6.1 Overall activity variations

Chromospheric plages produce flux variations in the emission cores of $Ca II H & K$ lines as observed on the Sun and Sun-like stars. Monitoring $Ca II$ emission on solar-type dwarfs, pioneered by O. Wilson at Mt. Wilson Observatory, has led to the detection of solar-like activity cycles in such stars (Baliunas et al., 1995 ). For a sample of stars (about 100) of spectral type G0-K5 V changes in rotation and chromospheric activity are found to occur on an evolutionary timescale. Young rapidly rotating stars exhibit high average levels of activity and rarely display a smooth, cyclic variation. Stars of intermediate age (approximately $1 -2 Gyr$ for $1Mo.$ ) have moderate levels of activity and occasional smooth cycles (see Figure 13

). Finally, stars as old as the Sun and older have slower rotation rates, lower activity levels and smooth cycles. Some stars showed no variations at all, which was first interpreted as stars being in the stage similar to the Maunder minimum on the Sun. Later it was shown though that these stars are probably subgiants evolved off the main-sequence (Wright, 2004 ). In addition, contemporaneous photometric and chromospheric $H & K$ emission time series for 35 stars revealed that the luminosity variation of young stars anti-correlates with their variation in chromospheric emission (Radick et al., 1998 ), i.e., young stars become fainter near their activity maxima, while older stars, including the Sun, become brighter at maximum activity. This suggests a shift from spot-dominated to faculae-dominated activity as stars evolve along the main-sequence. Such an evolution implies that activity cycles on young stars should be more prominent in spot patterns rather than in chromospheric plages.

Figure 13:

Chromospheric $Ca II$ emission cycles for Sun-like stars, illustrating the regular cyclic variation that is common in such stars. The $Ca II$ emission is plotted in Mount Wilson “S-Index” units. From Radick (2000 ).

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 $s$ Gem in Figure 14

). The stellar spot cycles are, therefore, reminiscent of the 11- $yr$ sunspot cycle.

Figure 14:

Spot cycles in the solar irradiance and $V$ magnitudes of the RS CVn binary $s$ Gem and two young solar analogues AB Dor and LQ Hya. Note that the maximum of the spot area corresponds to the maximum irradiance on the Sun and minimum brightness on the stars.

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.