Application of the coupled escape probability method to spherical clouds

Abstract

Aims. We present applications of one of the novel radiative transfer methods, called the coupled escape probability method, to spherical clouds. It provides efficient tools for analyzing molecular lines, but its implementation is limited to plane-parallel systems. For more general uses, we extend its capability to spherical systems.Methods. The spherical geometry means that the derivation of mean intensity has to be conducted numerically, whereas it was done more analytically in the original work. The new method was applied to spherical clouds and cores with various conditions, and systems with moderate systematic motion are tested in addition to static systems. The case of molecular transitions with hyperfine overlap is also investigated.Results. While this method is more complicated than the original method, it maintains the underlying simplicity and practicality. The excitation conditions derived for the various conditions are compared with those of the Monte Carlo methods, and the results are found to be in good agreement with each other. The new method is more efficient in computing time and number of iterations. © 2009 ESO.