An interstellar line coincident with the P(2,l)transition of hydronium (H₃O⁺)

Abstract

Gas-phase interstellar chemistry is thought to be dominated by ion–molecule reactions. Neutral molecular species abound in the dense interstellar clouds, but only seven molecular ions have been detected. Hydronium (H₃O⁺) is predicted in all ion–molecule reactions^1–3 to be an abundant molecular ion, forming OH and H₂O by dissociative electron recombination reactions. Here we report the detection, towards the Orion-KL nebula, of a weak emission spectral line coincident in frequency with the P(2,l) rotation–inversion transition of H₃O⁺. Although we cannot be certain that this single-line detection is H₃O⁺, we evaluate this possibility with regard to its observed line parameters and, using theoretical chemical models, derive an Orion fractional abundance for H₂O (relative to H₂) of 10⁻⁵ and 10⁻⁶ in thermal and non-thermal cases, respectively. We also report 1-mm-wavelength observations towards Orion which yield new interstellar transitions of CH₃OH, ³⁴SO₂, H₂CS, SO, OCS, SiO and two unidentified lines at 307,313 and 304,374 MHz.