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. Because the observed HD level population is not thermalized at the densities prevailing in the emitting region, the total HD column density is sensitive to the assumed gas density, temperature, and dissociation fraction. 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.
The observed emission presumably arises in the warm layers of partially dissociative magnetic shocks, where HD can be depleted relative to H2 due to an asymmetry in the deuterium-hydrogen exchange reaction. This leads to an average HD depletion relative to H2 of about 40%. Correcting for this chemical depletion, we derive a deuterium abundance in the warm shocked gas, [D]/[H]= (7.6±2.9)×10-6.
The derived deuterium abundance is not very sensitive to the dissociation fraction in the emitting region, since both the non-LTE and the chemical depletion corrections act in oppositite direction. 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.
Keywords: Shock waves -- ISM: abundances -- ISM:individual objects:Orion Peak 1 -- ISM: molecules -- Cosmology:observations --Infrared: ISM: lines and bands
Status: A&A accepted
Address: mailto: rosental@mpe.mpg.de
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Last update: 17-Jun-1999 by K.Seidenschwang