Accretion Processes in Magnetic Binaries

Lilia Ferrario , Jianke Li , Curtis Saxton , Kinwah Wu, PASA, 16 (3), 234.
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Contents Page: Volume 16, Number 3

Accretion Processes in Magnetic Binaries

Lilia Ferrario 1,2
Jianke Li 1,2
Curtis Saxton 3
Kinwah Wu 3

1 Department of Mathematics, The Australian National University, Canberra, ACT 0200
2 Astrophysical Theory Centre, The Australian National University, Canberra, ACT 0200,

3 Research Centre for Theoretical Astrophysics, School of Physics,
University of Sydney, NSW 2006,


In this paper, we give a brief summary of the talks on accretion processes in AM Herculis systems which were presented at the ANU Astrophysical Theory Centre workshop on ``Magnetic Fields and Accretion''. One of the topics to be discussed was the mechanism that leads to the formation of magnetically funnelled accretion flows in close interacting magnetic binaries. New solutions to the Bernoulli integral indicate that the field lines must be twisted and have a strong toroidal component at the base of the funnel for chanelled flow to be possible. The magnetic field pressure of these toroidal fields first lifts the material out of the orbital plane allowing it to ``levitate'' before freely falling along magnetic field lines towards the stellar surface. Results of recent calculations of the thermal structure and radiation properties of accretion funnels were also presented. These new 3-D calculations allow for heating by the soft X-rays originating from the accretion shock and by magnetic heating at the base of the funnel, and determine self-consistently the thermal structure, and the continuum and line emissions, allowing both for transfer of the external radiation field and for the trapping of radiation within the funnel. Calculations were also presented of the expected properties of H- and He-like Fe lines originating from the accretion shock itself at the stellar surface. These lines are predicted to be rather strong and can be used as diagnostic of the accretion flow. Finally, the stability of the accretion shock was also addressed. In particular, it was shown that radiative cooling may cause thermal instability and an oscillatory behaviour with two competing processes coming into play: bremsstrahlung cooling, which promotes instability, and cyclotron cooling which tends to dampen the oscillations.

Keywords: stars: binaries - stars: Cataclysmic Variables - magnetic fields - white dwarfs.

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