Far-Infrared Detection of H2D+ toward Sgr B2

We report on the first far-IR detection of H2D+, using the Infrared Space Observatory, in the line of sight toward Sgr B2 in the Galactic center. The transition at λ = 126.853 μm connecting the ground level of o-H2D+, 11, 1, with the 21, 2 level at 113 K is observed in absorption against the continuum emission of the cold dust of the source. The line is broad, with a total absorption covering 350 km s-1, i.e., similar to that observed in the fundamental transitions of H2O, OH, and CH at ~179, 119, and 149 μm, respectively. For the physical conditions of the different absorbing clouds, the H2D+ column density ranges from 2 to 5 × 1013 cm-2, i.e., near an order of magnitude below the upper limits obtained from ground-based submillimeter telescopes. The derived H2D+ abundance is of a few times 10-10, which agrees with chemical model predictions for a gas at a kinetic temperature of ≃20 K.