Accretion streams in AM Her type systems

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Abstract
The accretion stream in AM Her type systems is examined through model calculations and observations of the optical and UV emission lines. The magnetospheric radii for two specific systems, ST LMi and QQ Vul, are calculated using system parameters that have been recently obtained from observations. It is shown that the magnetosphere is well within the primary Roche lobe in QQ Vul, but is very close to the L1 point in ST LMi. The shape of the post-shock region is shown to be elongated through model calculations. The size and the blackbody temperature obtained in the models are consistent with the observational constraints available for QQ Vul and ST LMi. One particular model for ST LMi produces two post-shock regions separated by a region of low local accretion rate; such a two-spot structure has been suggested by the observations (Cropper).The properties of the optical emission lines are re-examined, and it is found that, in many systems, the emission lines have very complex structures with as many as four separate components. One broad component probably originates in the magnetically confined stream, and it is argued that the centre-of-light is determined by the balance between the excitation mechanism and the radiating surface area available. Another component seen in ST LMi and QQ Vul is likely to originate in the free-falling part of the stream near the threading region, and this may be a common feature in many AM Her type systems. The observed soft X-ray component from the white dwarf surface is shown to be sufficient to produce the strong UV emission lines in the magnetically confined stream.

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