Neutral atomic carbon in the globules of the Helix

Details Neutral atomic carbon in the globules of the Helix Neutral atomic carbon in the globules of the Helix

1997-03-18

arxiv.org/abs/astro-ph/9703116v1

Astronomy and Astrophysics

Astrophysics

5 pages, Latex, AAS macros, 3 EPS figures, to appear in Astrophysical Journal Letters

Scientific paper

10.1086/310677

We report detection of the 609u 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 13CO(2-1) spectra and a 135" x 135" map in CO(2-1). The velocity distribution of CI shows six narrow (1 -> 2 km/sec) components which are associated with individual globules traced in CO. The CI column densities are 0.5 -> 1.2 x 10^16/cm^2. CI is found to be a factor of ~6 more abundant than CO. Our estimate for the mass of the neutral envelope is an order of magnitude larger than previous estimates. The large abundance of CI in the Helix can be understood as a result of the gradual photoionisation of the molecular envelope by the central star's radiation field.

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