New Observations of the [HCO +]/[HOC +] Ratio in Dense Molecular Clouds

Details New Observations of the [HCO +]/[HOC +] Ratio in Dense Molecular Clouds New Observations of the [HCO +]/[HOC +] Ratio in Dense Molecular Clouds

May 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...481..800a&link_type=abstract

Astrophysical Journal v.481, p.800

Astronomy and Astrophysics

Astronomy

24

Ism: Molecules, Ism: Abundances, Ism: Clouds, Radio Lines: Ism

Scientific paper

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 Ntot ~ (7 x 1010)--(3 x 1012) cm-2, which correspond to fractional abundances of f~(2 x 10-13)--(3 x 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 360--6000. 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.

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