Observations of H3+ in the diffuse interstellar medium

B. J. McCall, K. H. Hinkle, T. R. Geballe, G. H. Moriarty-Schieven, N. J. Evans, K. Kawaguchi, S. Takano, V. V. Smith, T. Oka

Research output: Contribution to journalArticlepeer-review


Surprisingly large column densities of H3+ have been detected using infrared absorption spectroscopy in seven diffuse cloud sight lines (Cygnus OB2 12, Cygnus OB2 5, HD 183143, HD 20041, WR 104, WR 118, and WR 121), demonstrating that H3+ is ubiquitous in the diffuse interstellar medium. Using the standard model of diffuse cloud chemistry, our H3+ column densities imply unreasonably long path lengths (∼1 kpc) and low densities (∼3 cm-3). Complimentary millimeter-wave, infrared, and visible observations of related species suggest that the chemical model is incorrect and that the number density of H 3+ must be increased by 1-2 orders of magnitude. Possible solutions include a reduced electron fraction, an enhanced rate of H 2 ionization, and/or a smaller value of the H3 + dissociative recombination rate constant than implied by laboratory experiments.

Original languageEnglish (US)
Pages (from-to)391-406
Number of pages16
JournalAstrophysical Journal
Issue number1 I
StatePublished - Mar 1 2002


  • Cosmic rays
  • ISM: clouds
  • ISM: molecules
  • Infrared: ISM: lines and bands
  • Molecular processes

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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