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  • 1.
    Hackman, T.
    et al.
    Univ Helsinki, Dept Phys, POB 64, Helsinki 00084, Finland..
    Lehtinen, J.
    Univ Helsinki, Dept Phys, POB 64, Helsinki 00084, Finland..
    Rosén, Lisa
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Kapyla, M. J.
    Aalto Univ, Dept Comp Sci, ReSoLVE Ctr Excellence, POB 15400, Aalto 00076, Finland..
    Zeeman-Doppler imaging of active young solar-type stars2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 587, article id A28Article in journal (Refereed)
    Abstract [en]

    Context: By studying young magnetically active late-type stars, i.e. analogues to the young Sun, we can draw conclusions on the evolution of the solar dynamo.

    Aims: We determine the topology of the surface magnetic field and study the relation between the magnetic field and cool photospheric spots in three young late-type stars.

    Methods: High-resolution spectropolarimetry of the targets was obtained with the HARPSpol instrument mounted at the ESO 3.6 m telescope. The signal-to-noise ratios of the Stokes IV measurements were boosted by combining the signal from a large number of spectroscopic absorption lines through the least squares deconvolution technique. Surface brightness and magnetic field maps were calculated using the Zeeman-Doppler imaging technique.

    Results: All three targets show clear signs of magnetic fields and cool spots. Only one of the targets, V1358 Ori, shows evidence of the dominance of non-axisymmetric modes. In two of the targets, the poloidal field is significantly stronger than the toroidal one, indicative of an alpha(2)-type dynamo, in which convective turbulence effects dominate over the weak differential rotation. In two of the cases there is a slight anti-correlation between the cool spots and the strength of the radial magnetic field. However, even in these cases the correlation is much weaker than in the case of sunspots.

    Conclusions: The weak correlation between the measured radial magnetic field and cool spots may indicate a more complex magnetic field structure in the spots or spot groups involving mixed magnetic polarities. Comparison with a previously published magnetic field map shows that on one of the stars, HD 29615, the underlying magnetic field changed its polarity between 2009 and 2013.

  • 2.
    Kochukhov, Oleg
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Petit, P.
    Univ Toulouse, UPS OMP, IRAP, Toulouse, France.;CNRS, Inst Rech Astrophys & Planetol, Toulouse, France..
    Strassmeier, K. G.
    Leibniz Inst Astrophys Potsdam AIP, Potsdam, Germany..
    Carroll, T. A.
    Leibniz Inst Astrophys Potsdam AIP, Potsdam, Germany..
    Fares, R.
    Osserv Astrofis Catania, INAF, Catania, Italy..
    Folsom, C. P.
    Univ Toulouse, UPS OMP, IRAP, Toulouse, France.;CNRS, Inst Rech Astrophys & Planetol, Toulouse, France..
    Jeffers, S. V.
    Georg August Univ Gottingen, Inst Astrophys, Gottingen, Germany..
    Korhonen, H.
    Univ Copenhagen, Niels Bohr Inst, Copenhagen, Denmark..
    Monnier, J. D.
    Univ Michigan, Dept Astron, Ann Arbor, MI 48109 USA..
    Morin, J.
    Univ Montpellier, CNRS, LUPM, Montpellier, France..
    Rosén, Lisa
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Roettenbacher, R. M.
    Univ Michigan, Dept Astron, Ann Arbor, MI 48109 USA.;Stockholm Univ, Dept Astron, Stockholm, Sweden..
    Shulyak, D.
    Georg August Univ Gottingen, Inst Astrophys, Gottingen, Germany..
    Surface magnetism of cool stars2017In: Astronomical Notes - Astronomische Nachrichten, ISSN 0004-6337, E-ISSN 1521-3994, Vol. 338, no 4, p. 428-441Article in journal (Refereed)
    Abstract [en]

    Magnetic fields are essential ingredients of many physical processes in the interiors and envelopes of cool stars. Yet their direct detection and characterization is notoriously difficult, requiring high-quality observations and advanced analysis techniques. Significant progress has been recently achieved by several types of direct magnetic field studies on the surfaces of cool, active stars. In particular, complementary techniques of field topology mapping with polarization data and total magnetic flux measurements from intensity spectra have been systematically applied to different classes of active stars, leading to interesting and occasionally controversial results. In this paper, we summarize the current status of direct magnetic field studies of cool stars and investigations of surface inhomogeneities caused by the field, based on the material presented at the Cool Stars 19 splinter session.

