Detection of HD in the Orion molecular outflow

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We report a detection in the interstellar medium of an infrared transition within the electronic ground state of the deuterated hydrogen molecule, HD. Through a deep integration with the Short-Wavelength-Spectrometer on ISO, the pure rotational v=0-0 R(5) line at 19.43um was detected toward the Orion (OMC-1) outflow at its brightest H₂ emission region, Peak 1. The 20" beam-averaged observed flux of the line is (1.84 +- 0.4) 10-5 erg cm-2 s-1 sr-1. Upper flux limits were derived for sixteen other rotational and ro-vibrational HD lines in the wavelength range 2.5 to 38 um. We utilize the rich spectrum of H₂ lines observed at the same position to correct for extinction, and to derive a total warm HD column density under the assumption that similar excitation conditions apply to H₂ and HD. Accounting for non-LTE HD level populations in a partially dissociated gas, our best estimate for the total warm HD column density is N(HD)=(2.0+-0.75)10¹6 cm-2. The warm molecular hydrogen column density is (2.21+-0.24)10²1 cm-2, so that the relative abundance is [HD]/[H₂]=(9.0+-3.5)10-6. Accounting for HD depletion relative to H₂ in partially dissociative shocks we derive a deuterium abundance in the warm shocked gas, [D]/[H]= (7.6+-2.9)10-6. Our implied deuterium abundance is low compared to previous determinations in the local interstellar medium, but it is consistent with two other recent observations toward Orion, suggesting that deuterium may be significantly depleted there.