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 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.

Based on observations with the Infrared Space Observatory (ISO), an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the UK) and with participation of ISAS and NASA.