The optical spectrum of MCG-6-30-15 clearly displays the very
high-excitation forbidden lines of [FeX] 
, [FeXI] 
and [FeXIV] 
. These are the so-called iron coronal
lines. Previous studies of the coronal lines in Seyfert galaxies have
found them to have line widths intermediate between that of the
lower-excitation forbidden narrow lines (such as [OIII]) and the permitted
broad lines (Grandi 1978). These studies have also found the coronal lines
to be slightly blueshifted with respect to the narrow forbidden lines. As
shown in Table 2, the coronal lines in MCG-6-30-15 follow exactly these
trends. Note that the true width and flux of the [FeXIV]
line may be less than that presented in Table 2 due to blending with
[CaV] 
. Oliva et al. (1994) have examined various models for
the coronal line emission.
Such observations have led to the discussion of the coronal line region (CLR) which is distinct from the BLR or NLR. Here we address the possibility that the CLR in MCG-6-30-15 can be identified with the outer warm absorber.
The collision strengths for these transitions are poorly known (Osterbrock
& Fulbright 1996; Oliva 1996). Any calculations of line strengths that we
perform will be tainted by this basic uncertainty in the atomic physics.
Despite these uncertainties, we have used CLOUDY to examine the
coronal line emission from the inner and outer warm absorber.
Table 3 reports the predicted coronal line fluxes as a fraction of the
observed line flux. These predicted fluxes assume an optically-thin,
unobscured spherical shell with total covering fraction 
.
It is clear from Table 3 that the inner warm absorber cannot contribute
much to the observed coronal line emission. This is primarily due to the
fact that iron is too highly ionized, although collisional de-excitation is
also relevant in suppressing the coronal line emission. On the other hand, the
outer warm absorber can produce significant coronal emission. If we
hypothesize that all of the [FeX] and [FeXIV]
emission originates from the outer warm absorber, we deduce that 
. However, half of the optical emission from this region
may well be blocked by very optically-thick material (e.g. the molecular
torus). This is suggested by the fact that, in some other Seyfert nuclei,
even infrared coronal lines are observed to be blueshifted with respect to
the low-ionization narrow lines implying that any redshifted coronal
emission must be heavily extinguished. If this is the case, the true
obscuration-corrected coronal line flux maybe twice that
observed leading to a revised covering fraction of 
.
This compares well with the estimate of the covering fraction of the outer
warm absorber, 
, based on the analysis of ASCA
data for a sample of Seyfert galaxies (Reynolds 1997).
According to our CLOUDY calculations, the observed [FeXI] emission cannot be explained as originating from the same material as
the other coronal lines. Within our hypothesis, three possibilities
present themselves. First, the observed [FeXI] may have its
origins elsewhere. Any separate coronal line emitting component would then
be heavily constrained by the fact that it could not over-produce
[FeX] and [FeXIV] . Secondly, the uncertainties
in the atomic physics may lead CLOUDY to grossly underestimate the
[FeXI] flux and this line may, in fact, originate within the
same material as the other coronal lines. Thirdly, the uncertainties in
the atomic physics may have led CLOUDY to grossly overestimate the
[FeX] and [FeXIV] emission from the outer warm
absorber. In this case, either the warm absorber covering fraction is
large ( 
) or the coronal lines are emitted from a
completely distinct (and as yet unidentified) region. Further progress
in this area clearly requires better atomic parameters for these
transitions, such as those which will be provided by the IRON project
(Hummer et al. 1993).