Three-temperature radiation hydrodynamics with PLUTO: Tests and applications to protoplanetary disks

In circumstellar disks around T Tauri stars, visible and near-infrared stellar irradiation is intercepted by dust at the disk's optical surface and reprocessed into thermal infrared; this subsequently undergoes radiative diffusion through the optically thick bulk of the disk. The gas component -- overwhelmingly dominant by mass, but contributing little to the opacity -- is heated primarily by gas-grain collisions. In hydrodynamical simulations, however, typical models for this heating process (local isothermality, $\beta$-cooling, two-temperature radiation hydrodynamics) incorporate simplifying assumptions that limit their ranges of validity. To build on these methods, we develop a ``three-temperature" numerical scheme, which self-consistently models energy exchange between gas, dust, and radiation, as a part of the PLUTO radiation-hydrodynamics code. With a range of test problems in 0D, 1D, 2D, and 3D, we demonstrate the efficacy of our method, and make the case for its applicability to a wide range of problems in disk physics, including hydrodynamic instabilities and disk-planet interaction.