3D radiative transfer: Continuum and line scattering in non-spherical winds from OB stars

Abstract

Context. State of the art quantitative spectroscopy utilizes synthetic spectra to extract information from observations. For hot, massive stars, these synthetic spectra are calculated by means of 1D, spherically symmetric, NLTE atmosphere and spectrum-synthesis codes. Certain stellar atmospheres, however, show strong deviations from spherical symmetry, and need to be treated in 3D. Aims. We present and test a newly developed 3D radiative transfer code, tailored to the solution of the radiation field in rapidly expanding stellar atmospheres. We apply our code to the continuum transfer in wind-ablation models, and to the UV resonance line formation in magnetic winds. Methods. We have used a 3D finite-volume method for the solution of the time-independent equation of radiative transfer, to study continuum- and line-scattering problems, currently approximated by a two-level-atom. Convergence has been accelerated by coupling the formal solver to a non-local approximate Λ-iteration scheme. Particular emphasis has been put on careful tests, by comparing with alternative solutions for 1D, spherically symmetric model atmospheres. These tests allowed us to understand certain shortcomings of the methods, and to estimate limiting cases that can actually be calculated. Results. The typical errors of the converged source functions, when compared to 1D solutions, are of the order of 10-20%, and rapidly increase for optically thick (τ ≳ 10) continua, mostly due to the order of accuracy of our solution scheme. In circumstellar discs, the radiation temperatures in the (optically thin) transition region from wind to disc are quite similar to corresponding values in the wind. For MHD simulations of dynamical magnetospheres, the line profiles, calculated with our new 3D code, agree well with previous solutions using a 3D-SEI method. When compared with profiles resulting from the so-called analytic dynamical magnetosphere (ADM) model, however, significant differences become apparent. Conclusions. Due to similar radiation temperatures in the wind and the transition region to the disc, the same line-strength distribution can be applied within radiation hydrodynamic calculations for optically thick circumstellar discs in "accreting high-mass stars". To properly describe the UV line formation in dynamical magnetospheres, the ADM model needs to be further developed, at least in a large part of the outer wind.