Detection of HD in the Orion molecular outflow

We report a detection in the interstellar medium of an infrared transition within the electronic ground state of the deuterated hydrogen molecule, KD. Through a deep integration with the Short-Wavelength-Spectrometer (SWS) on board the Infrared Space Observatory (ISO), the pure rotational nu = 0 - 0 R(5) line at 19.33 mu m was detected toward the Orion (OMC-1) outflow at i ts brightest Hz emission region, Peak 1. The similar to 20" beam-averaged o bserved flux of the line is (1.84 +/- 0.4) x 10(-5) erg cm(-2) s(-1) sr(-1) . Upper flux limits were derived for sixteen other rotational and re-vibrat ional HD lines in the wavelength range 2.5 to 38 mu m. We utilize the rich spectrum of H-2 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 Ha and HD. Becau se the observed HD level population is not thermalized at the densities pre vailing in the emitting region, the total I-ID column density is sensitive to the assumed gas density, temperature, and dissociation fraction. Account ing for non-LTE HD level populations in a partially dissociated gas, our be st estimate for the total warm HD column density is N(HD) = (2.0 +/- 0.75) x 10(16) cm(-2). The warm molecular hydrogen column density is (2.21 +/- 0. 24) x 10(21) cm(-2), so that the relative abundance is [HD] /[H-2] = (9.0 /- 3.5) x 10(-6). The observed emission presumably arises in the warm layers of partially dis sociative magnetic shocks, where KD can be depleted relative to H-2 due to an asymmetry in the deuterium-hydrogen exchange reaction. This leads to an average HD depletion relative to H-2 of about 40%. Correcting for this chem ical depletion, we derive a deuterium abundance in the warm shocked gas, [D ]/[H]= (7.6 +/- 2.9) x 10(-6). The derived deuterium abundance is not very sensitive to the dissociation f raction in the emitting region, since both the non-LTE and the chemical dep letion corrections act in oppositite direction. Our implied deuterium abund ance is low compared to previous determinations in the local interstellar m edium, but it is consistent with two other recent observations toward Orion , suggesting that deuterium may be significantly depleted there.