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Magnetic fields of young solar twins
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Department of Physics, P.O. Box 64, FI-00014 University of Helsinki.
Department of Physics, P.O. Box 64, FI-00014 University of Helsinki;ReSoLVe Centre of Excellence, Aalto University, Department of Computer Science, PO Box 15400, 00076 Aalto, Finland.
2016 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 593, A35Article in journal (Refereed) Published
Abstract [en]

Aims. The goal of this work is to study the magnetic fields of six young solar-analogue stars both individually, and collectively, to search for possible magnetic field trends with age. If such trends are found, they can be used to understand magnetism in the context of stellar evolution of solar-like stars and, the past of the Sun and the solar system. This is also important for the atmospheric evolution of the inner planets, Earth in particular. Methods. We used Stokes IV data from two different spectropolarimeters, NARVAL and HARPSpol. The least-squares deconvolution multi-line technique was used to increase the signal-to-noise ratio of the data. We then applied a modern Zeeman-Doppler imaging code in order to reconstruct the magnetic topology of all stars and the brightness distribution of one of our studied stars. Results. Our results show a significant decrease in the magnetic field strength and energy as the stellar age increases from 100 Myr to 250 Myr while there is no significant age dependence of the mean magnetic field strength for stars with ages 250-650 Myr. The spread in the mean field strength between different stars is comparable to the scatter between different observations of individual stars. The meridional field component has the weakest strength compared to the radial and azimuthal field components in 15 out of the 16 magnetic maps. It turns out that 89-97% of the magnetic field energy is contained in l=1-3. There is also no clear trend with age and distribution of field energy into poloidal/toroidal and axisymmetric/non-axisymmetric components within the sample. The two oldest stars in this study do show a twice as strong octupole component compared to the quadrupole component. This is only seen in one out of 13 maps of the younger stars. One star, χ 1 Ori displays two field polarity switches during almost 5 years of observations suggesting a magnetic cycle length of either 2, 6 or 8 years.

Place, publisher, year, edition, pages
2016. Vol. 593, A35
Keyword [en]
polarisation – stars: magnetic fields – stars: late-type – stars: individual: EK Dra, HN Peg, π 1 UMa, χ 1 Ori, BE Cet, κ 1 Cet
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-283269DOI: 10.1051/0004-6361/201628443ISI: 000385820100070OAI: oai:DiVA.org:uu-283269DiVA: diva2:918966
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilSwedish National Space Board
Note

Originally included in thesis in manuscript form.

Available from: 2016-04-12 Created: 2016-04-12 Last updated: 2017-11-30Bibliographically approved
In thesis
1. Magnetic fields of cool active stars
Open this publication in new window or tab >>Magnetic fields of cool active stars
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Magnetic fields are present throughout the universe and are very important for many astrophysical processes. Magnetic field influences a star throughout its life and affects nearby objects such as planets. Stellar magnetic field can be detected by measuring the Zeeman splitting of spectral lines in the intensity spectra (Stokes I) if the field is strong, or by analyzing polarization spectra if the field is weak. Magnetic fields in stars similar to the Sun are ubiquitous but, in general, relatively weak. Until recently these fields were detected through circular polarization (Stokes V) only since linear polarization (Stokes QU) is significantly weaker. The information embedded in different Stokes spectra is used for reconstruction of the surface magnetic field topology with Zeeman Doppler imaging (ZDI) technique. However, cool stars often have complex field geometries and this, combined with a low field strength, partial Stokes parameter observations and the presence of cool spots, makes accurate magnetic mapping difficult. We have performed numerical tests of ZDI to investigate some of the problems of magnetic inversions and ways to overcome them. The most reliable results were found when magnetic field and temperature inhomogeneities were modelled simultaneously and all four Stokes parameters were included in the reconstruction process. We carried out observations of active cool stars in all four Stokes parameters trying to find an object with linear polarization signatures suitable for ZDI. The RS CVn star II Peg was identified as a promising target, showing exceptionally strong linear polarization signatures. We reconstructed the magnetic field in II Peg using full Stokes vector observations for the first time in a cool star. Compared to the magnetic maps recovered from the Stokes IV spectra, the four Stokes parameter results reveal a significantly stronger and more complex surface magnetic field and a more compact stellar magnetosphere. Spectropolarimetric observations and magnetic inversions can also be used to investigate magnetic activity of the young Sun and its implications for the solar system past. To this end, we studied a sample of six stars with parameters very similar to the present Sun, but with ages of only 100-650 Myr. Magnetic field maps of these young solar analogues suggest a significant decrease of the field strength in the age interval 100-250 Myr and a possible change in the magnetic field topology for stars older than about 600 Myr.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 67 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1367
Keyword
stars: magnetic field, stars: late-type, stars: activity, stars: imaging, polarisation, starspots
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:uu:diva-283357 (URN)978-91-554-9561-9 (ISBN)
Public defence
2016-06-02, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2016-05-11 Created: 2016-04-12 Last updated: 2016-06-01

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Rosén, LisaKochukhov, Oleg

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