Magnetic White Dwarfs

In this paper we review the current status of research on the observational and theoretical characteristics of isolated and binary magnetic white dwarfs (MWDs). Magnetic fields of isolated MWDs are observed to lie in the range 10³–10⁹ G. While the upper limit cutoff near 10⁹ G appears to be real, the lower limit is more difficult to investigate. The incidence of magnetism below a few 10³ G still needs to be established by sensitive spectropolarimetric surveys-conducted on 8 m class telescopes. Highly magnetic WDs tend to exhibit a complex and non-dipolar field structure with some objects showing the presence of higher order multipoles. There is no evidence that fields of highly magnetic WDs decay over time, which is consistent with the estimated Ohmic decay times scales of ∼10¹¹ yrs. The slow rotation periods (∼100 yrs) inferred for a large number of isolated MWDs in comparison to those of non-magnetic WDs (a few days) suggest that strong magnetic fields augment the braking of the stellar core. MWDs, as a class, also appear to be more massive (0.784±0.047 M_⊙) than their weakly or non-magnetic counterparts (0.663±0.136 M_⊙). MWDs are also found in binary systems where they accrete matter from a low-mass donor star. These binaries, called magnetic Cataclysmic Variables (MCVs), comprise about 20–25 % of all known CVs. Zeeman and cyclotron spectroscopy of MCVs have revealed the presence of fields in the range ∼7–230 MG. Complex field geometries have been inferred in the high field MCVs (the polars) whilst magnetic field strength and structure in the lower field group (intermediate polars, IPs) are much harder to establish. The MCVs exhibit an orbital period distribution which is similar to that of non magnetic CVs. Polars dominate the distribution at orbital periods ≲4 h and IPs at longer periods. It has been argued that IPs above the 2–3 hr CV period gap with magnetic moments ≳ 5×10³³ G cm³ may eventually evolve into polars. It is vital to enlarge the still incomplete sample of MCVs to understand not only their accretion processes but also their evolution. The origin of fields in MWDs is still being debated. While the fossil field hypothesis remains an attractive possibility, field generation within the common envelope of a binary system has been gaining momentum, since it would explain the absence of MWDs paired with non-degenerate companions and also the lack of relatively wide pre-MCVs.