Structure and light emission of a Stroemgren system

Abstract

Stroemgren defined a model made up of an extremely hot source plunged in a constant density, huge, static cloud of low pressure hydrogen. The present studies of this model apply qualitatively with relatively small and cold sources, but without these assumptions, we must take into account that: - the source has, at all frequencies, the spectral radiance of a laser at a single frequency, so that the multi-photon absorption of few lines involves the whole continuous spectrum; - a spherical shell containing large column densities of excited atoms emits strongly super-radiant beams selected by competition of modes; - the long paths in excited atoms allow observations which require in the labs the use of ultrashort laser pulses. A necessary, qualitative update watch that: - the whole continuous spectrum of the source pumps the atoms to excited states, so that almost all energy emitted by the source is transferred to a line spectrum; - the main fraction of this line spectrum is emitted by a strong super-radiance in a spherical shell where the temperature becomes cold enough for a notable de-ionization of the atoms; by competition of modes, a dotted ring appears; - inside the ring, a less bright region emits spontaneously lines made extremely broad by parametric transfers of energy between the light beams and thermal radiation; the intensities of these lines decrease down to zero, from increased laboratory frequencies to decreased frequencies. Supernova remnant 1987A shows these properties.