Millimeter observations of planetary nebulae

Aims. We present 43 GHz (7 mm) observations of a sample of radio-bright Planetary nebulae aimed to obtain, together with far-IR measurements (IRAS), reliable estimates of the fluxes emitted in the millimetre and sub-millimetre band, and, therefore, to test their detectability by the forthcoming ESA PLANCK mission. This spectral range, even though very important to constrain the physics of circumstellar environments, is still far from being completely exploited. Methods. The new millimetre 43 GHz observations were obtained by using the 32 m INAF-IRA Noto Radiotelescope. To estimate the millimetre and sub-millimetre fluxes, we extrapolated and summed the ionized gas (free-free radio emission) and dust (thermal emission) contributions in this frequency range. By comparison of the derived flux densities to the predicted sensitivity we investigate the possible detection of such source for all the channels of PLANCK Results. We conclude that almost 80% of our sample will be detected by PLANCK, with a higher detection rate in the higher frequency channels, where there is a good combination of brighter intrinsic flux from the sources and reduced extended Galactic foregrounds contamination despite poorer instrumental sensitivity. From the new 43 GHz, combined with single-dish 5 GHz observations from the literature, we derive radio spectral indexes, which are consistent with optically thin free-free nebula. This result indicates that the high frequency radio spectrum of our sample sources is dominated by thermal free-free, and other emissions, if present, are negligible