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Astron. Astrophys. 361, 952-958 (2000)


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The history and source of mass-transfer variations in AM Herculis

F.V. Hessman 1, B.T. Gänsicke 1 and J.A. Mattei 2

1 Universitäts-Sternwarte, Geismarlandstrasse 11, 37083 Göttingen, Germany
2 American Association Variable Star Observers, 25 Birch St., Cambridge, MA, USA

Received 14 March 2000 / Accepted 14 August 2000

Abstract

The optical brightness of magnetic cataclysmic variables without accretion discs is a direct measure of the near-instantaneous mass-transfer rates from the late-type secondary stars to the magnetic white dwarfs in these semi-detached binary systems. We derive the mass-transfer history of the magnetic cataclysmic variable AM Herculis from its long-term visual light curve and from bolometric corrections obtained from a number of X-ray observations covering various accretion states. On average, less than 25% of the maximum observed mass-transfer rate occurs. Assuming that the mass-transfer rate is modulated by stellar starspots on the secondary stars, we convert the derived mass-transfer rates into spot filling factors at the L1-point. A statistical model for the coverage fraction and size distribution of random spots near the L1-point in AM Her suggests that the spot filling factor is roughly 0.5 for a fitted power-law distribution of starspot radii, i.e. about half the surface of the star near the L1-point is covered with spots. This density can only be explained if the spottedness of the L1-point is unusual - for instance if spot groups are forced to wander towards the L1-point - or if a large-scale magnetic spot group produced by an [FORMULA]-dynamo slowly drifts in and out of the L1-region. The former solution predicts that the occurrence of long-term high- and low-states is random and the latter that the long-term light curves of polars are quasi-periodic; the light curve of AM Her may suggest periods of order a decade. Finally, we discuss the relevance of this result to the mass-transfer variations of other cataclysmic variables.

Key words: accretion, accretion disks – stars: binaries: general – stars: individual: AM Her – stars: magnetic fields – stars: novae, cataclysmic variables

Send offprint requests to: hessman@uni-sw.gwdg.de

© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000

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