uu.seUppsala University Publications
Change search
ReferencesLink to record
Permanent link

Direct link
Strong variable linear polarization in the cool active star II Peg
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
2013 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 436, no 1, L10-L14 p.Article in journal (Refereed) Published
Abstract [en]

Magnetic fields of cool active stars are currently studied polarimetrically using only circular polarization observations. Including linear polarization in the reconstruction of stellar magnetic fields allows more information about the magnetic field to be extracted and significantly improves the reliability of stellar magnetic field maps. The goal of this study is to initiate systematic observations of active stars in all four Stokes parameters and to identify cool stars for which linear polarization can be detected at a level sufficient for Zeeman-Doppler Imaging (ZDI). Four active RS CVn binaries, II Peg, HR 1099, IM Peg and sigma Gem, were observed with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope during a time period from 2012 February to 2013 January. The least-squares deconvolution procedure was applied to derive mean polarization profiles of all four Stokes parameters. Linear polarization was detected in all four stars in at least one observation. At the same time, II Peg showed an exceptionally strong and highly variable linear polarization signature throughout all observations. This establishes II Peg as the first promising target for ZDI in all four Stokes parameters and suggests the feasibility of such an analysis with existing equipment for at least a few of the most active cool stars.

Place, publisher, year, edition, pages
2013. Vol. 436, no 1, L10-L14 p.
Keyword [en]
polarization, stars: individual: II Peg, stars: individual: HR 1099, stars: individual: IM Peg, stars: individual: sigma Gem, stars: magnetic field
National Category
Natural Sciences
URN: urn:nbn:se:uu:diva-212329DOI: 10.1093/mnrasl/slt102ISI: 000326435100003OAI: oai:DiVA.org:uu-212329DiVA: diva2:677610
Available from: 2013-12-10 Created: 2013-12-09 Last updated: 2016-06-01
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

Open Access in DiVA

No full text

Other links

Publisher's full text

Search in DiVA

By author/editor
Rosén, LisaKochukhov, Oleg
By organisation
Department of Physics and AstronomyObservational Astronomy
In the same journal
Monthly notices of the Royal Astronomical Society
Natural Sciences

Search outside of DiVA

GoogleGoogle Scholar
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Altmetric score

Total: 516 hits
ReferencesLink to record
Permanent link

Direct link