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We present observations of planetary nebulae with the LOw Frequency ARray(LOFAR) between 120 and 168 MHz. The images show thermal free-free emissionfrom the nebular shells. We have determined the electron temperatures forspatially resolved, optically thick nebulae. These temperatures are 20 to 60%lower than those estimated from collisionally excited optical emission lines.This strongly supports the existence of a cold plasma component, whichco-exists with hot plasma in planetary nebulae. This cold plasma does notcontribute to the collisionally excited lines, but does contribute torecombination lines and radio flux. Neither of the plasma components arespatially resolved in our images, although we infer that the cold plasmaextends to the outer radii of planetary nebulae. However, more cold plasmaappears to exist at smaller radii. The presence of cold plasma should be takeninto account in modeling of radio emission of planetary nebulae. Modelling ofradio emission usually uses electron temperatures calculated from collisionallyexcited optical and/or infrared lines. This may lead to an underestimate of theionized mass and an overestimate of the extinction correction from planetarynebulae when derived from the radio flux alone. The correction improves theconsistency of extinction derived from the radio fluxes when compared toestimates from the Balmer decrement flux ratios.

Evidence for cold plasma in planetary nebulae from radio observations with the LOw Frequency ARray (LOFAR)