EDP Sciences
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Volume 481, Number 2, April II 2008
Page(s) 433 - 439
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20078556
Published online 04 February 2008

A&A 481, 433-439 (2008)
DOI: 10.1051/0004-6361:20078556

High and low states of the system AM Herculis

K. Wu1, 2 and L. L. Kiss2

1  Mullard Space Science Laboratory, University College London, Holmbury St Mary, Surrey RH5 6NT, UK
    e-mail: kw@mssl.ucl.ac.uk
2  School of Physics A28, University of Sydney, NSW 2006, Australia
    e-mail: l.kiss@physics.usyd.edu.au

(Received 27 August 2007 / Accepted 24 October 2007)

Context. We investigate the distribution of optically high and low states of the system AM Herculis (AM Her).
Aims. We determine the state duty cycles, and their relationships with the mass transfer process and binary orbital evolution of the system.
Methods. We make use of the photographic plate archive of the Harvard College Observatory between 1890 and 1953 and visual observations collected by the American Association of Variable Star Observers between 1978 and 2005. We determine the statistical probability of the two states, their distribution and recurrence behaviors.
Results. We find that the fractional high state duty cycle of the system AM Her is 63%. The data show no preference of timescales on which high or low states occur. However, there appears to be a pattern of long and short duty cycle alternation, suggesting that the state transitions retain memories. We assess models for the high/low states for polars (AM Her type systems). We propose that the white-dwarf magnetic field plays a key role in regulating the mass transfer rate and hence the high/low brightness states, due to variations in the magnetic-field configuration in the system.

Key words: accretion, accretion disks -- stars: binaries: close -- stars: individual: AM Herculis -- stars: magnetic fields -- stars: variables: general -- stars: novae, cataclysmic variables

© ESO 2008