Abstract
The 2.46 μm v = 1 - 0 R(5) line of deuterated molecular hydrogen (HD) has been detected in the Orion Peak 1 shock emission region, at a surface brightness of (8.5 ± 2.1) × 10-9Wm-2sr-1 over a 6 arcsec2 area. Comparison of the column density of HD (v = 1,J = 6) with the column density of HD (v = 0,J = 6) previously observed from the Infrared Space Observatory (ISO) and the H2 level column densities toward the same region implies that the excitation of HD is similar to that of H2 for these energy levels, despite much higher spontaneous transition rates for HD. We suggest that this rough equality is caused by the coupling of the HD levels to those of H2, due to strong reactive collisions, HD + H ↔ H2 + D, in warm, partially dissociated gas. The deuterium abundance implied by the combined ISO and UKIRT measurements toward Orion Peak 1 is [D]/[H] = (5.1 ± 1.9) × 10-6.
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