Magnetic fields in isolated and interacting white dwarfs
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The magnetic white dwarfs (MWDs) are found either isolated or in interacting binaries. The isolated MWDs divide into two groups: a high field group (105–109 G) comprising some 13±4% of all white dwarfs (WDs), and a low field group (B<105 G) whose incidence is currently under investigation. The situation may be similar in magnetic binaries because the bright accretion discs in low field systems hide the photosphere of their WDs thus preventing the study of their magnetic fields’ strength and structure. Considerable research has been devoted to the vexed question on the origin of magnetic fields. One hypothesis is that WD magnetic fields are of fossil origin, that is, their progenitors are the magnetic main-sequence Ap/Bp stars and magnetic flux is conserved during their evolution. The other hypothesis is that magnetic fields arise from binary interaction, through differential rotation, during common envelope evolution. If the two stars merge the end product is a single high field MWD. If close binaries survive and the primary develops a strong field, they may later evolve into the magnetic cataclysmic variables (MCVs). The recently discovered population of hot, carbon-rich WDs exhibiting an incidence of magnetism of up to about 70% and a variability from a few minutes to a couple of days may support the merging binary hypothesis. The fields in the low field MWDs may instead arise from a dynamo mechanism taking place in convective zones during post main-sequence evolution. Should this be the case, there may be a field strength below which all WDs are magnetic and thus fields are expected to always play a role in accretion processes in close binaries. Several studies have raised the possibility of the detection of planets around MWDs. Rocky planets may be discovered by the detection of anomalous atmospheric heating of the MWD when the unipolar inductor mechanism operates whilst large gaseous planets may reveal themselves through cyclotron emission from wind-driven accretion onto the MWD. Planetary remains have recently revealed themselves in the atmospheres of about 25% of WDs that are polluted by elements such as Ca, Si, and often also Mg, Fe, Na. This pollution has been explained by ongoing accretion of planetary debris. Interestingly, the incidence of magnetism is approximately 50% in cool, hydrogen-rich, polluted WDs, suggesting that these fields may be related to differential rotation induced by some super-Jupiter bodies that have plunged into the WD. The study of isolated and accreting MWDs is likely to continue to yield exciting discoveries for many years to come.
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