Evidence linking the photometric variability of solar-type stars to the sunspot phenomenon is provided by the fact that continuum variability seems to occur in anti-phase with variations in H and Ca ii H & K emission variations (Dorren and Guinan, 1982). The anticorrelation implies that the surface activity of such stars is confined to localised activity centres that include both emission plages and dark spots, similar to active regions observed on the Sun. Moreover, short-term stellar irradiance variations may be largely explained as rotational modulation by active regions which can persist for several rotation periods (Lockwood et al., 1984).
The onset of solar-type activity at F7 stars with subsurface convection zones was firmly established by the survey of Radick et al. (1982), while the reality of the limit at K2 seemed to be less certain. For instance, a higher precision of 0.001 mag achieved aboard the Hubble Space Telescope allowed for the detection of periodic brightness variations of the red dwarf Proxima Cen (V645 Cen, M5Ve) with an amplitude of 0.01 mag and a period of 41.6 d, which were interpreted as rotational modulation of starspots in the stellar photosphere (Benedict et al., 1993).
It was firmly established that magnetic activity in solar-type stars declines with age and that it is closely related to a loss of angular momentum throughout the main-sequence lifetime (Skumanich, 1972; Noyes et al., 1984; Baliunas et al., 1995; Güdel et al., 1997). Thus, young stars exhibit high average levels of activity and rapid rotation, while stars as old as the Sun and older have slower rotation rates and lower activity levels.
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