Doppler Imaging of stellar magnetic fields

Abstract

Abstract. We used the new magnetic Doppler Imaging code to reconstruct the magnetic field geometry and surface abundance distributions for the classical magnetic CP star α 2 CVn. High-resolution spectropolarimetric observations in the Stokes I and V parameters were collected with the SOFINéchelle spectrograph at the Nordic Optical Telescope. This superb observational material in combination with the advanced modelling technique allowed to achieve the first simultaneous self-consistent mapping of the vector magnetic field and the abundance distributions of six chemical elements. In recovering the stellar magnetic distribution no prior assumptions about the field geometry or strength were made. Instead, we restricted possible solutions of the inverse problem by means of the multipolar regularization method which searches for the field map close to a general non-axisymmetric multipolar configuration but allows departures from this geometry if that is required by the observational data. We found that the magnetic field of α 2 CVn is dominated by a dipolar component and has a minor quadrupole contribution. The surface distributions of the chemical species form symmetric patterns which closely follow the magnetic geometry. This discovery constitutes one of the first direct observational constraints on the horizontal diffusion processes acting in the upper envelope of a strongly magnetized stellar atmosphere. In addition to the extensive magnetic Doppler Imaging analysis, we derived new accurate estimates of the atmospheric parameters and basic physical properties of α 2 CVn using the energy distribution, hydrogen line profiles and the recent Hipparchos parallax.

Multi-element abundance Doppler imaging of the rapidly oscillating Ap star HR 3831

“…This led to a simple picture of chemical spots and rings, symmetric with respect to the magnetic field axis. The validity of this approach, which was, in fact, assumed rather than deduced from observations, was recently questioned by Kochukhov et al (2002). They found that a multitude of chemical abundance structures in the magnetic Ap star α 2 CVn cannot be reduced to a system of spots and rings and that the distribution of some of the elements exhibits a clear asymmetry with respect to the dipolar magnetic field geometry of the star.…”

“…They found that a multitude of chemical abundance structures in the magnetic Ap star α 2 CVn cannot be reduced to a system of spots and rings and that the distribution of some of the elements exhibits a clear asymmetry with respect to the dipolar magnetic field geometry of the star. Kochukhov et al (2002) suggested that phenomena other than a magnetic field play a substantial role in determining the geometry of abundance structures. Stellar rotation, as well as its possible effect on magnetically confined mass loss, is likely to be a physical effect overlooked in theoretical models.…”

A statistical analysis of the magnetic structure of CP stars

“…These considerations prompt for further searches for statistical properties of the magnetic structures of CP stars by making use of a modelling technique not limited by simplifying approximations on the magnetic geometry. A fully adequate modelling technique based on the inversion of Stokes profiles may exist: the Zeeman Doppler Imaging (ZDI; see Piskunov & Kochukhov 2002). However, only few stars have been modelled in detail through the inversion of Stokes profiles (e.g., α 2 CVn, see Kochukhov et al 2002).…”

Modified methods of stellar magnetic field measurements

“…The effective temperature of the star is equal to 11600±500 K (Kochukhov & Wade 2010), its luminosity ≈ 10 2 L ⊙ and the rotation velocity V sin i = 18 km/s (Kochukhov et al 2002). To test our methods of the magnetic field determination we used the polarization spectra of α 2 CVn obtained on February 2, 2009 at the 6-meter telescope of the Russian Special Astrophysical observatory by Chountonov & Rusomarov (2011).…”