Detection of a vibration - rotation emission line of hydrogen deuteride toward Orion Peak 1 : excitation coupling of HD to H

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 −9 Wm −2 sr −1 over a 6 arcsec 2 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 ISO and the H 2 level column densities toward the same region implies that the excitation of HD is similar to that of H 2 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 H 2 , due to strong reactive collisions, HD + H ↔ H 2 + 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 .