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 reactions1–3 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.