ORFEUS II Far-UV Spectroscopy of AM Herculis

Six high-resolution (λ/∆λ ≈ 3000) far-UV (λλ = 910–1210 Å) spectra of the magnetic cataclysmic variable AM Herculis were acquired in 1996 November during the flight of the ORFEUS-SPAS II mission. AM Her was in a high optical state at the time of the observations, and the spectra reveal emission lines of O VI λλ1032, 1038, C III λ977, λ1176, and He II λ1085 superposed on a nearly flat continuum. Continuum flux variations can be described as per Gänsicke et al. by a ≈ 20 kK white dwarf with a ≈ 37 kK hot spot covering a fraction f ∼ 0.15 of the surface of the white dwarf, but we caution that the expected Lyman absorption lines are not detected. The O VI emission lines have broad and narrow component structure similar to that of the optical emission lines, the C III and He II emission lines are dominated by the broad component, and the radial velocities of these lines are consistent with an origin of the narrow and broad components in the irradiated face of the secondary and the accretion funnel, respectively. The density of the narrowand broad-line regions is nnlr ∼ 3 × 10 10 cm and nblr ∼ 1 × 10 12 cm, respectively, yet the narrow-line region is optically thick in the O VI line and the broad-line region is optically thin; apparently, the velocity shear in the broad-line region allows the O VI photons to escape, rendering the gas effectively optically thin. Unexplained are the orbital phase variations of the emission-line fluxes. Subject headings: binaries: close — stars: individual (AM Herculis) — stars: magnetic fields — ultraviolet: stars — white dwarfs