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
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