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How reliable is Zeeman Doppler imaging without simultaneous temperature reconstruction?
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.
2012 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 548, A8- p.Article in journal (Refereed) Published
Abstract [en]

Context. Aims. The goal of this study is to perform numerical tests of Zeeman Doppler imaging (ZDI) to asses whether correct reconstruction of magnetic fields is at all possible without taking temperature into account for stars in which magnetic and temperature inhomogeneities are spatially correlated. Methods. We used a modern ZDI code employing a physically realistic treatment of the polarized radiative transfer in all four Stokes parameters. We generated artificial observations of isolated magnetic spots and of magnetic features coinciding with cool temperature spots and then reconstructed magnetic and temperature distributions from these data. Results. Using Stokes I and V for simultaneous magnetic and temperature mapping for the star with a homogeneous temperature distribution yields magnetic field strengths underestimated by typically 10-15% relative to their true values. When temperature is kept constant and Stokes I is not used for magnetic mapping, the underestimation is 30-60%. At the same time, the strength of magnetic field inside cool spots is underestimated by as much as 80-95% and the spot geometry is also poorly reconstructed when temperature variations are ignored. On the other hand, the inversion quality is greatly improved when temperature variations are accounted for in magnetic mapping. The field strength is underestimated by 40-70% for the radial and azimuthal spots and by 70-80% for the meridional spots. Inversions still suffer from significant crosstalk between radial and meridional fields at low latitudes. The azimuthal field component proves to be most robust since it suffers the least from crosstalk. When using all four Stokes parameters crosstalk is removed. In that case, the reconstructed field strength inside cool spots is underestimated by 30-40% but the spot geometry can be recovered very accurately compared to the experiments with circular polarization alone. Conclusions. Reliable magnetic field reconstruction for a star with high-contrast temperature spots is essentially impossible if temperature inhomogeneities are ignored. A physically realistic line profile modeling method, which simultaneously accounts for both types of inhomogeneities, is required for meaningful ZDI of cool active stars.

Place, publisher, year, edition, pages
2012. Vol. 548, A8- p.
Keyword [en]
Methods: numerical, Polarization, Stars: imaging, Stars: magnetic field, Starspots
National Category
Natural Sciences
URN: urn:nbn:se:uu:diva-188133DOI: 10.1051/0004-6361/201219972ISI: 000311901200008OAI: oai:DiVA.org:uu-188133DiVA: diva2:576318
Available from: 2012-12-12 Created: 2012-12-12 Last updated: 2016-06-01Bibliographically 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.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1367
stars: magnetic field, stars: late-type, stars: activity, stars: imaging, polarisation, starspots
National Category
Astronomy, Astrophysics and Cosmology
Research subject
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)
Available from: 2016-05-11 Created: 2016-04-12 Last updated: 2016-06-01

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