Planck intermediate results. XVIII The millimetre and sub-millimetre
  emission from planetary nebulae
  Late stages of stellar evolution are characterized by copious mass-loss
events whose signature is the formation of circumstellar envelopes (CSE).
Planck multi-frequency measurements have provided relevant information on a
sample of Galactic planetary nebulae (PNe) in the important and relatively
unexplored observational band between 30 and 857GHz. Planck enables the
assembly of comprehensive PNe spectral energy distributions (SEDs) from radio
{\bf to} far-infrared frequencies. Modelling of the derived SEDs provides us
with information on physical properties of CSEs and the mass content of both
main components: ionised gas, traced by the free-free emission at cm--mm waves;
and thermal dust, traced by the millimetre and far-IR emission. In particular,
the amount of ionised gas and dust has been derived here. Such quantities have
also been estimated for the very young PN CRL618, where the strong variability
observed in its radio and millimetre emission has previously prevented the
construction of its SED. A morphological study of the Helix Nebula has also
been performed. Planck maps reveal, for the first time, the spatial
distribution of the dust inside the envelope, allowing us to identify different
components, the most interesting of which is a very extended component (up to
1pc) that may be related to a region where the slow expanding envelope is
interacting with the surrounding interstellar medium.
