidates that fit the theory.