Variability of young solar-type stars : Spot cycles, rotation, and active longitudes

The outer convective envelopes of late-type stars are believed to support turbulent dynamos that are able to generate strong and dynamic magnetic fields. When these magnetic fields penetrate the stellar surface, they give rise to directly observable activity phenomena, such as dark spots and line emission from the chromospheric layers. Observing and analyzing these activity indicators allows us to characterize the behaviour of the dynamos on a wide range of different stars as well as to put the behaviour of the Sun into a wider context. This thesis presents a study of the activity of a sample of 21 young solar-type stars that can be seen as analogues of the Sun during the first few hundred million years of its existence. The aim is to characterize their activity behaviour on different time scales from months to decades as well as to derive estimates for the magnitude of their surface differential rotation. The results of the analysis provide important observational constraints for the dynamo theory. The study is primarily based on the time series analysis of up to three decades of photometric monitoring of the sample stars. The light curves of these stars display quasiperiodic variations that are induced by changing patterns of dark starspots on the stellar surface and the rotation of the star. The time series analysis results are supplemented by spectroscopic observations that are used to determine the chromospheric activity levels of the stars. The time series analysis is performed using the Continuous Period Search and Carrier Fit methods along with a selection of additional complementary tools. The development of the Continuous Period Search method forms a part of this thesis. This method is fully characterized and its performance is tested on noisy low amplitude light curve data. The photometric analysis reveals new results from both activity cycles and the longitudinal distribution of the spot activity on the stars. Furthermore, it confirms previously published results concerning these and the strength of the surface differential rotation using a new sample of stars. The results show that activity cycles of different lengths are common on active solar-type stars and that their lengths fall on a set of distinct branches. When the longitudinal distribution of the spot activity is compared to the activity level of the stars, the results show a clear domain shift between stars that have axisymmetric spot distributions and stars where the spot activity shows stable longitudinal concentration. Such concentration of activity, known as active longitudes, only appears on the faster rotating more active stars. This suggests a transition between axisymmetric and non-axisymmetric dynamo modes as a function of the activity level of a star. The active longitudes also show a tendency to follow prograde propagation with respect to the stellar rotation. This may be interpreted either as a signature of radial differential rotation or as an azimuthal dynamo wave.

Grantor dc:publisher

Helsingin yliopisto

Year dc:date.issued

2016

Author dc:creator

• Lehtinen, Jyri

• tähtitiede

Statement dc:rights

• Julkaisu on tekijänoikeussäännösten alainen. Teosta voi lukea ja tulostaa henkilökohtaista käyttöä varten. Käyttö kaupallisiin tarkoituksiin on kielletty.
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• Publikationen är skyddad av upphovsrätten. Den får läsas och skrivas ut för personligt bruk. Användning i kommersiellt syfte är förbjuden.

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/10138/161017

OAI record oai:identifier

https://helda.helsinki.fi/server/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Ahelda.helsinki.fi%3A10138%2F161017