The bound-bound and free-free radiative losses for the nonthermal distributions in solar and stellar coronae

Abstract

Context. The radiative-loss function is an important ingredient in the physics of the solar corona, transition region, and flares.Aims. We investigate the radiative losses due to the bound-bound transitions and bremsstrahlung for nonthermal κ - and n -distributions.Methods. The bound-bound radiative losses are computed by integrating synthetic spectra. An analytical expression is derived for nonthermal bremsstrahlung. The bremsstrahlung is computed numerically using accurate values of the free-free Gaunt factor.Results. We find that the changes in radiative-loss functions due to nonthermal distributions are several times greater than the errors due to the missing contribution of the free-bound continuum or errors in atomic data. For κ -distributions, the radiative-loss functions are in general weaker than for Maxwellian distribution, with a few exceptions caused by the behavior of Fe. The peaks of the radiative-loss functions are in general flatter. The situation is opposite for n -distributions, for which the radiative-loss functions have higher and narrower peaks. Local minima and maxima of the radiative-loss functions may also be shifted. The contribution from bremsstrahlung only changes by a few percent except in the extreme nonthermal case of κ = 2. Stability analysis reveals that the X-ray loops are stable against the radiatively-driven thermal instability.