A survey of N2H+ in dense clouds: Implications for interstellar nitrogen and ion-molecule chemistry

M. Womack, L. M. Ziurys, S. Wyckoff

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Abstract

Spectra of the N2H+ J = 1 → 0, J = 3 → 2 and 15NNH+ and N15NH+ J = 1 → 0 rotational transitions have been obtained toward a sample of star-forming and cold dark clouds in the Galaxy. Toward the starforming regions, line profiles are relatively narrow (typically 1-5 km s-1) and show no evidence of line wings, in contrast to the spectra of HCO+. The apparent absence of N2H+ in hot, shocked gas suggests that this ion may be a selective tracer of extended, quiescent material. Column densities of N2H+ were found to be Ntot ∼ 5 × 1012 cm-2 toward cold clouds and Ntot ∼ 1014 cm-2 toward warm clouds. These values correspond to fractional abundances, relative to H2, of f ∼ 4 × 10-10 toward both the warm and cold clouds. Such abundances agree well with predictions of quiescent cloud ion-molecule chemistry models, provided "steady state," not "early time" values are used. Thus, the abundance and distribution of N2H+ is well-explained by ion-molecule chemistry, provided it has reached steady state, as is found for interstellar NH3. In contrast, most abundances of simple interstellar molecules are better reproduced by models calculated for early times. The N2H+ results thus are further evidence that interstellar nitrogen chemistry is anomalous.

Original languageEnglish (US)
Pages (from-to)417-429
Number of pages13
JournalAstrophysical Journal
Volume387
Issue number1
DOIs
StatePublished - Jan 1 1992
Externally publishedYes

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Keywords

  • ISM: abundances
  • ISM: molecules
  • Molecular processes

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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