  • 3.
    Rosén, Lisa
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Magnetic fields of cool active stars2016Doctoral 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.

    List of papers
    1. How reliable is Zeeman Doppler imaging without simultaneous temperature reconstruction?
    Open this publication in new window or tab >>How reliable is Zeeman Doppler imaging without simultaneous temperature reconstruction?
    2012 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 548, p. A8-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.

    Keywords
    Methods: numerical, Polarization, Stars: imaging, Stars: magnetic field, Starspots
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:uu:diva-188133 (URN)10.1051/0004-6361/201219972 (DOI)000311901200008 ()
    Available from: 2012-12-12 Created: 2012-12-12 Last updated: 2017-12-07Bibliographically approved
    2. Strong variable linear polarization in the cool active star II Peg
    Open this publication in new window or tab >>Strong variable linear polarization in the cool active star II Peg
    2013 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 436, no 1, p. L10-L14Article 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.

    Keywords
    polarization, stars: individual: II Peg, stars: individual: HR 1099, stars: individual: IM Peg, stars: individual: sigma Gem, stars: magnetic field
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:uu:diva-212329 (URN)10.1093/mnrasl/slt102 (DOI)000326435100003 ()
    Available from: 2013-12-10 Created: 2013-12-09 Last updated: 2017-12-06
    3. First Zeeman Doppler Imaging of a Cool Star Using All Four Stokes Parameters
    Open this publication in new window or tab >>First Zeeman Doppler Imaging of a Cool Star Using All Four Stokes Parameters
    2015 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 805, no 2, article id 169Article in journal (Refereed) Published
    Abstract [en]

    Magnetic fields are ubiquitous in active cool stars, but they are in general complex and weak. Current Zeeman Doppler imaging (ZDI) studies of cool star magnetic fields chiefly employ circular polarization observations because linear polarization is difficult to detect and requires a more sophisticated radiative transfer modeling to interpret. But it has been shown in previous theoretical studies, and in the observational analyses of magnetic Ap stars, that including linear polarization in the magnetic inversion process makes it possible to correctly recover many otherwise lost or misinterpreted magnetic features. We have obtained phase-resolved observations in all four Stokes parameters of the RS CVn star II Peg at two separate epochs. Here we present temperature and magnetic field maps reconstructed for this star using all four Stokes parameters. This is the very first such ZDI study of a cool active star. Our magnetic inversions reveal a highly structured magnetic field topology for both epochs. The strength of some surface features is doubled or even quadrupled when linear polarization is taken into account. The total magnetic energy of the reconstructed field map also becomes about 2.1-3.5 times higher. The overall complexity is also increased as the field energy is shifted toward higher harmonic modes when four Stokes parameters are used. As a consequence, the potential field extrapolation of the four Stokes parameter ZDI results indicates that magnetic field becomes weaker at a distance of several stellar radii due to a decrease of the largescale field component.

    Keywords
    polarization, stars: individual (II Peg), stars: late-type, stars: magnetic field
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-258780 (URN)10.1088/0004-637X/805/2/169 (DOI)000356715400084 ()
    Available from: 2015-07-20 Created: 2015-07-20 Last updated: 2017-12-04Bibliographically approved
    4. Magnetic fields of young solar twins
    Open this publication in new window or tab >>Magnetic fields of young solar twins
    2016 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 593, article id 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.

    Keywords
    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:nbn:se:uu:diva-283269 (URN)10.1051/0004-6361/201628443 (DOI)000385820100070 ()
    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
  • 4.
    Rosén, Lisa
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    How reliable is Zeeman Doppler imaging without simultaneous temperature reconstruction?2012In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 548, p. A8-Article in journal (Refereed)
    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.

