Introduction to Non-LTE Radiative Transfer and Atmospheric Modeling

Introduction to Non-LTE Radiative Transfer and Atmospheric Modeling
(PDF file)

Lecture Notes

Eugene H. Avrett
Harvard-Smithsonian Center for Astrophysics
July, 2008

Putting It All Together (A Summary)

The modeling discussed here is 1-dimensional and time-independent. We have given the equations for plane-parallel geometry, and without flow velocites, but only a few modifications are needed to convert to spherical coordinates, and to include radial velocities.

A complete discussion of the spherical case, including flows, is given by Avrett, E. H. & Loeser, R. 1984, Line transfer in static and expanding spherical atmospheres, in Methods in Radiative Transfer, ed. W. Kalkofen, Cambridge Univ. Press, 341.

For a review of atmospheric modeling, including references to earlier work, see Avrett, E. H. 1996, Next generation model atmospheres, in Stellar Surface Structure, ed K. G. Strassmeier & J. L. Linsky, I.A.U. Symp. 176, (Dordrecht: Kluwer), 503.

And see the basic textbook of Mihalas, D. 1978, Stellar Atmospheres, 2nd ed. (San Francisco: Freeman).

Different types of model atmospheres can be calculated. The usual input is temperature as a function of some depth variable such as height, radius, column mass, or optical depth at some wavelength. If hydrostatic or pressure equilibrium (including flows) can be assumed (or if pressure is assumed constant, as might be assumed in an interstellar cloud), then the density distribution can be calculated. If energy balance can be imposed, either with radiative equilibrium alone, or including non-radiative heating and energy transfer, then the temperature can be calculated as well, and the model atmosphere and calculated spectrum depend only on parameters such as surface gravity, effective temperature (i.e., the overall energy flux), and the elemental abundances (that control opacities and ionization equilibrium).

(Editor's Note: Non-LTE is non-Local Thermodynamic Equilibrium. Visit Wikipedia for a very basic review of Thermodynamic Equalibrium.)