Infrared Space Observatory-long wavelength spectrometer detection of the 112 micron HD J=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 mu m. The detection was made toward the Orion Bar using the Fabry-Perot 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) x 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) x 10(17) cm-2. Assuming LTE excitation, the total HD column density is (2.9 +/- 0.8) x 10(17) cm-2 for temperatures between 85 and 300 K. Combined with the total, warm H-2 column density of similar to(1.5-3.0) x 10(22) cm(-2) derived from either the H-2 pure rotational lines, the (CO)-O-18 observations, or the dust continuum emission, the implied HD abundance, HD/H-2, ranges from 0.7 x 10(-5) to 2.6 x 10(-5), with a preferred value of (2.0 +/- 0.6) x 10(-5). The corresponding deuterium abundance of [D]/[H] = (1.0 +/- 0.3) x 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.