The detection of activity cycles in overall brightness variations due to cool spots on young solar analogues confirms the above implication. At least eight young dwarfs of the solar type and one weak-line T Tau star clearly show spot cycles (Amado et al., 2001; Berdyugina et al., 2002; Messina and Guinan, 2002, 2003; Stelzer et al., 2003; Järvinen et al., 2005b; Berdyugina and Järvinen, 2005). Two examples (AB Dor and LQ Hya) are shown in Figure 14. In contrast to the Sun, where the maximum spot area is reached at the maximum irradiance, on more active stars it occurs at minimum brightness. In addition, such stars exhibit periodic changes of spot rotation periods in phase with the spot cycle, which is consistent with the presence of a differential rotation. The latter may also vary with a cycle (Collier Cameron and Donati, 2002; Donati et al., 2003a). Variations of the total spottedness are also found on other types of cool active stars, including components of binary systems ((e.g., Henry et al., 1995); see plot for Gem in Figure 14). The stellar spot cycles are, therefore, reminiscent of the 11-yr sunspot cycle.
A sample of stars with different rotation rates and cycle frequencies provides an opportunity to investigate the likely evolution of the stellar dynamo, as was done by, e.g., Saar and Brandenburg (1999). For instance, it appears that there is only a weak correlation, if any, between cycle and rotational frequencies, while the cycle length probably correlates with the differential rotation shear (Messina and Guinan, 2003). However, in such an analysis it is important to distinguish between different cycle types as they can be associated with different dynamo modes, as discussed by Moss (2004) and Fluri and Berdyugina (2004).
A comparison of the activity patterns of the present Sun and young solar analogues allows us to infer possible evolution of the stellar dynamo on main-sequence stars. First of all, the overall activity level is reducing while the star evolves along the main-sequence and looses its angular momentum. Secondly, the activity is changing from spot-dominated to faculae-dominated. This implies that cycles on young stars are more prominent in spot patterns, while on stars of the solar age cycles become more apparent in chromospheric plages. Thirdly, young stars show conspicuous non-axisymmetric fields, which weaken on the Sun and solar-age stars and coexist with a strong axisymmetric component.
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