1 RADIO FREQUENCY CONTINUUM EMISSION FROM EVOLVED STARS(14)

The evolution of stars between the AGB and planetary nebula phases was investigated by sensitive radio continuum observations of a sample of 21 evolved stars with high mass loss rates and extended circumstellar envelopes, in a search for newly formed compa

14 atmospheres for planetary nebula nuclei calculated by Hummer and Mihalas (1970) gives a relatively low temperature for the central star of 30,000 K with a radius of about 1 R . The observations of four compact planetary nebulae surrounded by circumstellar molecular clouds show that the nebulae are small enough that they are likely to have formed recently. If we assume that planetary nebula formation occurs when mass loss stops, we can make rough age estimates for these nebulae. We assume that the inner regions of the envelope continue to coast away from the star at the wind out ow velocity as the central star begins to heat up, and estimate the evolution time for the radius of the HII region and the out ow speed of the envelope given by molecular line emission (Knapp et al. 1995). The results are given in Table 5, which lists the planetary nebula name, the distance, the radius of the HII region (assu

ming that it is spherical), the out ow speed of the envelope, the resulting timescale in years and the spectral type for the central star. These data show that ionization can begin very soon after mass loss ceases, 100 years, as is also shown by the rate of growth of the HII region in CRL 618. Several of the stars observed in the present work have fast molecular winds: these are CRL 618, IRAS 17423-1755, VY CMa, OH231.8+4.2, IRAS 09371+1212, CRL 2688 and M2-56. The radio continuum observations described herein show that only the rst two of these are compact planetary nebulae. These observations show that in at least some of these stars the formation of the fast wind precedes that of the ionized nebula.