Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: First Zeeman Doppler imaging of a cool star using all four Stokes
  parameters   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 towards
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 large-scale field component.

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The categories of this paper are: mtrl-th
5
The categories of this paper are: gr-qc
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The categories of this paper are: comp-gas
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The categories of this paper are: plasm-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: q-alg
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The categories of this paper are: adap-org
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The categories of this paper are: alg-geom
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The categories of this paper are: cs
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The categories of this paper are: cmp-lg
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The categories of this paper are: stat
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The categories of this paper are: q-fin
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The categories of this paper are: chao-dyn
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The categories of this paper are: quant-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-th