White Dwarf Planets as Unipolar Generators

The white dwarf star GD 356 exhibits a strange spectrum, where its only recognizable features are the Balmer series in Zeeman-split emission (B = 13 MG). GD 356 is also apparently isolated, with no detected stellar companion or active mass transfer. We recently discovered another white dwarf, SDSSJ125230.93−023417.72, which also exhibits Zeeman-split Balmer emission and is the fastest-rotating apparently isolated white dwarf discovered to date (P = 317 s, B = 5 MG). It is hypothesized that the emission in in these stars arises from the presence of an unseen conducting planet, whose orbit through the white dwarf's magnetosphere induces an electric current along the star's magnetic field lines, exciting the stellar atmosphere into emission. This model is a form of unipolar generator. We present here a summary of white dwarf unipolar generators, as inferred from the observed behavior of GD 356 and SDSSJ1252, and discuss their presumed origins and context for the type-Ia supernova progenitor question.