New observations of the [HCO+]/[HOC+] ratio in dense molecular clouds

A. J. Apponi, L. M. Ziurys

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41 Scopus citations


New measurements of the millimeter-wave transitions of HOC+ toward dense molecular clouds have been carried out using the NRAO 12 m telescope. The J = 1 → 0 transition of HOC+ at 89 GHz has been detected for the first time toward DR21 (OH), W51M, W3 (OH), NGC 7538, Orion (3N, 1E), NGC 2024, and G34.3. The J = 2 → 1 and J = 3 → 2 lines at 179 GHz and 268 GHz have also been observed in W51 and Orion KL. In conjunction with past observations of HOC+ toward Orion KL and SgrB2 (OH) by Ziurys & Apponi, these data confirm the presence of this ion in the interstellar medium. The column densities of HOC+ were found to be Niot ∼ (7 × 1010)-(3 × 1012) cm-2, which correspond to fractional abundances of f∼(2 × 10-13)-(3 × 10-12) relative to H2. Observations of the J = 1 → 0 transition of HC18O+ at 85 GHz have also been carried out, resulting in [HCO+]/[HOC+] ratios of 3606000. These values are in good agreement with recent modeling by Herbst & Woon and confirm that the reaction of HOC+ with H2 indeed has a substantial barrier at low (T < 50 K) temperatures. The widespread abundance of HOC+ additionally suggests that there is not a strong preference for synthesis of this ion in photon-dominated regions, although there may be a small enhancement toward such gas. Finally, confirmation of interstellar HOC+ has been accomplished through observations of weak emission (TR ≲, 50 mK) at 179 GHz, proving that measurements near the atmospheric water line at 183 GHz are not as difficult as previously thought.

Original languageEnglish (US)
Pages (from-to)800-808
Number of pages9
JournalAstrophysical Journal
Issue number2 PART I
StatePublished - 1997


  • ISM: abundances
  • ISM: clouds
  • ISM: molecules
  • Radio lines: ISM

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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