Physics Underlying the ATOMDB

The ATOMDB line and continuum results apply to an optically-thin thermal plasma with astronomical abundances (such as those of Anders & Grevesse, 1989, Geochimica et Cosmochimica Acta, 53, 197).

Calculating the X-ray/UV spectrum of a hot, collisionally-dominated optically-thin plasma requires knowledge of the atomic transition rates and energies of the ions involved, as well as a code to calculate the interplay between the different rates. Here we describe an outline of the relevant processes.

In most astrophysical plasmas, the following processes are important:

  1. Continuum emission processes
    1. Free-bound emission (radiative recombination)
    2. 2-photon emission
  2. Line emission process
    1. One-electron radiative transitions
    2. Dielectronic recombination satellite lines
    3. Inner-shell ionization

This list omits a number of processes:

The appropriate model depends not only on the temperature of the plasma, but also its density. At sufficiently high densities, collisions completely determine the level population. As the density drops, a collisional-radiative model must be used and finally the purely radiative coronal/nebular approximation can be used. The breakpoints between these models are discussed below:

Local Thermodynamic Equilibrium (LTE)

LTE rarely applies to an optically-thin astrophysical hot collisional plasma, but when it does, the level populations determined only by collisional processes. It requires:


Collisional-Radiative Model (CR Model)

This is the most general case, where collisional excitation and de-excitation compete with radiative transitions.

Coronal and Nebular Models

The low-density approximation, where collisional de-excitation can be ignored, so every excited level will decay via a radiative transition.

Other Common Simplifying Assumptions

Most of these assumptions break down somewhere in astrophysics. In fact, most of them break down in our own Sun! The ability to calculate more general cases is limited by availability and accuracy of atomic data (and/or by computational limits). The ability to parameterize an astrophysical plasma in full detail is a different, often more difficult, problem.
  • Ionization/recombination may be solved separately from excitation/de-excitation
  • Either collisional processes dominate or radiative processes dominate
  • Optical depth effects may be treated in a simple way:
    escape probability formalism
  • Low density
    • Ion population mostly in the ground state
      • Coronal approximation (collisionally ionized plasmas)
      • Nebular approximation (photoionized plasmas)
    • Rate coefficients are not density-sensitive
  • Time-independent
  • Maxwellian electrons
  • Electric and magnetic field effects are ignored
  • No diffusion