Far-infrared Detection of H2d + toward Sgr B2 1

We report on the first far–IR detection of H 2 D + , 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-H 2 D + , 1 1,1 , with the 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 H 2 O, OH and CH at ∼179, 119 and 149 µm respectively. For the physical conditions of the different absorbing clouds the H 2 D + column density ranges from 2 to 5×10 13 cm −2 , i.e., near an order of magnitude below the upper limits obtained from ground based submillimeter telescopes. The derived H 2 D + abundance is of a few 10 −10 , which agrees with chemical models predictions for a gas at a kinetic temperature of ≃20 K.