O. Kochukhov, G. A. Wade, D. Shulyak
Magnetic Doppler imaging is currently the most powerful method of
interpreting high-resolution spectropolarimetric observations of stars. This
technique has revealed the presence of unexpected small-scale magnetic fields
on the surfaces of Ap stars. These studies were recently criticisied by Stift
et al. (2012), who claimed that magnetic inversions are not robust and are
undermined by neglecting a feedback on the Stokes line profiles from the local
atmospheric structure in the regions of enhanced metal abundance. We show that
Stift et al. misinterpreted published magnetic Doppler imaging results and
neglected some of the most fundamental principles behind magnetic mapping. We
demonstrate that the variation of atmospheric structure across the surface of a
star with chemical spots affects the local continuum intensity but is
negligible for the normalised local Stokes profiles. For the disk-integrated
spectra of an Ap star with extreme abundance variations, we find that the
assumption of a mean model atmosphere leads to moderate errors in Stokes I but
is negligible for polarisation spectra. Employing a new magnetic inversion
code, which incorporates the horizontal variation of atmospheric structure, we
reconstructed new maps of magnetic field and Fe abundance for the Ap star
alpha^2 CVn. The resulting distribution of chemical spots changes
insignificantly compared to the previous modelling based on a single model
atmosphere, while the magnetic field geometry does not change at all. This
shows that the assertions by Stift et al. are exaggerated as a consequence of
unreasonable assumptions and extrapolations, as well as methodological flaws
and inconsistencies of their analysis. Our discussion proves that published
magnetic inversions based on a mean stellar atmosphere are robust and reliable,
and that the presence of small-scale magnetic field structures on the surfaces
of Ap stars is real.
View original:
http://arxiv.org/abs/1201.1902
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