The Formula line polarization in magnetic white dwarfs

The π subcomponents of hydrogen line transitions only absorb light that is linearly polarized parallel to a local magnetic field. In the spectra of magnetic white dwarfs, the circular polarization in these π lines is usually expected and often observed to be reduced in magnitude relative to the neighbouring continuum. However, observations of some strongly magnetized white dwarfs show some π lines that have circular polarization equal to that of the adjacent continuum. We show that this is a consequence of the effect of magnetic anomalous dispersion (magneto-optical effects) on radiative transfer. The observed equality between continuum and π line polarization is obtained, in theory, at adequately high fields and for special field geometries.