LTE spectrum synthesis in magnetic stellar atmospheres

Abstract

With the aim of establishing a benchmark for the detailed calculation of the polarised line profiles of magnetic stars, we describe an intercomparison of LTE Stokes profiles calculated using three independent, state-of-the-art magnetic spectrum synthesis codes: Cossam, ı10 and Zeeman2. We find, upon establishing a homogeneous basis for the calculations (identical definitions of the Stokes parameters and the magnetic and stellar reference frames, identical input model stellar atmosphere, identical input atomic data, and identical chemical element abundances and magnetic field distributions), that local and disc-integrated Stokes IQUV profiles of Fe calculated using the three codes agree very well. For the illustrative case of disc-integrated profiles calculated for abundance , dipole magnetic field intensity kG, and projected rotational velocity km s-1, Stokes I profiles (depth ∼40% of the continuum flux Ic) agree to within about 0.05% rms of Ic, Stokes V profiles (full amplitude ∼10% ) to within about 0.02% rms of Ic, and Stokes Q and U profiles (full amplitudes ∼2% ) at the sub-0.01% rms level. These differences are sufficiently small so as to allow for congruent interpretation of the best spectropolarimetric data available, as well as for any data likely to become available during the near future. This indicates that uncertainties in modeling Stokes profiles result overwhelmingly from uncertainties in input atomic and physical data, especially the state and structure of model stellar atmospheres.