Neutral Atomic Carbon in the Globules of the Helix
1997
We report detection of the 609 μm3P1 →3P0 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 13CO(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) × 1016 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.
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