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Density dependent recombination rates

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 10$^4$K and 10$^7$ 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($\xi $)=2 with an $\varepsilon^{-1}$ 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 $\sim$ a few beginning at densities greater than 10$^6$-10$^8$.


next up previous contents
Next: Level populations vs. density Up: Sample Results: High Density Previous: Sample Results: High Density   Contents
Tim Kallman 2014-09-12