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Issue A&A
Volume 414, Number 2, February I 2004
Page(s) 613 - 632
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20031595



A&A 414, 613-632 (2004)
DOI: 10.1051/0004-6361:20031595

Magnetic Doppler imaging of 53 Camelopardalis in all four Stokes parameters

O. Kochukhov1, S. Bagnulo2, G. A. Wade3, L. Sangalli3, N. Piskunov1, J. D. Landstreet4, P. Petit5 and T. A. A. Sigut4

1  Uppsala Astronomical Observatory, Box 515, 751 20 Uppsala, Sweden
2  European Southern Observatory, Casilla 19001, Santiago 19, Chile
3  Department of Physics, Royal Military College of Canada, Box 17000, Kingston, Ontario, K7K 4B4 Canada
4  Physics and Astronomy Department, University of Western Ontario, London, Ontario, N6A 3K7 Canada
5  Observatoire Midi-Pyrénées, 14 avenue Édouard Belin, 31400 Toulouse, France

(Received 30 June 2003 / Accepted 10 October 2003)

Abstract
We present the first investigation of the structure of the stellar surface magnetic field using line profiles in all four Stokes parameters. We extract the information about the magnetic field geometry and abundance distributions of the chemically peculiar star 53 Cam by modelling time-series of high-resolution spectropolarimetric observations with the help of a new magnetic Doppler imaging code. This combination of the unique four Stokes parameter data and state-of-the-art magnetic imaging technique makes it possible to infer the stellar magnetic field topology directly from the rotational variability of the Stokes spectra. In the magnetic imaging of 53 Cam we discard the traditional multipolar assumptions about the structure of magnetic fields in Ap stars and explore the stellar magnetic topology without introducing any global a priori constraints on the field structure. The complex magnetic model of 53 Cam derived with our magnetic Doppler imaging method achieves a good fit to the observed intensity, circular and linear polarization profiles of strong magnetically sensitive $\ion{Fe}{ii}$ spectral lines. Such an agreement between observations and model predictions was not possible with any earlier multipolar magnetic models, based on modelling Stokes  I spectra and fitting surface averaged magnetic observables (e.g., longitudinal field, magnetic field modulus, etc.). Furthermore, we demonstrate that even the direct inversion of the four Stokes parameters of 53 Cam assuming a low-order multipolar magnetic geometry is incapable of achieving an adequate fit to our spectropolarimetric observations. Thus, as a main result of our investigation, we discover that the magnetic field topology of 53 Cam is considerably more complex than any low-order multipolar expansion, raising a general question about the validity of the multipolar assumption in the studies of magnetic field structures of Ap stars. In addition to the analysis of the magnetic field of 53 Cam, we reconstruct surface abundance distributions of Si, Ca, Ti, Fe and Nd. These abundance maps confirm results of the previous studies of 53 Cam, in particular dramatic antiphase variation of Ca and Ti abundances.


Key words: stars: chemically peculiar -- stars: magnetic fields -- polarization -- stars: individual: 53 Cam

Offprint request: O. Kochukhov, oleg.kochukhov@astro.uu.se

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© ESO 2004


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