Radio Emission

Extract

I understand that my task is to summarize the present state of radio observation of planetary nebulae, and to give us radio observers our marching orders for the next few years. Where, then, do we stand at the end of this Symposium? Well over 100 planetary nebulae have been looked at by one or more groups at one or more frequencies, and nearly that number have been detected. Most nebulae which have been observed at several frequencies show optically thin spectra, about eight to a wavelength of 40 cm, and three to 70 cm. The fluxes of these nebulae agree well with the values predicted from the observed Hβ radiation, after correction for interstellar extinction. Some are clearly optically thick at long wavelengths, but none have been shown to remain so at the shortest wavelengths. NGC 6853 becomes optically thin near 75 cm, and NGC 6857 may become thin near 5 cm. Eight further nebulae become thin at intermediate wavelengths.