Detection of Interstellar Ortho-D₂H⁺ with SOFIA

Abstract

We report on the detection of the ground-state rotational line of ortho-D₂H⁺ at 1.477 THz (203 μm) using the German REceiver for Astronomy at Terahertz frequencies (GREAT) on board the Stratospheric Observatory For Infrared Astronomy (SOFIA). The line is seen in absorption against far-infrared continuum from the protostellar binary IRAS 16293-2422 in Ophiuchus. The para-D₂H⁺ line at 691.7 GHz was not detected with the APEX telescope toward this position. These D₂H⁺ observations complement our previous detections of para-H₂D⁺ and ortho-H₂D⁺ using SOFIA and APEX. By modeling chemistry and radiative transfer in the dense core surrounding the protostars, we find that the ortho-D₂H⁺ and para-H₂D⁺ absorption features mainly originate in the cool (T < 18 K) outer envelope of the core. In contrast, the ortho-H₂D⁺ emission from the core is significantly absorbed by the ambient molecular cloud. Analyses of the combined D₂H⁺ and H₂D⁺ data result in an age estimate of ∼5 × 10⁵ yr for the core, with an uncertainty of ∼2 × 10⁵ yr. The core material has probably been pre-processed for another 5 × 10⁵ years in conditions corresponding to those in the ambient molecular cloud. The inferred timescale is more than 10 times the age of the embedded protobinary. The D₂H⁺ and H₂D⁺ ions have large and nearly equal total (ortho+para) fractional abundances of ∼10^-9 in the outer envelope. This confirms the central role of {{{H}}}₃⁺ in the deuterium chemistry in cool, dense gas, and adds support to the prediction of chemistry models that also {{{D}}}₃⁺ should be abundant in these conditions.