Disc-Magnetosphere interactions in Cataclysmic Variable Stars

TL;DR

This review explores the observational understanding of how accretion discs interact with magnetic fields in cataclysmic variable stars, covering flow regimes, magnetic threading, and effects of outbursts.

Contribution

It synthesizes current observational insights into disc-magnetosphere interactions in cataclysmic variables, including the effects of outbursts and dynamo-driven magnetospheres.

Findings

Accretion flow transitions from stream to disc near certain stellar parameters.

Magnetic field distortions relate to accretion torques and white dwarf spin changes.

Disc outbursts influence the disc-magnetosphere interaction dynamics.

Abstract

I review, from an observational perspective, the interactions of accretion discs with magnetic fields in cataclysmic variable stars. I start with systems where the accretion flows via a stream, and discuss the circumstances in which the stream forms into an accretion disc, pointing to stars which are close to this transition. I then turn to disc-fed systems and discuss what we know about how material threads on to field lines, as deduced from the pattern of accretion footprints on the white dwarf. I discuss whether distortions of the field lines are related to accretion torques and the changing spin periods of the white dwarfs. I also review the effect on the disc-magnetosphere interaction of disc-instability outbursts. Lastly, I discuss the temporary, dynamo-driven magnetospheres thought to occur in dwarf-nova outbursts, and whether slow-moving waves are excited at the inner edges of the disc.