1 RADIO FREQUENCY CONTINUUM EMISSION FROM EVOLVED STARS(9)

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

9 are given as 5 . CRL 2688 and IRAS 21282+5050 are found to be extended with respect to the 300 beam. Possible weak extended sources were also found within 3000 of IRC+10011 and OH231.8+4.2 and are listed in Table 3. Radio emission at 3.6 cm was detected from 9 of the 21 stars observed. However, compact planetary n

ebulae are only one possible source of radio frequency continuum emission from evolved stars; others are the stellar photosphere, the chromosphere, and circumstellar dust. The weak emission from CRL 2688 may be due to circumstellar dust, as discussed in Paper 1. To decide among these possibilities for the other stars, we compare the 3.6 cm ux densities with data on molecular line emission from the circumstellar material. Figure 1 shows the histogram of the distance-independent ratio R of the 3.6 cm ux density to the integrated intensity ICO of the CO(3-2) line in K km s?1. The group with the higher values of R contains IRAS 21282+5050, CRL 915, CRL 618 and IRAS 17423-1755, and the group with lower values contains VY CMa, IRC+10216, R Leo and CIT 6. The expected values of R for two models are also shown in Figure 1. The CO line ux is calculated for a star losing mass at a rate of 10?5 M yr?1 (at the high end of the observed range for AGB stars) at an out ow speed of Vo= 15 km s?1 and with CO=H2= 3 10?4, the value typical of oxygen stars (Knapp et al. 1995). The model radio continuum ux density is calculated for a black body sphere using equation (1). We consider two cases (1) photospheric emission from a star of temperature 2500 K and luminosity 104 L and (2) a star which has just begun to form a planetary nebula by ionizing the inner regions of the circumstellar shell. The central HII region is assumed to be optically thick, with a temperature of 104 K and a radius of 1014 cm. This model gives a lower limit to the expected value of R; it is calculated for a high mass loss rate and the model HII region is smaller