3D continuum radiative transfer in complex dust configurations around stellar objects and active galactic nuclei I. Computational methods and capabilities

We present the new grid-based code STEINRAY which has been developed to solve the full 3D continuum radiative transfer problem generally arising in the analysis of star-forming regions, matter around evolved stars, starburst galaxies, or tori around active galactic nuclei. The program calculates the intensity emerging from these complicated structures using a combination of step-size controlled ray-tracing and adaptive multi-wavelength photon transport grids. Along with a 2nd order finite differencing approach, the grids are optimized to reduce the discretization error and provide global error control. The full wavelength-dependent problem is solved without any flux approximation, and for arbitrary scattering properties of the dust. The program is designed to provide spatially resolved images, visibilities, and spectra of complex dust distributions even without any symmetry for wavelengths ranging from the UV to FIR and allows for multiple internal and external sources. In this paper, the algorithm is described and the capabilities of the code are illustrated by the treatment of 1D and 3D test cases. Analyzing the properties of typical cosmic dust, we discuss the wavelength range for which the time-consuming solution on adaptive grids can be omitted. The temperature is calculated self-consistently using standard accelerated (lambda)-iteration.