Starspots

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.

Erratum: Physical properties, star-spot activity, orbital obliquity and transmission spectrum of the Qatar-2 planetary system from multicolour photometry

“…10 versus the temperature of the photosphere of the corresponding star. The spectral class for most of the stars is also reported and allows us to see that the temperature difference between photosphere and star-spots is not strongly dependent on spectral type, as already noted by Strassmeier (2009).…”

Modeling the Subsurface Structure of Sunspots

“…Their model was based on a large set of observations (radio, optical, EUV, X-ray). The photospheric structure was taken largely from Stellmacher and Wiehr (1975), while its lower chromospheric stratification is similar to that of Teplitskaya, Grigor'eva, and Skochilov (1977). They also introduced two components (hot and cold) in higher layers.…”

“…Overturning convection would instead expel a magnetic field that is imposed from above, through the well-known process of convective expulsion (Zel'dovich, 1956;Weiss, 1966). The process is seen in action in numerical simulations of horizontal magnetic fields overlying granulation by Steiner et al (2008Steiner et al ( , 2009.…”

Using SONG to probe rapid variability and evolution of starspots

“…The use of Doppler imaging techniques has revealed the surface structure (magnetic and otherwise) of a large number of stars (Berdyugina 2005, Strassmeier 2009). For the most part, these have been based on observations from a single site, with data obtained over several rotations to achieve full phase coverage.…”