Accretion processes in AM Herculis systems
Date
1989
Authors
Ferrario, L.
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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.
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