3D spectral synthesis and rotational line broadening

Context. Spectral synthesis calculations based on three-dimensional stellar atmosphere models are limited by the affordable angular resolution of the radiation field. This hampers an accurate treatment of rotational line broadening. Aims. We aim to find a treatment of rotational broadening of a spherical star when the radiation field is only available at a modest number of limb-angles. Methods. We apply a combination of analytical considerations of the line-broadening process and numerical tests. Results. We obtain a method which is closely related to classical flux convolution and which performs noticeably better than a previously suggested procedure. It can be applied to rigid as well as differential rotation.