1 RADIO FREQUENCY CONTINUUM EMISSION FROM EVOLVED STARS(13)

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

13 compact HII region. d. IRAS 21282+5050 This young planetary nebula has a WC11 central star surrounded by a cool circumstellar shell showing strong emission in the PAH 3 m features and the CO rotational lines (de Muizon et al. 1986; Cohen and Jones 1987; Likkel et al. 1988). High-resolution molecular line, infrared and radio-frequency observations reveal a toroidal structure (Shibata et al. 1989; Kwok et al. 1993; Meixner et al. 1993; Likkel et al. 1994). The distance is estimated to be 2 kpc (cf. Likkel et al. 1988 and Shibata et al. 1989). The 3.6 cm measurements from the present paper show that the nebulosity is slightly extended on the 300 scale (Table 4), with a total ux density at 3.6 cm of 5.3 0.3 mJy. The deconvolved source size is 3:800 2:700 at position angle 153o, in excellent agreement with the higher resolution results at 6 cm and 2 cm by Likkel et al. (1994). Likkel et al. (1994) nd ux densities of 6.9 and 6.8 mJy at 6 cm and 2 cm, while Meixner et al. (1993) report a 6 cm ux density of 8 mJy. These results show that the spectrum is nearly at in the 2 - 6 cm range, consistent with optically thin bremsstrahlung emission. The optical magnitudes measured by Cohen and Jones (1987), the 2 m - 12 m ux densities given by Meixner et al. (1993) and the IRAS ux densities give a bolometric ux corresponding to a luminosity of 3300 L . If we assume that the HII region is optically thin at 3.6 cm and has a temperature of 104 K, the corresponding number of Lyman continuum photons is 2 1045 s?1, using the analysis of Churchwell and Walmsley (1973). Comparison with the model