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Atomic Rates and Cross Sections

We begin by displaying some of the atomic rates which are notable due to their departure from previous work, or to their effects on the model results. Figure 1 shows the ground state photoionization cross sections we adopt. Each panel contains the cross sections for a given element, with various curves for the respective ions. In most cases the various subshells of a given ion are also plotted as separate non-overlapping curves. Resonance structure near threshold of outer shells is apparent, particularly in ions with Z$\geq$10. The photoionization cross sections from many excited levels also show resonance structure. This is illustrated in Figure 2, which shows a few of the excited level cross sections for O VII. Notable are the resonance features near 650 eV, corresponding to the 1-2 transitions in the O VIII ion. Although cross sections with comparable resolution are available for many ions from the opacity project, we adopt Gaussian average fits to these for the great majority of excited levels. For O VII we include all available cross sections at high resolution for ground and excited levels with principle quantum number $n \leq$4 in order to illustrate the potential importance of the resonance structures in observed spectra.

Ground state collisional ionization rate coefficients are shown in Figure 3. Each panel contains the rates for a given element as a function of temperature.

Figure 4 shows the radiative recombination rates we adopt. We emphasize that these are calculated by performing a Milne integral (equation 3) over the photoionization cross section for each of the bound levels of the recombined ion, and then summing over those rates. This is in contrast to the more typical nebular treatment in which such a sum is fit to an analytic formula as was done by, e.g. [Aldrovandi and Pequignot 1973], and has the advantage that it causes all rates to go to detailed balance ratios in the proper limit. Each panel in Figure 4 contains the rates for a given element. Also shown, as the dashed curves, are the rates adopted in XSTAR v.1, i.e. those of [Gould and Thakur 1970] (hydrogenic ions) [Arnaud and Raymond 1992] (for iron), and [Aldrovandi and Pequignot 1973] (all others). Differences are prominent for elements such as C, O, and Fe, and primarily reflect differences between the previous dielectronic recombination rates and those adopted here (e.g. [Nahar 2000] and references therein).


next up previous contents
Next: Ionization Balance Up: Sample Results: Low Density Previous: Sample Results: Low Density   Contents
Tim Kallman 2014-09-12