Accretion funnels in AM Herculis systems - I. Model characteristics

Abstract

We present calculations of the thermal structure of accretion funnels heated by hard X-ray emission from the accretion shock, soft X-ray reprocessed radiation from the white dwarf surface and magnetic heating in the matter-field interaction region close to the orbital plane. The calculations determine self-consistently the thermal structure of the funnel, allowing for radiative transfer, electron scattering and the trapping of radiation within the funnel, and represent a substantial improvement on previous purely kinematical models. We show that, while models that allow only for X-ray heating can explain the observed intensities of the Balmer lines, they cannot, at the same time, explain the intensities of the He I and He II lines in the optical spectra. These lines appear to be formed mainly in the magnetically heated transition region near the orbital plane, with this region playing a role similar to the hotspot in accretion discs. We show that, with the inclusion of this region, models can be constructed that are in close agreement with the optical line and continuum emission observed in AM Herculis systems: that is, they exhibit a flat or inverted Balmer decrement, He I lines, a strong He II λ4686 line and complex emission-line profiles which vary dramatically in velocity and shape over the orbital period of the white dwarf. We also show that the continuum emission from the accretion funnel provides an important source of unpolarized background radiation, which reduces the degree of polarization of the cyclotron radiation from the accretion shocks, and produces the polarization standstills that are a well-known characteristic of these systems.