2002ApJ...566..905R

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)x10^–9W.m^–2.sr^–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)x10^–6.