Far-Infrared Detection of H 2 D + toward Sgr B2

We report on the first far-IR detection of H2D+, using the Infrared SpaceObservatory, in the line of sight toward Sgr B2 in the galactic center. Thetransition at 126.853 um connecting the ground level of o-H2D+, 1_1,1 with thethe 2_1,2 level at 113 K, is observed in absorption against the continuumemission of the cold dust of the source. The line is broad, with a totalabsorption covering 350 km s^-1, i.e., similar to that observed in thefundamental transitions of H2O, OH and CH at ~179, 119 and 149 um respectively.For the physical conditions of the different absorbing clouds the H2D+ columndensity ranges from 2 to 5x10^13 cm^-2, i.e., near an order of magnitude belowthe upper limits obtained from ground based submillimeter telescopes. Thederived H2D+ abundance is of a few 10^-10, which agrees with chemical modelspredictions for a gas at a kinetic temperature of ~20K.Comment: Accepted in ApJ letters. Non edite