Magnetism in white dwarfs

Ten strongly magnetic white dwarfs are now known, of which half have spectra which show Zeeman splitting in lines of H, He, or CH in magnetic fields of 5 to 25 MG. Two of these have sharply defined Zeeman subcomponents, indicative of very uniform surface fields. The remaining five have still stronger fields, such that the spectral features, if present, are weak and of uncertain origin. In these objects the magnetic field is identified by the elliptical polarization of the optical continuum, and is of the order of 100 MG. Within the small sample of 10 there is some evidence that the magnetic field modifies the normal extreme atmospheric compositions of white dwarfs. Two cases (Feige 7 and G99-37) show mixed compositions not seen in other white dwarfs. Most of the magnetic white dwarfs show no spectral or polarization variations and may be rotating very slowly. However, two are identified as oblique rotators with periods of the order of hours, in line with estimates of nonmagnetic white dwarfs. Other measurable properties of magnetic white dwarfs do not seem remarkably different from white dwarfs in general. The fact that they are not especially hot means that the time scale for field decay is comparable to or longer than that for cooling.