Accretion processes in AM Herculis systems

Abstract

We begin by presenting a theoretical model for the radial velocity and velocity dispersion of the broad emission line component in AM Herculis systems assuming that it originates mainly in the gas which is diverted out of the orbital plane and funneled on to the white dwarf surface along magnetic field lines. The model is used to locate the line forming region in three AM Her variables using as constraints the radial velocity and velocity dispersion data. This work shows that the material is threaded by the magnetic field in a very azimuthally extended coupling region located 0.5 - 0. 75 of the way between the white dwarf and the inner Lagrange point. This implies that the cyclotron emission region on the stellar surface is extended, probably arc-shaped, and offset from the magnetic pole. Models for cylindrically extended and arc-shaped cyclotron emission regions displaced from the magnetic poles are then generated, taking into account the effects of field spread across the shock. The properties of cylindrically extended models with density and temperature structure and geometrical extension are also investigated. These models are an improvement on previous point source models and have characteristics that are in better overall agreement with the properties of AM Herculis systems. Phas dependent spectropolarimetric observations of VV Puppis which show, for the first time, cyclotron lines from both poles are presented. The rnain and secondary emission regions have fields of 30.5 MG and 56 MG respectively. The observations have been interpreted in terms of a dipole which is offset by ~ 0.1 white dwarf radii from the center of the star in the direction of the dipole axis with the emission regions located near the foot points of closed field lines. Spectroscopic and circular polarization observations of the soft X-ray clipsing binary EXO 033319 - 2554.2 which confirm it as a member of the AM Herculis class are presented. Broad resolvable and variable cyclotron harmonics from both poles are seen during the bright phase corresponding to magn tic fields of 56 MG (main region) and 28 MG (secondary region). As is the case for VV Puppis, the large difference in magnetic field str ngth of the two regions implies a more complex field structure than that of a cent red dipole field distribution, if it is assumed that the regions are connected by closed field lines. U BV RI and long term plate photometry of the faint and highly erratic cataclysmic variable discovered from a sequence of UK Schmidt measurements are presented. We show that this object ( designated Grus Vl) is almost certainly a new AM Herculis variable, and develop a model in which the light curves are explained as originating from two nonpolar cyclotron em1ss1on regions.