SKIRT 9: redesigning an advanced dust radiative transfer code to allow kinematics, line transfer and polarization by aligned dust grains

The open source SKIRT Monte Carlo radiative transfer code has been used for more than 15 years to model the interaction between radiation and dust in various astrophysical systems. In this work, we present version 9 of the code, which has been substantially redesigned to support long-term objectives. We invite interested readers to participate in the development, testing and application of new features such as including gas media types in addition to dust, performing line transfer in addition to continuum radiation transfer, and modeling polarization by non-spherical dust grains aligned by magnetic fields. We describe the major challenges involved in preparing the code for these and other extensions, as well as their resolution, including a completely new treatment of wavelengths to support kinematics. SKIRT 9 properly runs over 400 handcrafted functional tests and successfully performs all relevant benchmarks. The source code and all documentation is publicly available for use and ready for further collaborative development.