Emission

Emission lines can for different reasons occur in astrophysical spectra of various objects. In general, the density in the line-forming region is lower than in stellar atmospheres. For example,

1. Stellar spectra may turn from photospheric absorption to chromospheric emission outside the range of the star’s blackbody radiation.

2. Forbidden lines from metastable atomic states appear in regions with very low densities, e.g. planetary nebulae. The plasma is ionized by stellar radiation and the metastable states populated by collisions.

3. Fluorescence lines in complex spectra, like Fe II, occur in spectra of e.g. symbiotic stars. The lines are generated in a PAR (photoexcitation by accidental resonance) process, similar to the well-known Bowen mechanism. Strong resonance lines of e.g. H I, C IV, N V, O VI pump selectively certain Fe II levels, which decay by fluorescence.

4. Stimulated emission lines appear in extended low-density clouds close to hot, eruptive stars, e.g. Eta Carinae. Some very special Fe II energy levels, so called “pseudo-metastable (PM)” states, at relatively high excitation energy have unexpectedly long lifetimes. The decay of Lya pumped levels cause an inverted population in these PM states, opening stimulated emission channels.

We work, in general, on the interpretation of anomalous emission line intensities in spectra of various objects. In particular we work on the identification of fluorescence channels in symbiotic stars (RR Tel, AG Peg, etc), blue luminous variables (Eta Car), cool star chromospheres (the Sun, g Cru ) and also detailed modeling of some of the pumping lines. We also study and model the conditions for stimulated emission in gas blobs in Eta Car, as well as the time behaviour of fluorescence lines.