1 RADIO FREQUENCY CONTINUUM EMISSION FROM EVOLVED STARS(3)

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

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1. INTRODUCTIONStellar evolution on the red giant branches, particularly on the asymptotic giant branch (AGB) is dominated by mass loss. The extended circumstellar shells produced by mass loss can be investigated using line emission from circumstellar molecules and continuum emission from circumstellar dust: mass loss rates of 10?8 to 10?4 M yr?1 and lifetimes of 2 105 years are found (Young, Phillips and Knapp 1993). At the end of the mass loss phase, it is likely that the central hot remnant ionizes the circumstellar material and a planetary nebula is formed (e.g. Schonberner and Blocker 1993). This paper describes a radio wavelength search for young planetary nebulae in a sample of 21 well-studied evolved stars selected to cover most of the range of properties for these objects, with emphasis on stars which may be evolved past the AGB phase. The detection of new-formed planetary nebulae in the centers of extended circumstellar envelopes allows the investigation of the conditions which attend the termination of stellar mass loss and the formation of a planetary nebula. Further, stellar models show that the transition between the AGB and the planetary nebula phases can be very short, about 100 years, for the highest-mass remnant stars (Schonberner and Blocker 1993). Hence, in these cases, the onset of planetary nebula formation and the initial evolution may be rapid enough to observe. The obs

ervations were made with the Very Large Array (VLA) at its most sensitive wavelength, 3.6 cm (8.4 GHz). Radio continuum observations are una ected by dust extinction, which can be very high, up to several hundred magnitudes, in these dusty circumstellar envelopes. Further, the great sensitivity of the VLA can