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 lambda = 126.853 mu m connecting the ground level of o-H2D+, 1(1, 1), with the 2(1, 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 similar to 179, 119, and 149 mu m, respectively. For the physical conditions of the different absorbing clouds, the H2D+ column density ranges from 2 to 5 x 10(13) 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 similar or equal to 20 K.