The radio and infrared spectrum of DR 21

The radio and infrared spectrum of DR 21 is established over a wide range of frequencies (from 10² to 10⁸ MHz). Two physical processes, free-free emission from the ionized hydrogen at radio wavelengths and reradiation at infrared wavelengths of the original stellar ultraviolet radiation by dust grains have to be considered in the explanation of the derived spectrum. Physical parameters of the object deduced from its radio emission are also presented.