Far-Infrared Detection of H[TINF]3[/TINF]O[TSUP]+[/TSUP] in Sagittarius B2

We report the first far-infrared detection of the hydronium ion (H 3 O + ) toward the Sagittarius B2 molecular cloud, near the Galactic center. Using the Infrared Space Observatory Long-Wavelength Spectrometer, we have detected three lines arising from the ν 2 ground-state inversion mode (0 + → 0 - ) at 55.3 cm -1 . All transitions are observed in absorption against the optically thick infrared continuum emission of the dust. Two different absorption regions can be distinguished, the H 3 O + associated with the Sgr B2 extended envelope and that of foreground gas along the line of sight. The derived abundances for both components are rather similar, ≲1 × 10 -9 , and may explain the high water abundance found in the low-density gas toward Sgr B2 as its dissociative recombination leads to the formation of important molecules OH and H 2 O. Contribution of the Q (1, 1) line of para-H 3 O + to the 2 12 -1 01 line of H 2 18 O is also analyzed. This allows a better determination of water abundance from far-infrared observations. The corrected H 2 O abundances are ≤10 -6 for the diffuse clouds along the line of sight and a few times 10 -6 for the gas associated with the Sgr B2 envelope, in good agreement with water vapor observations from the Submillimeter Wave Astronomy Satellite .