[ITAL]Infrared Space Observatory[/ITAL]–Long Wavelength Spectrometer Detection of the 112 Micron HD [ITAL]J[/ITAL] = 1 → 0 Line toward the Orion Bar

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)×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±0.2)×1017 cm−2. Assuming LTE excitation, the total HD column density is (2.9±0.8)×1017 cm−2 for temperatures between 85 and 300 K. Combined with the total, warm H2 column density of ~(1.5-3.0)×1022 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 0.7×10−5 to 2.6×10−5, with a preferred value of (2.0±0.6)×10−5. The corresponding deuterium abundance of [D]/[H] = (1.0±0.3)×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.