1999ApJ...515L..29W

We report the first detection outside of the solar system of the lowest pure rotational J=1⟶0 transition of the HD molecule at 112 µm. The detection was made toward the Orion Bar using the Fabry-Pérot interferometer of the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory. The line appears in emission with an integrated flux of (0.93±0.17)x10^–19 W.cm^–2 in the LWS beam, implying a beam-averaged column density in the v=0, J=1 state of (1.2±0.2)x10¹⁷ cm^–2. Assuming LTE excitation, the total HD column density is (2.9±0.8)x10¹⁷ cm^–2 for temperatures between 85 and 300 K. Combined with the total, warm H₂ column density of ~(1.5-3.0)x10²² cm^–2 derived from either the H₂ pure rotational lines, the C¹⁸O observations, or the dust continuum emission, the implied HD abundance, HD/H₂, ranges from 0.7x10^–5 to 2.6x10^–5, with a preferred value of (2.0±0.6)x10^–5. The corresponding deuterium abundance of [D]/[H] = (1.0±0.3)x10^–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.