Journal Article

Magnetic braking in magnetic binary stars

Jianke Li and Dayal T. Wickramasinghe

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 300, issue 3, pages 718-732
Published in print November 1998 | ISSN: 0035-8711
Published online November 1998 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-8711.1998.t01-1-01913.x
Magnetic braking in magnetic binary stars

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Abstract

The role of an external magnetic field in the magnetic braking of a star with a dipolar field is investigated. In a magnetic cataclysmic variable system (i.e. the primary compact star has a strong magnetic field), the field external to the braking star (a late-type main-sequence star with a dynamo-generated field) originates from the compact star. A closed field region — the system dead zone — is formed within the binary system, and it does not take part in magnetic braking. The overall braking rate depends on the extent of this region and of the open flux, and is dependent on centrifugal effects. In the case of two interacting dipoles, the dipole orientations relative to the spin axes and to each other are found to be important, leading to different amounts of open flux and therefore of magnetic braking, owing to different centrifugal effects on closed field regions. However, in circumstances consistent with observations and dynamo theory, the white dwarf's field reduces the magnetic braking of the secondary significantly, a finding qualitatively similar to the results previously obtained for two anti-aligned dipoles perpendicular to the orbital plane. In the cases where the two dipole axes are not perpendicular to the orbital plane, but are inclined in the plane that links them, the ‘cut-off’ in magnetic braking is less abrupt, and this effect is more obvious as the inclinations increase. Only in the extreme cases when the two dipole axes are aligned in the orbital plane does the braking increase with white dwarf field strength. We conclude that detailed evolutionary modelling of AM Herculis systems needs to take account of the inclination effect.

Keywords: magnetic fields; binaries: close; stars: magnetic fields; novae, cataclysmic variables

Journal Article.  0 words. 

Subjects: Astronomy and Astrophysics

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