How reliable is Zeeman Doppler imaging without simultaneous temperature reconstruction?

Rosén, Lisa; Kochukhov, O.

doi:10.1051/0004-6361/201219972

Cited by 36 publications

(43 citation statements)

References 18 publications

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“…This has been shown in several numerical experiments, see e.g. Kochukhov & Piskunov (2002) and Rosén & Kochukhov (2012). The inclusion of linear polarization will also remove the crosstalk.…”

Section: Introductionsupporting

confidence: 53%

Strong variable linear polarization in the cool active star II Peg

2013

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Abstract. Magnetic fields of cool active stars are currently studied polarimetrically using only circular polarization observations. This provides limited information about the magnetic field geometry since circular polarization is only sensitive to the line-of-sight component of the magnetic field. Reconstructions of the magnetic field topology will therefore not be completely trustworthy when only circular polarization is used. On the other hand, linear polarization is sensitive to the transverse component of the magnetic field. By including linear polarization in the reconstruction the quality of the reconstructed magnetic map is dramatically improved. For that reason, we wanted to identify cool stars for which linear polarization could be detected at a level sufficient for magnetic imaging. Four active RS CVn binaries, II Peg, HR 1099, IM Peg, and σ Gem were observed with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope. Mean polarization profiles in all four Stokes parameters were derived using the multiline technique of least-squares deconvolution (LSD). Not only was linear polarization successfully detected in all four stars in at least one observation, but also, II Peg showed an extraordinarily strong linear polarization signature throughout all observations. This qualifies II Peg as the first promising target for magnetic Doppler imaging in all four Stokes parameters and, at the same time, suggests that other such targets can possibly be identified.

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Section: Introductionsupporting

confidence: 53%

Strong variable linear polarization in the cool active star II Peg

2013

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“…Only for the case of V1358 Ori do we observe such a weak trend, and even then this is most likely biased by the poor phase coverage. On the other hand, Rosén & Kochukhov (2012) showed that the ZDI method is capable of reproducing both the magnetic field and cool spot structures in cases where they coincide. In the case of our maps of AH Lep and especially of V1358 Ori, the poor phase coverage makes the comparison of magnetic and brightness maps uncertain.…”

Section: Discussionmentioning

confidence: 99%

Zeeman-Doppler imaging of active young solar-type stars

Hackman

Lehtinen

Rosén

et al. 2016

A&A

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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 α 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.

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“…As inferred from the previous numerical tests of ZDI inversions (Donati & Brown 1997;Rosén & Kochukhov 2012), reconstruction of the magnetic field distributions from the Stokes I and V data alone suffers from some ambiguities. The most important problem is a cross-talk between the radial and meridional field components at low latitudes (i.e.…”

Section: Zeeman Doppler Imaging Code Invers13mentioning

confidence: 98%

“…Skelly et al 2010;Waite et al 2011;Marsden et al 2011) still systematically neglect effects of temperature spots in the magnetic inversions. As demonstrated by the numerical tests of Rosén & Kochukhov (2012), the lack of self-consistency in temperature and magnetic mapping can lead to severe artefacts if cool spots coincide with major concentrations of the magnetic flux. Calculation of the local Stokes parameter profiles by Invers13 is based on a prescribed line list of atomic and molecular lines, normally obtained from the VALD and MARCS databases, and a grid of model atmospheres.…”

Section: Zeeman Doppler Imaging Code Invers13mentioning

confidence: 99%

Magnetic field topology of the RS CVn star II Pegasi

Kochukhov

Mantere

Hackman

et al. 2013

A&A

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Context. The dynamo processes in cool active stars generate complex magnetic fields responsible for prominent surface stellar activity and variability at different time scales. For a small number of cool stars magnetic field topologies were reconstructed from the time series of spectropolarimetric observations using the Zeeman Doppler imaging (ZDI) method, often yielding surprising and controversial results. Aims. In this study we follow a long-term evolution of the magnetic field topology of the RS CVn binary star II Peg using a more self-consistent and physically more meaningful modelling approach compared to previous ZDI studies. Methods. We collected high-resolution circular polarisation observations of II Peg using the SOFIN spectropolarimeter at the Nordic Optical Telescope. These data cover 12 epochs spread over 7 years, comprising one of the most comprehensive spectropolarimetric data sets acquired for a cool active star. A multi-line diagnostic technique in combination with a new ZDI code is applied to interpret these observations. Results. We have succeeded in detecting clear magnetic field signatures in average Stokes V profiles for all 12 data sets. These profiles typically have complex shapes and amplitudes of ∼10 −3 of the unpolarised continuum, corresponding to mean longitudinal fields of 50-100 G. Magnetic inversions using these data reveals evolving magnetic fields with typical local strengths of 0.5-1.0 kG and complex topologies. Despite using a self-consistent magnetic and temperature mapping technique, we do not find a clear correlation between magnetic and temperature features in the ZDI maps. Neither do we confirm the presence of persistent azimuthal field rings found in other RS CVn stars. Reconstruction of the magnetic field topology of II Peg reveals significant evolution of both the surface magnetic field structure and the extended magnetospheric field geometry on the time scale covered by our observations. From 2004 to 2010 the total field energy drastically declined and the field became less axisymmetric. This also coincided with the transition from predominantly poloidal to mainly toroidal field topology. Conclusions. A qualitative comparison of the ZDI maps of II Peg with the prediction of dynamo theory suggests that the magnetic field in this star is produced mainly by the turbulent α 2 dynamo rather than the solar αΩ dynamo. Our results do not show a clear active longitude system, nor is there evidence of the presence of an azimuthal dynamo wave.

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How reliable is Zeeman Doppler imaging without simultaneous temperature reconstruction?

Cited by 36 publications

References 18 publications

Strong variable linear polarization in the cool active star II Peg

Strong variable linear polarization in the cool active star II Peg

Zeeman-Doppler imaging of active young solar-type stars

Magnetic field topology of the RS CVn star II Pegasi

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