Abstract
Radiative transfer out of local thermodynamic equilibrium (NLTE) has been increasingly adressed, mostly numerically, for about six decades now. However, the standard NLTE problem most often refers to the only deviation of the distribution of photons from their equilibrium, that is to say a Planckian distribution. Hereafter we revisit after Oxenius (1986, Kinetic theory of particles and Photons – Theoretical Foundations of non–LTE Plasma Spectroscopy, Springer) the so-called full NLTE problem, which considers coupling and therefore solving self–consistently for deviations from equilibrium distributions of photons as well as for massive particles constituting the atmospheric plasma.
Highlights
The “standard” nonlocal thermodynamic equilibrium (NLTE) radiation transfer problem considers that the distribution of photons in a given “atmosphere”, that is, more generally, whatever sample of celestial body material where light–matter interactions are taking place, may depart from the equilibrium distribution described by the Planck law (Planck 1900)
The vast majority of these problems still rely on what became a somewhat implicit assumption that the distribution of atoms both responsible for, and experiencing light–matter interactions, remain a priori known and characterized by the Maxwell– Boltzmann equilibrium distribution. Another issue related to the standard NLTE radiation transfer problem concerns the redistribution of photons after scattering onto these very massive particles, at least in frequencies if we restrict ourselves to isotropic scattering
It is important to realize that, in the vast majority of the cases, and even though the problem of partial frequency redistribution has been addressed quite early in the 1960s (e.g., Avrett & Hummer 1965; Auer 1968 for pioneering works), the standard NLTE problem remains limited to the frame of complete redistribution (CRD) in most cases
Summary
The “standard” nonlocal thermodynamic equilibrium (NLTE) radiation transfer problem considers that the distribution of photons in a given “atmosphere”, that is, more generally, whatever sample of celestial body material where light–matter interactions are taking place, may depart from the equilibrium distribution described by the Planck law (Planck 1900). The vast majority of these problems still rely on what became a somewhat implicit assumption that the distribution of atoms both responsible for, and experiencing light–matter interactions, remain a priori known and characterized by the Maxwell– Boltzmann equilibrium distribution Another issue related to the standard NLTE radiation transfer problem concerns the redistribution of photons after scattering onto these very massive particles, at least in frequencies if we restrict ourselves to isotropic scattering. We believe that the last decades of evolution of numerical methods (see e.g., Lambert et al 2016 and references therein, as well as Noebauer & Sim 2019 for a recent review on Monte– Carlo radiative transfer) for radiation transfer should allow one to reconsider this issue, and evaluate up to which level of difficulty one could reasonably be able to achieve nowadays This new evaluation should give us new hints about which kind of astrophysical problems should be revisited, using full NLTE. We discuss future work in that somewhat forgotten physical frame for the unpolarized radiation transfer problem
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