The Origin of High Magnetic Fields in White Dwarfs

The lack of evidence for Zeeman splitting of the hydrogen lines in the spectra of the 1,253 close but detached binary systems consisting of a white dwarf and a nondegenerate star, a sample that includes the pre‐Cataclysmic Variables, identified in the Sloan Digital Sky Survey indicates that there are no identifiable progenitors for the Magnetic Cataclysmic Variables (MCVs), even though these comprise some 25per cent of all Cataclysmic Variables (CVs). Indeed, all high‐field white dwarfs appear to be either single stars or components of AM Her systems. This suggests that all such white dwarfs have a binary origin. We resolve this dilemma by postulating that the 106–108 G magnetic fields that are observed in the white dwarfs in the MCVs are generated in the common envelope phase of pre‐CV evolution in systems which almost merge. Systems that merge in the common envelope phase yield a population of isolated magnetic white dwarfs with fields of 106–109 G that make up the entire single magnetic white dwarf pop...