Magnetic activity similar to that of the Sun is observed on a variety of cool stars with external convection envelopes. Stellar rotation coupled with convective motions generate strong magnetic fields in the stellar interior and produce a multitude of magnetic phenomena including starspots in the photosphere, chromospheric plages, coronal loops, UV, X-ray, and radio emission and flares. Here I review the phenomenon of starspots on dierent types of cool stars, observational tools and diagnostic techniques for studying starspots as well as starspot properties including their temperatures, areas, magnetic field strengths, lifetimes, active latitudes and longitudes, etc. Evolution of starspots on various time scales allows us to investigate stellar dierential rotation, activity cycles, and global magnetic fields. Together these constitute the basis for our understanding of stellar and solar dynamos and provide valuable constraints for theoretical models.

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Starspots: A Key to the Stellar Dynamo

The main aim of this paper is to introduce the most important observables that help us to investigate stellar dynamos and compare those to the modeling results. We give an overview of the available observational methods and data processing techniques that are suitable for such purposes, with touching upon examples of inadequate interpretations as well. Stellar observations are compared to the solar data in such a way, which ensures that the measurements are comparable in dimension, wavelength, and timescale. A brief outlook is given to the future plans and possibilities. A thorough review of this topic was published nearly a decade ago (Berdyugina in Living Rev. Sol. Phys. 2:8, 2005), now we focus on the experiences that have been gathered since that time.

Observing Dynamos in Cool Stars

In this thesis solar active longitudes of X-ray flares and sunspots are studied. The fact that solar activity does not occur uniformly at all heliographic longitudes was noticed by Carrington as early as in 1843. The longitude ranges where solar activity occurs preferentially are called active longitudes. Active longitudes have been found in various manifestations of solar activity, such as sunspots, flares, radio emission bursts, surface and heliospheric magnetic fields, and coronal emissions. However, the active longitudes found when using different rigidly rotating reference frames differ significantly from each other. One reason is that the whole Sun does not rotate rigidly but differentially at different layers and different latitudes. The other reason is that the rotation of the Sun also varies with time. Earlier studies used a dynamic rotation frame for the differential rotation of the Sun and found two persistent active longitudes of sunspots in 1878-1996. However, the migration of active longitudes with respect to the Carrington rotation was treated there rather coarsely. We improved the accuracy of migration to less than one hour. Accordingly, not only the rotation parameters for each class of solar flares and sunspots are found to agree well with each other, but also the non-axisymmetry of flares and sunspots is systematically increased. We also studied the long-term variation of solar surface rotation. Using the improved analysis, the spatial distribution of sunspots in 1876-2008 is analyzed. The statistical evidence for different rotation in the northern and southern hemispheres is greatly improved by the revised treatment. Moreover, we have given consistent evidence for the periodicity of about one century in the north-south difference.

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Solar active longitudes and their rotation

We present an analysis of a sequence of light curves of the RS CVn-type binary II Pegasi extending from 1974 to 1998. The distribution of the spotted area versus longitude is derived by Maximum Entropy and Tikhonov regularized maps, assuming a constant spot temperature (Lanza et al. 1998a). The spot pattern on the active K2 IV star can be subdivided into a component uniformly distributed in longitude and a second unevenly distributed component, which is responsible for the observed photometric modulation. The uniformly distributed component appears to be possibly modulated with an activity cycle of ∼ 13.5 yr. The unevenly distributed component is mainly concentrated around three major active longitudes. The spot activity appears practically permanent at one longitude, but the spot area changes with a cycle of ∼ 9.5 yr. On the contrary, the spot activity is discontinuous at the other two longitudes, and it switches back and forth between them with a cycle of ∼ 6.8 yr. However, before each switching is completed, a transition phase of ∼ 1.05 yr, during which both longitudes are active, occurs. After this transient phase, spot activity remains localized at one of the two longitudes for ∼ 4.7 yr untill another switching event occurs, which re-establishes spot activity at the other longitude. The longitude separation between the permanent and the switching active longitudes is closest during the switching phases and it varies along the ∼ 6.8 yr cycle. Different time scales characterize the activity at the permanent longitude and at the switching longitudes: a period of ∼ 9.5 yr is related to the activity cycle at the permanent longitude, and a period of ∼ 4.3 yr characterizes the spot life time at the switching longitudes in between switching events. The photometric period of the active star changes from season to season with a relative amplitude of 1.5% and a period of ∼ 4.7 yr. Such a variation of the photometric period may be likely associated with the phase shift of the light curves produced by the switching of spot activity from one active longitude to the other. The permanently active longitude shows a steady migration towards decreasing orbital phases, with an oscillating migration rate along the 9.5 yr cycle period and nearly in phase with the variation of its spotted area. The amplitude of the differential rotation derived from such a behaviour is of the order of ∼ 0.023%, about one order of magnitude smaller than estiSend offprint requests to: S.Messina (sme@sunct.ct.astro.it) mated by Henry et al. (1995). The other two active longitudes migrates also towards decreasing orbital phase, but at a discontinuous rate. There appears to be no correlation between the location of the active longitudes with respect to the line joining the two components of the system and their activity level.

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M. Rodonò

Papers

The magnetic activity cycle of II Pegasi: results from twenty-five years of wide-band photometry