1 RADIO FREQUENCY CONTINUUM EMISSION FROM EVOLVED STARS(6)

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

6 (e.g. Likkel et al. 1987: Volk and Kwok 1989); the envelopes are cool because the mass loss rate is very high, and/or the envelope is very extended, and/or the inner regions of the envelope are detached from the central star. The stars in this category are R Scl, U Cam, CRL 618, CRL 915, OH231.8+4.2, IRAS 09371+1212, IRAS 17423-1755, IRAS 21282+5050, and M2-56. c. Stars with Fast Molecular Winds Molecular line observations show that some stars have fast as well as slow molecular winds. While the relationship between the evolutionary state of the star and fast winds is not understood, they are seen in some young planetary nebulae such as CRL 618 (Cernicharo et al. 1989; Gammie et al. 1989). Several stars with fast molecular winds are included in the present sample: these are CRL 618, VY CMa, OH231.8+4.2, IRAS 17423-1755, CRL 2688 and M2-56 (Knapp et al. 1995). d. Stars on the AGB and Supergiants The sample includes several`normal' evolved giants with a range of mass loss rates; these are IRC+10011, R Lep, CRL 809, R Leo, and IRC+10216. Two M supergiants, VY CMa and VX Sgr, were also observed. The observed stars are listed in Table 1, which gives the name, the position of the phase center of the observations, the most accurate available stellar position, the position uncertainty in each coordinate, the proper motions in and (where known), references for the position, and the stellar spectral type from the SIMBAD data base. Most of the stars have positions measured at several wavelengths. In two