  Radiative transfer calculations have predicted intensity enhancements for
optically thick emission lines, as opposed to the normal intensity reductions,
for astrophysical plasmas under certain conditions. In particular, the results
are predicted to be dependent both on the geometry of the emitting plasma and
the orientation of the observer. Hence in principle the detection of intensity
enhancement may provide a way of determining the geometry of an unresolved
astronomical source. To investigate such enhancements we have analyzed a sample
of active late-type stars observed in the far ultraviolet spectral region.
Emission lines of O VI in the FUSE satellite spectra of epsilon Eri, II Peg and
Prox Cen were searched for intensity enhancements due to opacity. We have found
strong evidence for line intensity enhancements due to opacity during active or
flare-like activity for all three stars. The O VI 1032/1038 line intensity
ratios, predicted to have a value of 2.0 in the optically thin case, are found
to be up to ~30% larger during several orbital phases. Our measurements,
combined with radiative transfer models, allow us to constrain both the
geometry of the O VI emitting regions in our stellar sources and the
orientation of the observer. A spherical emitting plasma can be ruled out, as
this would lead to no intensity enhancement. In addition, the theory tells us
that the line-of-sight to the plasma must be close to perpendicular to its
surface, as observations at small angles to the surface lead to either no
intensity enhancement or the usual line intensity decrease over the optically
thin value. For the future, we outline a laboratory experiment, that could be
undertaken with current facilities, which would provide an unequivocal test of
predictions of line intensity enhancement due to opacity, in particular the
dependence on plasma geometry.
