Possible Discovery of the 70 micron H 3O + 4 3 - 3 3 + Transition in Orion BN-IRc2

We report the first astronomical detection of a far-IR emission line at (69.524 +/- 0.008) mu m from aboard the Kuiper Airborne Observatory (KAO). We tentatively assign this line to the R-branch transition 4^-₃ --> 3^{+}₃ of H3O+. The wavelength of the discovered line lies between -112 and -60 km s-1 from predicted values. The position at which the emission line was detected is located about 20" north of Orion BN, which is close to the shocked molecular hydrogen peak 1. We exclude hot shocked gas as the possible origin of the H3O+ emission. Hydrogen densities of >~5 x 108 cm-3 and temperatures >~100 K are required to match the observed integrated intensity of the 70 mu m line. A speculative explanation for the 70 mu m {H}^{+}₃{O} emission is that it might originate from very dense clumps.