2007ApJ...657L..21C

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⁺, 1_1,1, with the 2_1,2level 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, 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 5x10¹³/cm2, 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.