Detection of HOC+ toward the orion bar and M17-SW: Enhanced abundances in photon-dominated regions?

A. J. Apponi, T. C. Pesch, L. M. Ziurys

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21 Scopus citations


New measurements of HOC+ emission have been carried out toward several photon-dominated regions using the National Radio Astronomy Observatory 12 m telescope. Toward the Orion bar, the J = 1 → 0, 2 → 1, and 3 → 2 transitions of HOC+ at 89, 179, and 268 GHz were detected, as well as the J = 1 → 0 line in M17-SW. In conjunction with observations of HC18O+, [HCO+]/[HOC+] abundance ratios have been established in these objects. Toward M17-SW, a ratio of ∼2260 was found. In the Orion bar, on the other hand, this ratio was determined to be [HCO+]/[HOC+] ∼ 270, both from J = 1 → 0 and J = 2 → 1 measurements - the lowest ratio thus far found in the Galaxy, and remarkably different from the value of 1800 found in nearby OMC-1. Moreover, the fractional abundance of HOC+ was found to be f ≳ 7 × 10-12 in this source, the highest value yet observed for this molecular ion. The Orion bar observations suggest that the abundance of HOC+ is enhanced relative to HCO+ because of the unique chemistry of photon-dominated regions.

Original languageEnglish (US)
Pages (from-to)L89-L92
JournalAstrophysical Journal
Issue number1 PART 2
StatePublished - Jul 1 1999


  • ISM: abundances
  • ISM: molecules
  • Line: identification

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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