Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000a%26a...361..952h&link_type=abstract
Astronomy and Astrophysics, v.361, p.952-958 (2000)
Astronomy and Astrophysics
Astrophysics
45
Accretion, Accretion Disks, Stars: Binaries: General, Stars: Individual: Am Her, Stars: Magnetic Fields, Stars: Novae, Cataclysmic Variables
Scientific paper
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 alpha 2-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.
Gänsicke Boris T.
Hessman Frederic V.
Mattei Janet A.
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