The effects of density on recombination rates
are illustrated in figure 9, which shows recombination rate as a function
of density for various ions of H, He and O. The curves correspond to temperatures
logarithmically spaced between 10K and 10
K, and the dashed curve shows the
XSTAR v.1 value. Stimulated recombination can cause large enhancements in the rates at high
density, but its effects are dependent on the shape of the assumed ionizing spectrum;
therefore it has been excluded from the results until the end of
this section for illustrative purposes. In the
H and He-like ions the lowering of the continuum is apparent at moderate densities, and
the effect of 3-body recombination is apparent in H I and He I at the highest densities.
This is an illustration of the fact that, at a given density, 3 body recombination is
greater for ions with lower free charge, and is greater at lower temperature ([Bautista and Kallman 2000]).
The calculations shown in this figure were done at a fixed ionization parameter of
log(
)=2 with an
ionizing continuum. As a result the lower ionization
stages, of oxygen, O I, O II, and O III have low abundance and are not included in the full
multilevel matrix calculation and their recombination rates are treated using the total
rates shown in figure 3. Other ions show the effects of continuum lowering,
which causes a decrease in rate by a factor of up to
a few
beginning at densities greater than 10
-10
.