Nonequilibrium corrections in the pressure tensor due to an energy flux

The physical interpretation of the nonequilibrium corrections in the pressure tensor for radiation submitted to an energy flux obtained in some previous works is revisited. Such pressure tensor is shown to describe a moving equilibrium system but not a real nonequilibrium situation.Comment: 4 pages, REVTeX, Brief Report to appear in PRE Dec 9

Four-dimensional beam tomography

A computer code has been developed to reconstruct the 4-D transverse phase-space distribution of an accelerator beam from a set of linear profiles measured at different angles at three or more stations along the beam line. The code was applied to wire-scan data obtained on the low-intensity H/sup -/ beam of the LAMPF accelerator. A 4-D reconstruction was obtained from 10 wire-scan profiles; 2-D projections of the reconstruction agree fairly well with slit-and-collector measurements of the horizontal and vertical emittance distributions

Radiation hydrodynamics with Adaptive Mesh Refinement and application to prestellar core collapse. I Methods

Radiative transfer has a strong impact on the collapse and the fragmentation of prestellar dense cores. We present the radiation-hydrodynamics solver we designed for the RAMSES code. The method is designed for astrophysical purposes, and in particular for protostellar collapse. We present the solver, using the co-moving frame to evaluate the radiative quantities. We use the popular flux limited diffusion approximation, under the grey approximation (one group of photon). The solver is based on the second-order Godunov scheme of RAMSES for its hyperbolic part, and on an implicit scheme for the radiation diffusion and the coupling between radiation and matter. We report in details our methodology to integrate the RHD solver into RAMSES. We test successfully the method against several conventional tests. For validation in 3D, we perform calculations of the collapse of an isolated 1 M_sun prestellar dense core, without rotation. We compare successfully the results with previous studies using different models for radiation and hydrodynamics. We have developed a full radiation hydrodynamics solver in the RAMSES code, that handles adaptive mesh refinement grids. The method is a combination of an explicit scheme and an implicit scheme, accurate to the second-order in space. Our method is well suited for star formation purposes. Results of multidimensional dense core collapse calculations with rotation are presented in a companion paper.Comment: 16 pages, 9 figures, A&A accepte

Protostellar collapse: radiative and magnetic feedbacks on small-scale fragmentation

Context. Both radiative transfer and magnetic field are understood to have strong impacts on the collapse and the fragmentation of prestellar dense cores, but no consistent calculation exists on these scales. Aims: We perform the first radiation-magneto-hydrodynamics numerical calculations on a prestellar core scale. Methods: We present original AMR calculations including that of a magnetic field (in the ideal MHD limit) and radiative transfer, within the flux-limited diffusion approximation, of the collapse of a 1 M_ȯ dense core. We compare the results with calculations performed with a barotropic EOS. Results: We show that radiative transfer has an important impact on the collapse and the fragmentation, by means of the cooling or heating of the gas, and its importance depends on the magnetic field. A stronger field yields a more significant magnetic braking, increasing the accretion rate and thus the effect of the radiative feedback. Even for a strongly magnetized core, where the dynamics of the collapse is dominated by the magnetic field, radiative transfer is crucial to determine the temperature and optical depth distributions, two potentially accessible observational diagnostics. A barotropic EOS cannot account for realistic fragmentation. The diffusivity of the numerical scheme, however, is found to strongly affect the output of the collapse, leading eventually to spurious fragmentation. Conclusions: Both radiative transfer and magnetic field must be included in numerical calculations of star formation to obtain realistic collapse configurations and observable signatures. Nevertheless, the numerical resolution and the robustness of the solver are of prime importance to obtain reliable results. When using an accurate solver, the fragmentation is found to always remain inhibited by the magnetic field, at least in the ideal MHD limit, even when radiative transfer is included