  • 5.
    Rosén, Lisa
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Alecian, E.
    Univ Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France.
    Neiner, C.
    Univ Paris Diderot, UPMC Univ Paris 06, Sorbonne Univ,LESIA, PSL Res Univ,CNRS,Sorbonne Paris Cite,Observ Pari, 5 Pl Jules Janssen, F-92195 Meudon, France.
    Morin, J.
    Univ Montpellier, CNRS, LUPM, Pl Eugene Bataillon, F-34095 Montpellier, France.
    Wade, G. A.
    Royal Mil Coll Canada, Dept Phys & Space Sci, POB 17000, Kingston, ON K7K 7B4, Canada.
    Magnetic field topology of the cool, active, short-period binary system sigma(2) Coronae Borealis2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 613, article id A60Article in journal (Refereed)
    Abstract [en]

    Aims. The goal of this work is to study the cool, active binary star sigma(2) CrB, focussing on its magnetic field. The two F9-G0 components of this system are tidally locked and in a close orbit, increasing the chance of interaction between their magnetospheres. Methods. We used Stokes IV data from the twin spectropolarimeters Narval at the TBL and ESPaDOnS at the CFHT. The leastsquares deconvolution multi-line technique was used to increase the signal-to-noise ratio of the data. We then applied a new binary Zeeman-Doppler imaging code to reconstruct simultaneously the magnetic topology and brightness distribution of both components of sigma(2) CrB. This analysis was carried out for two observational epochs in 2014 and 2017. Results. A previously unconfirmed magnetic field of the primary star has been securely detected. At the same time, the polarisation signatures of the secondary appear to have a systematically larger amplitude than that of the primary. This corresponds to a stronger magnetic field, for which the magnetic energy of the secondary exceeds that of the primary by a factor of 3.3-5.7. While the magnetic energy is similar for the secondary star in the two epochs, the magnetic energy is about twice as high in 2017 for the primary. The magnetic field topology of the two stars in the earlier epoch (2014) is very different. The fractions of energy in the dipole and quadrupole components of the secondary are similar and thereafter decrease with increasing harmonic angular degree l. At the same time, for the primary the fraction of energy in the dipole component is low and the maximum energy contribution comes from l = 4. However, in the 2017 epoch both stars have similar field topologies and a systematically decreasing energy with increasing l. In the earlier epoch, the magnetic field at the visible pole appears to be of opposite polarity for the primary and secondary, suggesting linked magnetospheres. The apparent rotational periods of both sigma(2) CrB components are longer than the orbital period, which we interpret as an evidence of a solar-like differential rotation. Conclusions. Despite their nearly identical fundamental parameters, the components of sigma(2) CrB system exhibit different magnetic field properties. This indicates that the magnetic dynamo process is a very sensitive function of stellar parameters.

  • 6.
    Rosén, Lisa
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Hackman, Thomas
    Department of Physics, P.O. Box 64, FI-00014 University of Helsinki.
    Lehtinen, Jyri
    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.
    Magnetic fields of young solar twins2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 593, article id A35Article in journal (Refereed)
    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.

  • 7.
    Rosén, Lisa
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Wade, G. A.
    First Zeeman Doppler Imaging of a Cool Star Using All Four Stokes Parameters2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 805, no 2, article id 169Article in journal (Refereed)
    Abstract [en]

    Magnetic fields are ubiquitous in active cool stars, but they are in general complex and weak. Current Zeeman Doppler imaging (ZDI) studies of cool star magnetic fields chiefly employ circular polarization observations because linear polarization is difficult to detect and requires a more sophisticated radiative transfer modeling to interpret. But it has been shown in previous theoretical studies, and in the observational analyses of magnetic Ap stars, that including linear polarization in the magnetic inversion process makes it possible to correctly recover many otherwise lost or misinterpreted magnetic features. We have obtained phase-resolved observations in all four Stokes parameters of the RS CVn star II Peg at two separate epochs. Here we present temperature and magnetic field maps reconstructed for this star using all four Stokes parameters. This is the very first such ZDI study of a cool active star. Our magnetic inversions reveal a highly structured magnetic field topology for both epochs. The strength of some surface features is doubled or even quadrupled when linear polarization is taken into account. The total magnetic energy of the reconstructed field map also becomes about 2.1-3.5 times higher. The overall complexity is also increased as the field energy is shifted toward higher harmonic modes when four Stokes parameters are used. As a consequence, the potential field extrapolation of the four Stokes parameter ZDI results indicates that magnetic field becomes weaker at a distance of several stellar radii due to a decrease of the largescale field component.

