The Spotted Flare Stars BY Dra and CC Eri: A model for the spots and some Astrophysical Implications

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Abstract
The photometric behaviour of the flare stars BY Dra and CC Eri outside flare activity is discussed. Excellent agreement with the photometric light curves is possible if we interpret them as due to the rotation of a star having a cool, dark spot on its surface. The orbital solution of the BY Dra system is presented. The system has a very high eccentricity and is presumably young. The inclination of the orbital plane places strong restrictions on the location of the spots. A model for the spotted areas is developed, incorporating four free parameters: longitude extent, latitude extent, limb darkening, and temperature. It is shown that the spots must be large, covering up to 20 per cent of the stellar hemisphere and with effective temperatures near 2000 K. In addition, the spots must extend to high latitudes (up to 60°). The model effectively explains the general lack of ( B – V ) variations over the photometric cycle: the spot is black at these wavelengths relative to the stellar photosphere. Estimates of the magnetic field necessary for the storage of part or all of the energy dammed back by the spots are in the range of a few thousand gauss. An energy balance argument suggests that the essence of the flare phenomenon is the release through magnetic field collapse of energy temporarily stored in the field.

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