1 RADIO FREQUENCY CONTINUUM EMISSION FROM EVOLVED STARS(5)

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

5 loss are discussed in Appendix A.

2. SAMPLE SELECTIONEvolved AGB stars and their circumstellar envelopes show a range of properties. About 10% of the stars are carbon stars, while the remainder have O]> C]. The mass loss rates are observed to lie in the range of 10?8 to 10?4 M yr?1 with out ow speeds typically within a factor of three of 15 km s?1 (Loup et al. 1993). The infrared colors of the dusty circumstellar envelopes measured by IRAS become cooler with increasing mass loss rate (van der Veen and Habing 1988). Extended circumstellar envelopes sometimes show bipolar structure, and fast molecular winds are found for some stars. The 21 stars observed in the present work were selected to sample most of this range of properties. There is a fair amount of overlap among the stars in each of the following categories, all of which contain examples of both carbon and`oxygen' stars. a. Stars of Spectral Type Ea

rlier than M or C Stars on the AGB are cool, with spectral types of M or C. Several stars with extensive dusty circumstellar envelopes are known to be of earlier type and therefore may be starting their evolution towards the planetary nebula stage. Objects in this category are CRL 618, CRL 915, CRL 2343, CRL 2688, IRAS 21282+5050 and M2-56. b. Stars with Very Cool Circumstellar Envelopes Evolved stars whose circumstellar envelopes have low color temperatures at IRAS wavelengths are considered to be good candidates for protoplanetary nebulae