In addition to enhancing the total recombination rate,
high densities enhance the importance of collisional processes relative to
radiative processes in bound-bound transitions. Level populations
approach their LTE values, which may greatly exceed the recombination
values for levels with dipole allowed decays.
This is illustrated in figure 10, which shows the ratio of level populations to
LTE populations (departure coefficients) as functions
of density for H I at log()=-5 and T=10
K (panel a) and for O VIII
at log(
)=-5 and T=10
K (panel b). Departure coefficients of all bound spectroscopic levels
decrease proportional to density, approaching assymptotic values at densities
greater than 10
cm
. The superlevels exhibit slower dependence on
density, reflecting the fact that they are likely to be in LTE with the continuum
at lower density than the spectroscopic levels.