EXTENDED COLD DUST EMISSION AT 1.3 mm FROM EVOLVED STARS
C.Sánchez Contreras 
, J.Alcolea 
, V.Bujarrabal and R.Neri 
Observatorio Astronómico Nacional (IGN), Apartado 1143, E-28800
Alcalá de Henares, Spain
Departamento de Astrofısica, Facultad C. Fısicas,
Universidad Complutense, E-28040 Madrid, Spain
IRAM, 300 rue de la Piscine, F-38406 St Martin d'Hères, France
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
objects, two of which are new detections at this wavelength: M
1-92 and, tentatively, 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 direction of their
symmetry axis up to distances from the central star
10 
cm. We argue that most of this radio emissio is
arising from cold dust present in the bipolar lobes. Extended
emission has not been found in the direction perpendicular to the
nebular axis (except probably for NGC 7027), therefore the
equatorial torus/disk of dust 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 of the circumstellar disk of dust
observed at shorter wavelengths in this source. In cases were
extended components have been found, we estimate, 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
mass is 2 10 
, while NGC 7027 seems to
have a larger dust content, 10 
. For M
2-9 and OH 231.8+4.2 the uncertainty 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 the 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 total dust
mass in the envelope ( 
50%) and is
probably composed by relatively big grains (radii larger than 1 
). We caution that the analysis of radio continuum emission can be
very uncertain when not enough data on extent and spectral flux
distribution exist.
To appear in A&A. Preprints can be obtained from sanchez@oan.es