A Generalization of the Sobolev Method for Radiation Transport with Local and Nonlocal Line Overlap
Abstract
We have generalized the Sobolev or large velocity gradient method for solving radiative transfer problems in moving media to treat spectral lines that overlap locally or nonlocally. When the rest frequencies of two spectral lines differ by less than their thermal Doppler width, then the lines overlap locally and photons from one line can influence the populations of the levels of the other line. Similarly, in a medium with velocity gradients and velocity differences enough to Doppler shift one line onto the other, the lines overlap nonlocally and one line can affect the level populations of the other. We have derived expressions for both the intensity and the mean intensity when there is local or nonlocal line overlap. These expressions are general and can be used for any temperature as long as the large velocity gradient approximation is satisfied. The mean intensity is essential for the radiative rates that enter into the statistical equilibrium equations, and the accurate inclusion of the effects of line overlap is crucial.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 1996
- DOI:
- 10.1086/177604
- Bibcode:
- 1996ApJ...467..292P
- Keywords:
-
- MASERS;
- RADIATIVE TRANSFER;
- ISM: MOLECULES