EXTENDED COLD DUST EMISSION AT 1.3 MM FROM EVOLVED STARS

We have performed maps of the 1.3 mm continuum emission from a sample of 16 evolved stars. We have detected emission from a total of 11 obje cts, two of which are new detections at this wavelength: M 1-92 and, t entatively, M 1-91. 4 objects in the sample, the bipolar nebulae M 2-9 , OH 231.8+4.2, NGC 7027 and CRL 2688, show extended emission in the d irection of their symmetry axis up to distances from the central star similar to 10(17) cm. We argue that most of this radio emission is ari sing from cold dust present in the bipolar lobes. Extended emission ha s not been found in the direction perpendicular to the nebular axis (e xcept probably for NGC 7027), therefore the equatorial torus/disk of d ust probably present in this type of objects is not extended enough to be detected by our observations. The 1.3 mm emission map of NGC 7027 shows an extended structure elongated approximately in the equatorial plane. This component extends up to a distance from the nebula center of about 15 '', and we think it could correspond to the outer region o f the circumstellar disk of dust observed at shorter wavelengths in th is source. In cases were extended components have been found, we estim ate, assuming simplifying hypotheses, the temperature and mass of the dust. In the sources M 2-9, OH 231.8+4.2 and CRL 2688, the cold dust m ass is similar to 2 10(-3) M., while NGC 7027 seems to have a larger d ust content, similar to 10(-2) M.. For M 2-9 and OH 231.8+4.2 the unce rtainty factors of our estimations have values between 2 and 3.5. For CRL 2688 the errors can be as high as a factor 10, and for NGC 7027 th e dust mass given could just be a lower limit. In all the well studied cases, the cold dust component represents a large fraction of the tot al dust mass in the envelope (greater than or similar to 50%) and is p robably composed by relatively big grains (radii larger than I mu m) W e caution that the analysis of radio continuum emission can be very un certain when not enough data on extent and spectral flux distribution exist.