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Accretion Processes in Magnetic Binaries*

Published online by Cambridge University Press:  05 March 2013

Lilia Ferrario
Affiliation:
Department of Mathematics, Australian National University, Canberra, ACT 0200, Australia Astrophysical Theory Centre, Australian National University, Canberra, ACT 0200, Australia; lilia@maths.anu.edu.au
Jianke Li
Affiliation:
Department of Mathematics, Australian National University, Canberra, ACT 0200, Australia Astrophysical Theory Centre, Australian National University, Canberra, ACT 0200, Australia; ljk@maths.anu.edu.au
Curtis Saxton
Affiliation:
Research Centre for Theoretical Astrophysics, School of Physics, University of Sydney, NSW 2006, Australia; kinwah@physics.usyd.edu.au
Kinwah Wu
Affiliation:
Research Centre for Theoretical Astrophysics, School of Physics, University of Sydney, NSW 2006, Australia; saxton@physics.usyd.edu.au
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Abstract

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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 in order for channelled 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 3D 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 for both transfer of the external radiation field and 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 diagnostics 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.

Type
Research Article
Copyright
Copyright © Astronomical Society of Australia 1999

Footnotes

*

Refereed paper based on two separate contributions to the Workshop on Magnetic Fields and Accretion, held at the Astrophysical Theory Centre, Australian National University, on 12–13 November 1998.

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