Spot activity of late-type stars : a study of II Pegasi and DI Piscium

All stars that have outer convection zones show magnetic activity. This activity strengthens with angular velocity and depth of the convection zone. Cool starspots are believed to be caused by local magnetic field concentrations on the surfaces of stars. They show up as dark regions against a bright photosphere and are observable manifestations of the internal dynamo activity. Therefore the solar dynamo is not unique, only one example of a cyclic dynamo. This makes studies of magnetic activity in other stars important. The convection zone is believed to be the source area of the solar dynamo. The distributed dynamo paradigm is relying on magnetic field generation throughout the convection zone, while the flux-transport paradigm is based on the Babcock-Leighton -effect near the surface in which the mean poloidal field is produced by the merging of twisted magnetic loops. Mean-field dynamo models can produce many observed features both of the solar cycle and some of the features seen in active rapid rotators, but it remains yet under debate which dynamo paradigm is correct. Especially in the case of the Sun, the kinematic mean-field models of different types can lead to a satisfactory reproduction of the solar cycle main properties. These two prevailing dynamo paradigms are briefly introduced in this thesis, as well as their current challenges. For II Peg our time series covers both states of high and low activity. Furthermore we discover a drift of the active region. This drift is also confirmed from photometry, with the carrier fit analysis. The most natural explanation for it would be an azimuthal dynamo wave. DI Psc is a rapidly rotating single giant, which has an interestingly high lithium concentration. We examine the spot behaviour for a two years time-span, and retrieve changes in the activity, which could indicate a fast activity cycle of only a few years.

Grantor dc:publisher

Helsingin yliopisto

Year dc:date.issued

2014

Author dc:creator

• Lindborg, Marjaana

• tähtitiede

Statement dc:rights

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Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/10138/144167

OAI record oai:identifier

https://helda.helsinki.fi/server/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Ahelda.helsinki.fi%3A10138%2F144167