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Detection of HD in the Orion molecular outflow *
Frank Bertoldi 1,2,
Ralf Timmermann 2,
Dirk Rosenthal 2,
Siegfried Drapatz 2 and
Christopher M. Wright 3,4
Received 5 November 1998 / Accepted 18 February 1999
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 (SWS) on board the Infrared Space Observatory (ISO), the pure rotational R(5) line at 19.43 µm was detected toward the Orion (OMC-1) outflow at its brightest H2 emission region, Peak 1. The beam-averaged observed flux of the line is . Upper flux limits were derived for sixteen other rotational and ro-vibrational HD lines in the wavelength range 2.5 to 38 µm.
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 . The warm molecular hydrogen column density is , so that the relative abundance is .
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]= .
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.
Key words: shock waves ISM: abundances ISM: individual objects: Orion Peak 1 ISM: molecules cosmology: observations infrared: ISM: lines and bands
* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the United Kingdom) and with the participation of ISAS and NASA.
Send offprint requests to: F. Bertoldi
Online publication: May 6, 1999