Neutral Atomic Carbon in the Globules of the Helix

We report detection of the 609 μm 3 P 1 → 3 P 0 line of neutral atomic carbon in globules of the Helix Nebula. The measurements were made toward the position of peak CO emission. At the same position, we obtained high-quality CO(2-1) and 13 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 -1 ) components, which are associated with individual globules traced in CO. The C I column densities are (0.5-1.2) × 10 16 cm -2 . 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 photoionization of the molecular envelope by the central star's radiation field.