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

Aims. In our series of papers presenting the Herschel imaging of evolved planetary nebulae, we present images of the dust distribution in the Helix nebula (NGC 7293). Methods. Images at 70, 160, 250, 350, and 500 μm were obtained with the PACS and SPIRE instruments on board the Herschel satellite. Results. The broadband maps show the dust distribution over the main Helix nebula to be clumpy and predominantly present in the barrel wall. We determined the spectral energy distribution of the main nebula in a consistent way using Herschel, IRAS, and Planck flux values. The emissivity index of β = 0.99 ± 0.09, in combination with the carbon rich molecular chemistry of the nebula, indicates that the dust consists mainly of amorphous carbon. The dust excess emission from the central star disk is detected at 70 μm and the flux measurement agrees with previous measurement. We present the temperature and dust column density maps. The total dust mass across the Helix nebula (without its halo) is determined to be 3.5 × 10^-3 M_⊙ at a distance of 216 pc. The temperature map shows dust temperatures between 22 K and 42 K, which is similar to the kinetic temperature of the molecular gas, confirming that the dust and gas co-exist in high density clumps. Archived images are used to compare the location of the dust emission in the far infrared (Herschel) with the ionized (GALEX and Hβ) and molecular (H₂) component. The different emission components are consistent with the Helix consisting of a thick walled barrel-like structure inclined to the line of sight. The radiation field decreases rapidly through the barrel wall.