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In our series of papers presenting the Herschel imaging of evolved planetarynebulae, we present images of the dust distribution in the Helix nebula (NGC7293). Images at 70, 160, 250, 350, and 500 micron were obtained with the PACSand SPIRE instruments on board the Herschel satellite. The broadband maps showthe dust distribution over the main Helix nebula to be clumpy and predominantlypresent in the barrel wall. We determined the spectral energy distribution ofthe main nebula in a consistent way using Herschel, IRAS, and Planck fluxvalues. The emissivity index of 0.99 +/- 0.09, in combination with the carbonrich molecular chemistry of the nebula, indicates that the dust consists mainlyof amorphous carbon. The dust excess emission from the central star disk isdetected at 70 micron and the flux measurement agree with previous measurement.We present the temperature and dust column density maps. The total dust massacross the Helix nebula (without its halo) is determined to be 0.0035 solarmass at a distance of 216 pc. The temperature map shows dust temperaturesbetween 22 and 42 K, which is similar to the kinetic temperature of themolecular gas, strengthening the fact that the dust and gas co-exist in highdensity clumps. Archived images are used to compare the location of the dustemission in the far infrared (Herschel) with the ionized (GALEX, Hbeta) andmolecular hydrogen component. The different emission components are consistentwith the Helix consisting of a thick walled barrel-like structure inclined tothe line of sight. The radiation field decreases rapidly through the barrelwall.

Herschel imaging of the dust in the Helix Nebula (NGC 7293)