The Cyclotron Lines in the Optical Spectra of the Stars of AM Herculis Group: Observations and Interpretation

Abstract

The stars of the AM Herculis group (AM Her, VV Pup, AN UMa and 2A 0311-227) are close binaries containing a mass losing, nondegenerate star and an accreting degenerate dwarf. Their main properties are: the large linear and circular polarization of the optical light, high and low luminosity states, the variable emission line, spectra of H, He and other elements and the identification of these objects with X-ray sources. It is generally accepted that the strong magnetic field of the degenerate dwarf is responsible for these peculiar properties and for the distinction between these objects and the cataclysmic variables (Mitrofanov 1978, 1979a). The polarized optical continuum may be emitted by the accreted magnetized plasma (e.g. Chanmugam and Wagner, 1979), by the magnetized photosphere of the degenerate dwarf (Mitrofanov et al. 1977), or by both sources (Mitrofanov, 1979b). To explain the observed X-rays, Lamb and Masters (1979) showed that a magnetic field about 108 gs is necessary. Unfortunately, the basic prediction of their model - the strong ultraviolet continuum in the spectrum of AM Herculis - appears to be absent (Raymond et al. 1979). Chanmugam and Wagner (1979) proposed a rather different estimate for B of 2 · 108/m*gs (m* = 5 ÷ 25). For further investigations of the AM Herculis-type stars it seems useful to find a direct observational method for measuring the dwarfs′ magnetic fields.