Investigation of the magnetic fields of a young Sun-like star ∏1 UMa

University essay from Uppsala universitet/Institutionen för fysik och astronomi

Abstract: In astronomy, the Sun has an important role. It keeps the solar-system together and is thesource for life, heat, light and energy to Earth. As any other star or planet, the Sun has amagnetic field. The magnetic field of the Sun has a great impact on the Sun itself as well asits surrounding. The magnetic field shapes solar wind, causes flares and drives coronal massejections radiating towards the Earth (and other planets). The Sun's magnetic field is still notfully understood, and therefore it is useful to study other stars with properties similar to theSun. So by studying young solar-type stars, the evolution of the Sun can be more easilyunderstood. The aim of this project is to study the surface magnetic field in a youngsolar-type star, π1 UMa to see how the magnetic field is distributed and if there are anypatterns like polarity reversals. Magnetic field generates polarisation and with Stokes vector Iand V, polarisation can be described. Earlier measurements from two time-epochs (2014 and2015) of Stokes I and V have been obtained from the spectropolarimeter NARVAL. To getthe desired mean polarisation profiles of the star, a technique called least squaredeconvolution was applied which increases the signal-to-noise level. To reconstruct themagnetic topology the Zeeman-Doppler imaging technique was used. Then we obtained thesurface magnetic field maps of both measurements. No change of the polarity of magneticfield at the visible stellar pole was found. Most of the magnetic field energy was contained inthe spherical harmonic modes with angular degrees l=1-3. The star shows dominance in thetoroidal component so the study seem to agree with the previously established trend thatyounger and faster rotating stars have predominantly toroidal magnetic fields and older starswith slower rotation rate, like the Sun, have predominantly poloidal field. Looking at themagnetic field plots, the star show dominance in the azimuthal field component, and themean magnetic field strength is similar to one found in the previous study. The results of thesurface magnetic field in our study thus agrees with previous study of the same star. With thiswe can conclude that the Sun's magnetic field probably been different when it was younger, and possibly similar to the star analyzed in this study.

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