The radio continuum properties of a sample of 31 evolved red giants with high mass loss rates, plus NGC 7027, were investigated by sensitive observations at 5 GHz using the VLA. Emission was detected from four stars (CRL 618, R Aqr, and, tentatively, IRC +10216 and ο Ceti). R Aqr was found to be a double radio source, with a second continuum component about 6arcsec from the star. It is suggested that the second source is a dense clump in stellar wind. The present data are examined for evidence of planetary nebula formation. Estimates are given of the time scales for ionization of the circumstellar material using simple models of the propagation of an ionization front into the circumstellar cloud, with evolution of the central star included in the calculations. The properties of CRL 618 and NGC 7027 are discussed in the context of these models.
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Spergel et al. (1983) studied this question.