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Paper: Recent Advances in Stellar Cycle Research
Volume: 27, The Solar Cycle: Proceedings of the National Solar Observatory/Sacramento Peak 12th Summer Workshop
Page: 150
Authors: Saar, S. H.; Baliunas, S. L.
Abstract: We review recent work on stellar cycles, focusing on a preliminary analysis of the first 25 years of data from the Mount Wilson Ca II program. Cyclic variations are generally solar-like (rapid increase, slow decline), but some stars show multiple cycle periods. About 10-15% of the stars may be in the stellar equivalent of "Maunder minima": epochs when cycles, but not all magnetic activity, temporarily cease. Well-determined cycle periods show no clear dependence on single stellar parameters, but do show correlations with more complex formulations (e.g., alpha-Omega dynamo number) when normalized to the magnetic diffusion timescale. The relation between this normalized cycle frequency (Omega*_cyc) and dynamo number appears to change with activity or age. Cycle amplitudes also correlate with Omega*_cyc, and tend to increase with convection zone depth and P_rot. Giants in young clusters also exhibit many of these phenomena, suggesting similar, dynamo-related origins. Stellar differential rotation can differ markedly from the Sun in both amplitude and form. Photometric variability increases rapidly with increasing Ca II emission, first reversing, and eventually eliminating the correlation between brightness and activity. Dynamos of active stars thus appear to produce a larger spot-to-plage ratio than inactive stars; more high-latitude spots are also seen. Surface convective properties may also change during the cycle.
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