THE PI-LINE POLARIZATION IN MAGNETIC WHITE-DWARFS

The pi subcomponents of hydrogen line transitions only absorb light th at is linearly polarized parallel to a local magnetic field. In the sp ectra of magnetic white dwarfs, the circular polarization in these pi lines is usually expected and often observed to be reduced in magnitud e relative to the neighbouring continuum. However, observations of som e strongly magnetized white dwarfs show some pi lines that have circul ar 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 pi line polarization is obtained, in theory, at adequately high fields and for special field geometries.