Non-LTE radiation hydrodynamics in PLUTO

Modeling the dynamics of most astrophysical structures requires an adequate description of the radiation-matter interaction. Several numerical (magneto)hydrodynamics codes were upgraded with a radiation module to fulfill this request. However, those among them that use either the flux-limited diffusion (FLD) or the M1 radiation moment approaches are restricted to the local thermodynamic equilibrium (LTE). This assumption may be not valid in some astrophysical cases. We present an upgraded version of the LTE radiation-hydrodynamics module implemented in the PLUTO code, originally developed by Kolb et al. (2013), which we have extended to handle non-LTE regimes. Starting from the general frequency-integrated comoving-frame equations of radiation hydrodynamics (RHD), we have justified all the assumptions made to obtain the non-LTE equations actually implemented in the module, under the FLD approximation. An operator-split method is employed, with two substeps: the hydrodynamic part is solved with an explicit method by the solvers already available in PLUTO, the non-LTE radiation diffusion and energy exchange part is solved with an implicit method. The module is implemented in the PLUTO environment. It uses databases of radiative quantities that can be provided independently by the user: the radiative power loss, the Planck and Rosseland mean opacities. Our implementation has been validated through different tests, in particular radiative shock tests. The agreement with the semi-analytical solutions (when available) is good, with a maximum error of 7%. Moreover, we have proved that non-LTE approach is of paramount importance to properly model accretion shock structures. Our radiation FLD module represents a step toward the general non-LTE RHD modeling. The module is available, under request, for the community.