Observations of Planetary Nebulae at Three Microwave Frequencies

The 125 ft × 83 ft fully steerable Mark II radio telescope at Jodrell Bank has been used in a recent attempt to detect the radio emission at three frequencies, 4995 Mc/s, 2695 Mc/s and 1420 Mc/s, from twenty-four of the brighter planetary nebulae. An additional twenty-six planetary nebulae have been observed at two frequencies, 4995 Mc/s and 2695 Mc/s, and a further forty-four planetary nebulae have been observed at 2695 Mc/s only. Two Wolf–Rayet stars, HD 193793 and HD 16523 have also been observed at 2695 Mc/s. The observations suggest that the majority of the nebulae are thermal radio sources. The radio flux densities for those nebulae which have been shown to have become optically thin in this frequency range agree well with values predicted from recombination theory.