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Paper: The high field magnetic CVs
Volume: 261, The Physics of Cataclysmic Variables and Related Objects
Page: 82
Authors: Wickramasinghe, D. T.; Ferrario, L.
Abstract: The magnetic cataclysmic variables are comprised of ~60 AM Herculis type variables and ~26 Intermediate Polars (IPs). All but three of the AM Hers have fields in the narrow field range ~7-80 MG, while the IPs as a class have lower fields. High field AM Hers are rare with the highest known field being that of AR UMa (~230 MG). The field distribution of the AM Hers contrasts with that of the high field isolated magnetic white dwarfs where roughly equal numbers are found in the field ranges ~10-100 MG and ~100-1000 MG. At very high fields (~500-1000 MG) efficient cyclotron cooling will ensure that a hydrodynamic shock does not form -- rather the photosphere will be heated directly by particle bombardment. The optical band will lie in the red wing of the optically thick cyclotron fundamental, and any emission associated with the impact region is likely to be unpolarised and featureless. Nevertheless, the impact heated photospheric layers may be cool enough to give rise to high field Zeeman absorption features during a high state. A characteristic feature of the high field (~100 - 1000) MG AM Hers during most states of activity would be the triplet type Zeeman splitting of emission lines arising from the low field (~10-100 kG) regions at the base of an accretion funnel. The emission lines will show reversals in the sign of circular polarisation across line centres, and at sufficiently high fields may even be seen as resolved Zeeman triplets in the flux spectra. We also review results of recent evolutionary calculations which argue that the observed differences in the period distributions of the AM Hers and of other CVs provide strong support for the disrupted magnetic braking model of the 2-3 hr period gap.
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