  We report on the detection of the ground-state rotational line of ortho-D2H+
at 1.477 THz (203 micron) using the German REceiver for Astronomy at Terahertz
frequencies (GREAT) onboard 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 ~500 000 yr for
the core, with an uncertainty of ~200 000 yr. The core material has probably
been pre-processed for another 500 000 years in conditions corresponding to
those in the ambient molecular cloud. The inferred time scale is more than ten
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 H3+ in the deuterium
chemistry in cool, dense gas, and adds support to the prediction of chemistry
models that also D3+ should be abundant in these conditions.
