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Issue A&A
Volume 369, Number 3, April III 2001
Page(s) 889 - 907
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20010101



A&A 369, 889-907 (2001)
DOI: 10.1051/0004-6361:20010101

A study of polarized spectra of magnetic CP stars: Predicted vs. observed Stokes IQUV profiles for $\beta$ CrB and 53 Cam

S. Bagnulo1, G. A. Wade2, J.-F. Donati3, J. D. Landstreet4, F. Leone5, D. N. Monin6 and M. J. Stift1

1  Institut für Astronomie, Universität Wien, Türkenschanzstrasse 17, 1180 Wien, Austria
    e-mail: bagnulo@astro.univie.ac.at & stift@fedelma.astro.univie.ac.at
2  Astronomy Department, University of Toronto at Missisauga, L5L 1CS Ontario, Canada
3  Observatoire Midi-Pyrénées, 14 avenue Édouard Belin, 31400 Toulouse, France
    e-mail: donati@obs-mip.fr
4  Physics & Astronomy Department, The University of Western Ontario, London, N6A 3K7 Ontario, Canada
    e-mail: jlandstr@astro.uwo.ca
5  Osservatorio Astrofisico di Catania, Città Universitaria, 95125 Catania, Italy
    e-mail: fleone@ct.astro.it
6  Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 357147, Russia
    e-mail: mosha@sao.ru

(Received 20 September 2000 / Accepted 12 January 2001 )

Abstract
We present a comparison of observed and calculated Stokes IQUV spectra of two well-known magnetic chemically peculiar stars, $\beta$ Coronae Borealis and 53 Camelopardalis. The observed Stokes spectra were recently described by Wade et al. (2000a), and have been complemented with additional circularly polarized spectra obtained at the Special Astrophysical Observatory. The calculated spectra represent the predictions of new and previously published magnetic field models derived from the analysis of some surface averaged field estimates (e.g., longitudinal field, magnetic field modulus, etc.). We find that these magnetic models are not sufficient to account fully for the observed Stokes profiles -particularly remarkable is the disagreement between the predicted and observed Stokes Q and U profiles of 53 Cam. We suggest that this should be interpreted in terms of magnetic morphologies which are significantly more complex than the second-order multipolar expansions assumed in the models. However, it is clear that some of our inability to reproduce the detailed shapes of the Stokes IQUV profiles is unrelated to the magnetic models. For many metallic ions, for both stars, we found it impossible to account for the strengths and shapes of the observed spectral line profiles when we adopted a unique value for the individual ion abundance. We suggest that this results from strongly non-uniform distributions of these ions as a function of optical depth (i.e., chemical stratification), a hypothesis that is supported by comparison with simple chemically stratified models.


Key words: stars: magnetic fields -- polarization -- stars: chemically peculiar -- stars: individual: $\beta$ CrB -- stars: individual: 53 Cam

Offprint request: S. Bagnulo

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© ESO 2001


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