Far-Infrared Detection of H₂D⁺ toward Sgr B2

Abstract

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_1,2 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.

Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the UK) and with participation of ISAS and NASA.