H. Socas-Navarro[1], G. Israelian[1], M.Friedjung[2]
[1] Instituto de Astrofisica de Canarias, Spain
[2] Institut d'Astrophysique, Paris, France
The semi-empirical self absorption curve (SAC) method enables physical information to be obtained about circumstellar regions producing emission lines of ions with complex spectra, without previously assumimg a detailed model. Theoretical calculations of their shape have started, using the MULTI code.