Infrared Space Observatory-Long Wavelength Spectrometer Detection of the 112 Micron HD J = 1 --> 0 Line toward the Orion Bar

Astronomy and Astrophysics – Astronomy

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Ism: Abundances, Ism: Individual: Name: Orion Bar, Ism: Molecules, Infrared: Ism: Lines And Bands

Scientific paper

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 mum. 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)x10^17 cm^-2. Assuming LTE excitation, the total HD column density is (2.9+/-0.8)x10^17 cm^-2 for temperatures between 85 and 300 K. Combined with the total, warm H_2 column density of ~(1.5-3.0)x10^22 cm^-2 derived from either the H_2 pure rotational lines, the C^18O observations, or the dust continuum emission, the implied HD abundance, HD/H_2, 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.

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