Radiative Transfer Effects

• Transfer effects are important when  > 1

  • There are three important “levels” of opacity sources
    • Line absorption/scattering ( ~ 10-16 cm2)
    • Continuum absorption ( ~ 10-18 cm2)
    • Electron scattering ( ~ 10-24 cm2)

• Most codes assume complete redistribution / escape probability methods for treating resonance line transfer

  • Although this approximation is appropriate for isolated lines with moderate optical depths ( ≤ 10), it does not adequately describe line transfer when absorption and scattering in the damping wings become non-negligible (i.e., when   100 - 1000).
  • It is also difficult to apply this method when other opacity sources (e.g. continuum absorption, line overlap) are important as well.
  • In this formalism, a correct treatment of radiative transfer is nearly hopeless when there are abundance and temperature gradients.