The 2.46 \mu m 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 \pm 2.1 ) \times 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 \leftrightarrow 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 \pm 1.9 ) \times 10 ^ { -6 } .