We report the first detection outside of the solar system of the lowest pure rotational J =1 \to 0 transition of the HD molecule at 112 \mu m. The detection was made toward the Orion Bar using the Fabry-Pérot of the Long Wavelength Spectrometer ( LWS ) on board the Infrared Space Observatory ( ISO ) .
The line appears in emission with an integrated flux of ( 0.93 \pm 0.17 ) \times 10 ^ { -19 } W cm ^ { -2 } in the LWS beam , implying a beam–averaged column density in the v =0 , J =1 state of ( 1.2 \pm 0.2 ) \times 10 ^ { 17 } cm ^ { -2 } .
Assuming LTE excitation , the total HD column density is ( 2.9 \pm 0.8 ) \times 10 ^ { 17 } cm ^ { -2 } for temperatures between 85 and 300 K. Combined with the total warm H _ { 2 } column density of \sim ( 1.5 - 3.0 ) \times 10 ^ { 22 } cm ^ { -2 } derived from either the H _ { 2 } pure rotational lines , C ^ { 18 } O observations or dust continuum emission , the implied HD abundance , HD/H _ { 2 } , ranges from 0.7 \times 10 ^ { -5 } to 2.6 \times 10 ^ { -5 } , with a preferred value of ( 2.0 \pm 0.6 ) \times 10 ^ { -5 } .
The corresponding deuterium abundance of [ D ] / [ H ] = ( 1.0 \pm 0.3 ) \times 10 ^ { -5 } is compared with recent values derived from ultraviolet absorption line observations of atomic H i and D i in interstellar clouds in the solar neighborhood and in Orion .