  • 8.
    Rosén, Lisa
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Wade, G. A.
    Strong variable linear polarization in the cool active star II Peg2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 436, no 1, p. L10-L14Article in journal (Refereed)
    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.

  • 9.
    See, V.
    et al.
    Univ St Andrews, Sch Phys & Astron, SUPA, St Andrews KY16 9SS, Fife, Scotland..
    Jardine, M.
    Univ St Andrews, Sch Phys & Astron, SUPA, St Andrews KY16 9SS, Fife, Scotland..
    Vidotto, A. A.
    Univ Geneva, Observ Geneve, CH-1290 Sauverny, Switzerland..
    Donati, J. -F
    Folsom, C. P.
    Univ Grenoble Alpes, IPAG, F-38000 Grenoble, France.;CNRS, IPAG, F-38000 Grenoble, France..
    Saikia, S. Boro
    Univ Gottingen, Inst Astrophys, D-37077 Gottingen, Germany..
    Bouvier, J.
    Univ Grenoble Alpes, IPAG, F-38000 Grenoble, France.;CNRS, IPAG, F-38000 Grenoble, France..
    Fares, R.
    INAF Osservatorio Astrofis Catania, I-95123 Catania, Italy..
    Gregory, S. G.
    Hussain, G.
    ESO, D-85748 Garching, Germany..
    Jeffers, S. V.
    Marsden, S. C.
    Univ So Queensland, Computat Engn & Sci Res Ctr, Toowoomba, Qld 4350, Australia..
    Morin, J.
    CNRS, LUPM UMR5299, F-34095 Montpellier, France.;Univ Montpellier 2, F-34095 Montpellier, France..
    Moutou, C.
    CNRS, Canada France Hawaii Telescope Corp, Kamuela, HI 96743 USA.;Aix Marseille Univ, CNRS, LAM, UMR 7326, F-13388 Marseille, France..
    do Nascimento, J. D., Jr.
    Univ Fed Rio Grande do Norte, Dept Fis Teor & Expt, BR-59072970 Natal, RN, Brazil.;Harvard Smithsonian Ctr Astrophys, Cambridge, MA 02138 USA..
    Petit, P.
    Univ Toulouse, Inst Rech Astrophys & Planetol, UPS OMP, F-31400 Toulouse, France.;CNRS, Inst Rech Astrophys & Planetol, F-31400 Toulouse, France..
    Rosén, Lisa
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Waite, I. A.
    Univ So Queensland, Computat Engn & Sci Res Ctr, Toowoomba, Qld 4350, Australia..
    The energy budget of stellar magnetic fields2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 453, no 4, p. 4301-4310Article in journal (Refereed)
    Abstract [en]

    Spectropolarimetric observations have been used to map stellar magnetic fields, many of which display strong bands of azimuthal fields that are toroidal. A number of explanations have been proposed to explain how such fields might be generated though none are definitive. In this paper, we examine the toroidal fields of a sample of 55 stars with magnetic maps, with masses in the range 0.1-1.5M(circle dot). We find that the energy contained in toroidal fields has a power-law dependence on the energy contained in poloidal fields. However the power index is not constant across our sample, with stars less and more massive than 0.5M(circle dot) having power indices of 0.72 +/- 0.08 and 1.25 +/- 0.06, respectively. There is some evidence that these two power laws correspond to stars in the saturated and unsaturated regimes of the rotation-activity relation. Additionally, our sample shows that strong toroidal fields must be generated axisymmetrically. The latitudes at which these bands appear depend on the stellar rotation period with fast rotators displaying higher latitude bands than slow rotators. The results in this paper present new constraints for future dynamo studies.

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