1 RADIO FREQUENCY CONTINUUM EMISSION FROM EVOLVED STARS(18)

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

18 when detected, is in excess by factors of several over the expected photospheric emission. Warm circumstellar dust is a plausible source of some of the excess emission from some of the stars, but cannot

explain the excess in all cases. The emission may be due to extended partially ionized chromospheres such as that seen around Ori (Newell and Hjellming 1982). The strength of the 3.6 cm excess suggests that the chromospheric radii are 2? 4 R?. These regions are extended enough and the outer regions are cool enough for dust to form and to initiate radiation pressure driven mass loss (cf. Bowen 1988). Finally, note that the data suggest that the strength of the`chromosphere' may di er from star to star. For example, 3.6 cm emission is detected from one of the two supergiants in the sample, VY CMa, but not from the other, VX Sgr.