Effect of Source Morphology on Thermal Radio Continuum Spectra

Abstract

The thermal radio continuum spectra derived from a selection of nebular models are compared. One class of models arises from three-dimensional distributions of emissivity, while the remainder are specified in a more abstract way. It is shown that the most important distinguishing feature between the models is the proportion of the nebular solid angle occupied by very faint emission. This finding is developed into a measure of nebular clumping. Practical aspects of the modelling are examined by applying a selection of the model spectra to the flux densities observed for the planetary nebula IC418.