Magnetism in Isolated and Binary White Dwarfs

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Abstract
Since the discovery of the first isolated magnetic white dwarf (MWD) Grw +70°8047 nearly 60 years ago, the number of stars belonging to this class has grown steadily. There are now some 65 isolated white dwarfs classified as magnetic, and a roughly equal number of MWDs are found in the close interacting binaries known as the magnetic cataclysmic variables (MCVs). The isolated MWDs comprise ~5% of all WDs, while the MCVs comprise ~25% of all CVs. The magnetic fields range from ~ 3 × 104–109 G in the former group with a distribution peaking at 1.6 × 107 G, and ~ 107–3 × 108 G in the latter group. The space density of isolated magnetic white dwarfs with fields in the range ~ 3 × 104–109 G is estimated to be ~ 1.5 × 10-4 pc−3. The MCVs have a space density that is about a hundred times smaller. About 80% of the isolated MWDs have almost pure H atmospheres and show only hydrogen lines in their spectra (the magnetic DAs), while the remainder show He i lines (the magnetic DBs) or molecular bands of C2 and CH (magnetic DQs) and have helium as the dominant atmospheric constituent, mirroring the situation in the nonmagnetic white dwarfs. The incidence of stars of mixed composition (H and He) appears to be higher among the MWDs. There is growing evidence based on trigonometric parallaxes, space motions, and spectroscopic analyses that the isolated MWDs tend as a class to have a higher mass than the nonmagnetic white dwarfs. The mean mass for 16 MWDs with well‐constrained masses is 0.95 M⊙. Magnetic fields may therefore play a significant role in angular momentum and mass loss in the post–main‐sequence phases of single star evolution affecting the initial‐final mass relationship, a view supported by recent work on cluster MWDs. The progenitors of the vast majority of the isolated MWDs are likely to be the magnetic Ap and Bp stars. However, the discovery of two MWDs with masses within a few percent of the Chandrasekhar limit, one of which is also rapidly rotating (Pspin = 12 minutes), has led to the proposal that these may be the result of double‐degenerate (DD) mergers. An intriguing possibility is that magnetism, through its effect on the initial‐final mass relationship, may also favor the formation of more massive double degenerates in close binary evolution. The magnetic DDs may therefore be more likely progenitors of Type Ia supernovae. A subclass of the isolated MWDs appear to rotate slowly with no evidence of spectral or polarimetric variability over periods of tens of years, while others exhibit rapid rotation with coherent periods in the range of tens of minutes to hours or days. There is a strong suggestion of a bimodal period distribution. The "rapidly" rotating isolated MWDs may include as a subclass stars which have been spun up during a DD merger or a previous phase of mass transfer from a companion star. Zeeman spectroscopy and polarimetry, and cyclotron spectroscopy, have variously been used to estimate magnetic fields of the isolated MWDs and the MWDs in MCVs and to place strong constraints on the field structure. The surface field distributions tend in general to be strongly nondipolar and to a first approximation can be modeled by dipoles that are offset from the center by ~10%–30% of the stellar radius along the dipole axis. Other stars show extreme spectral variations with rotational phase which cannot be modeled by off‐centered dipoles. More exotic field structures with spot‐type field enhancements appear to be necessary. These field structures are even more intriguing and suggest that some of the basic assumptions inherent in most calculations of field evolution, such as force‐free fields and free ohmic decay, may be oversimplistic.