  A complete model of helium-like line and continuum emission has been
incorporated into the plasma simulation code Cloudy. All elements between He
and Zn are treated, any number of levels can be considered, and radiative and
collisional processes are included. This includes photoionization from all
levels, line transfer including continuum pumping and destruction by background
opacities, scattering, and collisional processes. The model is calculated
self-consistently along with the ionization and thermal structure of the
surrounding nebula. The result is a complete line and continuum spectrum of the
plasma. Here we focus on the ions of the He I sequence and reconsider the
standard helium-like X-ray diagnostics. We first consider semi-analytical
predictions and compare these with previous work in the low-density,
optically-thin limit. We then perform numerical calculations of helium-like
X-ray emission (such as is observed in some regions of Seyferts) and predict
line ratios as a function of ionizing flux, hydrogen density, and column
density. In particular, we demonstrate that, in photoionized plasmas, the
$R$-ratio, a density indicator in a collisional plasma, depends on the
ionization fraction and is strongly affected by optical depth for large column
densities. We also introduce the notion that the $R$-ratio is a measure of the
incident continuum at UV wavelengths. The $G$-ratio, which is
temperature-sensitive in a collisional plasma, is also discussed, and shown to
be strongly affected by continuum pumping and optical depth as well. These
distinguish a photoionized plasma from the more commonly studied collisional
case.

Astrophysics