Ion chemistry in photon-dominated regions: Examining the [HCO +]/[HOC+]/[CO-] chemical network

C. Savage, L. M. Ziurys

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51 Scopus citations


HOC+ and CO+ have been detected toward two well-known photon-dominated regions (PDRs), S140 and NGC 2023, using the Arizona Radio Observatory 12 m telescope. The J = 1 → 0 transition of HOC+ at 89 GHz and the two spin components of the N = 2 → 1 line of CO+ near 236 GHz were observed, as well as the J = 1 → 0 transitions of H13CO+ and HC18O-. The J = 3 → 2 line of HOC- at 268 GHz was also mapped across the Orion Bar. The [HCO+]/[HOC-] ratios determined in S140 and NGC 2023 were ∼12,408 and 1913, respectively, values indicative of quiescent molecular gas rather than PDR sources, where the abundance of HOC - is thought to be enhanced. However, the beam in both these measurements may contain material from the adjoining molecular cloud, favoring HCO-. Alternatively, the [HCO+]/[HOC+] ratio may vary with Av in PDR regions. The [CO+]/[HOC +] ratio in S140 and NGC 2023, at several positions in the Orion Bar, and in other PDRs, on the other hand, falls uniformly in the range ∼1-10. In addition, the line profiles of CO- and HOC+ in the Orion Bar look remarkably similar. The syntheses of HOC- and CO - appear to be correlated in PDRs, most likely through the common precursor, C+. The reaction of C- and H2O is thought to preferentially create HOC-, as opposed to HCO -, and C+ + OH or O2 leads favorably to CO +.

Original languageEnglish (US)
Pages (from-to)966-975
Number of pages10
JournalAstrophysical Journal
Issue number2 I
StatePublished - Dec 1 2004


  • Abundances - ISM
  • Astrochemistry - H ii regions - ISM
  • Clouds - ISM
  • ISM
  • Molecules -radio lines

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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