Dust in the neutral globules of the Helix nebula, NGC 7293*

Images of the ionized knots and spokes within the helical structure of the Helix nebula have been obtained with subarcsecond resolution in the light of the Hα + [N II], [O III]5007 Å and [O II]3727 Å emission lines. The [O III] image reveals the knots and their tails in absorption. The globules and tails must be dusty and are seen against the diffuse, central [O III]-emitting volume of the Helix nebula. The curved heads of these neutral knots are radiatively ionized by the light of the central star to produce spatially resolved, bow-shaped Hα + [N II] and [O II]-emitting structures displaced (measured for the most prominent globule) by 1.65 arcsec |(3.1×10¹⁵\,cm)| towards the central star from the peak of the absorption. Assuming a standard interstellar dust/gas ratio and reddening laws, we find that the total mass in one globule is |≥ 1.0×10⁻⁵M_|⁠, and the mean gas density |NH\,is\,6.2×10⁻⁵\,cm⁻³|⁠. Considerations on the nature and survival of the globules and their tails are also discussed.