  Aims. We present a new radiation hydrodynamics code, called "ARK-RT" which
uses a two-moment model with the M1 closure relation for radiative transfer.
This code aims at being ready for high-performance computing, on exascale
architectures. Methods. The two-moment model is solved using a finite volume
scheme. The scheme is asymptotic preserving to capture accurately both
optically thick and thin regimes. We also propose a well-balanced
discretization of the radiative flux source term able to capture constant flux
steady states with discontinuities in opacity. We use the library Trilinos for
linear algebra and the package Kokkos allows us to reach high-performance
computing and portability across different architectures, such as multi-core,
many-core, and GP-GPU. Results. ARK-RT is able to reproduce standard tests in
both free-streaming and diffusive limits, including purely radiative tests and
radiation hydrodynamics ones. Using a time-implicit solver is profitable as
soon as the time step given by the hydrodynamics is 50-100 times larger than
the explicit time step for radiative transfer, depending on the preconditioner
and the architecture. Albeit more work is needed to ensure stability in all
circumstances. Using ARK-RT, we study the propagation of an ionization front in
convective dense cores. We show that the ionization front is strongly stable
against perturbations even with destabilizing convective motions. As a result,
the presence of instabilities should be interpreted with caution. Overall,
ARK-RT is well-suited to study many astrophysical problems involving convection
and radiative transfer such as the dynamics of H ii regions in massive
pre-stellar dense cores and future applications could include planetary
atmospheres.
