Chemical complexity in the helix Nebula: Multi-line observations of H 2CO, HCO+, and CO

L. N. Zack, L. M. Ziurys

Research output: Contribution to journalArticle

23 Scopus citations

Abstract

Observations of CO, HCO+, and H2CO have been carried out at nine positions across the Helix Nebula (NGC 7293) using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory. Measurements of the J = 1 → 0, 2 → 1, and 3 →2 transitions of CO, two transitions of HCO+ (J = 1 → 0 and 3 →2), and five lines of H2CO (J Ka, Kc = 10, 1 → 0 0, 0, 21, 2 → 11, 1, 20, 2 → 10, 1, 21, 1 → 11, 0, and 3 0, 3 →20, 2) were conducted in the 0.8, 1, 2, and 3 mm bands toward this highly evolved planetary nebula. HCO+ and H 2CO were detected at all positions, along with three transitions of CO. From a radiative transfer analysis, the kinetic temperature was found to be T K ∼ 15-40 K across the Helix with a gas density of n(H 2) ∼ 0.1-5 × 105 cm-3. The warmer gas appears to be closer to the central star, but high density material is distributed throughout the nebula. For CO, the column density was found to be N tot ∼ 0.25-4.5 × 1015 cm-2, with a fractional abundance of f (CO/H2) ∼ 0.3-6 × 10 -4. Column densities for HCO+ and H2CO were determined to be N tot ∼ 0.2-5.5 × 1011 cm -2 and 0.2-1.6 × 1012 cm-2, respectively, with fractional abundances of f (HCO+/H2) ∼ 0.3-7.3 × 10-8 and f (H2CO/H2) ∼ 0.3-2.1 × 10-7 - several orders of magnitude higher than predicted by chemical models. Polyatomic molecules in the Helix appear to be well-protected from photodissociation and may actually seed the diffuse interstellar medium.

Original languageEnglish (US)
Article number112
JournalAstrophysical Journal
Volume765
Issue number2
DOIs
StatePublished - Mar 10 2013

Keywords

  • ISM: molecules
  • astrochemistry
  • planetary nebulae: individual (NGC 7293)
  • radio lines: ISM

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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