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

J. Meaburn, J. R. Walsh, R. E. S. Clegg, N. A. Walton, D. Taylor, D. S. Berry

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 $(3.1\times{10}^{15}\,\text{cm})$ 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 $\geq 1.0\times{10}^{-5}{M}_{\odot}$, and the mean gas density ${N}_\text{H}\,\text {is}\,6.2\times{10}^{-5}\,\text{cm}^{-3}$. Considerations on the nature and survival of the globules and their tails are also discussed.