1 RADIO FREQUENCY CONTINUUM EMISSION FROM EVOLVED STARS(12)

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

12 The 3.6 cm emission is probably due to ionized gas (Figure 1) and comparison with the 1.3 mm ux density measured by Walmsley et al. (1991) shows that the source is optically thick at 3.6 cm: the observed ux density then gives a radius of only 2:5 1014 cm at a distance of 1 kpc, showing that the formation of the planetary nebula is very recent. The 3.6 cm ux density also requires ionization corresponding to a star of type earlier than B3 (following the analysis by Churchwell and Walmsley 1973). This is considerably earlier than the B8 - A0 type of HD 44179. However, Leinart and Haas (1989) show that HD 44179 is o set from the center of the optical nebulosity. The exciting star of the Red Rectangle may therefore not be HD 44179 but a hotter, unseen companion. c. IRAS 17423-1755 (He3-1475) This is a Be star (Henize 1976) surrounded by a compact planetary nebula (Bobrowski 1993). The distance is estimated to be 2.5 kpc (Parsatharathy and Pottasch 1989). The 3.6 cm ux density gives an excitation parameter of 1.8, suggesting a star of type B3, in agreement with the very low excitation measured by Bobrowski (1993). The CO(2{1) and the 1667 MHz OH line pro les have two components, a wea

k, broad ( 60 km s?1) component and a narrow emission feature of width 12 km s?1 blueshifted with respect to the central velocity of the broad component (te Lintel Hekkert 1991; Knapp et al. 1995). These line shapes resemble those seen in OH231.8+4.2 (Morris, Bowers and Turner 1982) and the envelope of He3-1475 may have the same sort of attened mass distribution. It is at a slightly later stage of evolution than is OH231.8+4.2, as shown by the detection of the small central