  Iron K\alpha emission from photoionized and optically thick material is
observed in a variety of astrophysical environments including X-ray binaries,
active galactic nuclei, and possibly gamma-ray bursts. This paper presents
calculations showing how the equivalent width (EW) of the Fe K line depends on
the iron abundance of the illuminated gas and its ionization state -- two
variables subject to significant cosmic scatter. Reflection spectra from a
constant density slab which is illuminated with a power-law spectrum with
photon-index \Gamma are computed using the code of Ross & Fabian. When the Fe K
EW is measured from the reflection spectra alone, we find that it can reach
values greater than 6 keV if the Fe abundance is about 10 times solar and the
illuminated gas is neutral. EWs of about 1 keV are obtained when the gas is
ionized. In contrast, when the EW is measured from the incident+reflected
spectrum, the largest EWs are ~800 keV and are found when the gas is ionized.
When \Gamma is increased, the Fe K line generally weakens, but significant
emission can persist to larger ionization parameters. The iron abundance has
its greatest impact on the EW when it is less than 5 times solar. When the
abundance is further increased, the line strengthens only marginally.
Therefore, we conclude that Fe K lines with EWs much greater than 800 eV are
unlikely to be produced by gas with a supersolar Fe abundance. These results
should be useful in interpreting Fe K emission whenever it arises from
optically thick fluorescence.
