Abstract
Starspots are created by local magnetic fields on the surfaces of stars, just as sunspots. Their fields are strong enough to suppress the overturning convective motion and thus block or redirect the flow of energy from the stellar interior outwards to the surface and consequently appear as locally cool and therefore dark regions against an otherwise bright photosphere (Biermann in Astronomische Nachrichten 264:361, 1938; Z Astrophysik 25:135, 1948). As such, starspots are observable tracers of the yet unknown internal dynamo activity and allow a glimpse into the complex internal stellar magnetic field structure. Starspots also enable the precise measurement of stellar rotation which is among the key ingredients for the expected internal magnetic topology. But whether starspots are just blown-up sunspot analogs, we do not know yet. This article is an attempt to review our current knowledge of starspots. A comparison of a white-light image of the Sun (G2V, 5 Gyr) with a Doppler image of a young solar-like star (EK Draconis; G1.5V, age 100 Myr, rotation 10 × Ω Sun) and with a mean-field dynamo simulation suggests that starspots can be of significantly different appearance and cannot be explained with a scaling of the solar model, even for a star of same mass and effective temperature. Starspots, their surface location and migration pattern, and their link with the stellar dynamo and its internal energy transport, may have far reaching impact also for our understanding of low-mass stellar evolution and formation. Emphasis is given in this review to their importance as activity tracers in particular in the light of more and more precise exoplanet detections around solar-like, and therefore likely spotted, host stars.
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Acknowledgments
Thanks are due to many colleagues working in the field and who made this topic a continuously flourishing one. In particular I would like to thank my mentors Douglas “Doug” Hall and William “Bill” Wehlau who got me going on this topic a number of years ago. Most recently, I have benefited a lot from many discussions with John Rice and Thorsten Carroll on issues of Doppler imaging and also with Katalin Oláh and Thomas Granzer on issues of light curve interpretations. On MHD and dynamo issues, Rainer Arlt shed some light into the far corners in the back of my brain. Not unimportantly, several grants along the way had made all this more manageable, in particular from the Austrian Fond zur Förderung der wissenschaftlichen Forschung (FWF), the Deutsche Forschungsgemeinschaft (DFG), the German Bundesministerium für Bildung und Forschung (BMBF), and theMinisterium fürWissenschaft, Forschung und Kultur (MWFK) of the State of Brandenburg in Germany. Finally, without my family I would probably herd cattle in the mountains of upper Styria (nothing bad with that though).
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Strassmeier, K.G. Starspots. Astron Astrophys Rev 17, 251–308 (2009). https://doi.org/10.1007/s00159-009-0020-6
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