Neutral atomic carbon in the globules of the Helix

We report detection of the 609 μm P1 → P0 line of neutral atomic carbon in globules of the Helix nebula. The measurements were made towards the position of peak CO emission. At the same position, we obtained high-quality CO(2–1) and CO(2–1) spectra and a 135 × 135 map in CO(2–1). The velocity distribution of C I shows six narrow (1–2 km s) components which are associated with individual globules traced in CO. The C I column densities are 0.5–1.2×10 cm. C I is found to be a factor of ∼6 more abundant than CO. The large abundance of C I in the Helix can be understood as a result of the gradual photoionisation of the molecular envelope by the central star’s radiation field. Subject headings: planetary nebulae: individual: (NGC 7293) — radio lines: stars — stars: AGB and post-AGB — stars: mass loss