Spitzer observations of hydrogen deuteride

We report the detection of interstellar hydrogen deuteride (HD) toward the supernova remnant IC 443, and the tentative detection of HD toward the Herbig-Haro objects HH 54 and HH 7 and the star-forming region GGD 37 (Cep A West). Our detections are based upon spectral line mapping observations of the R(3) and R(4) rotational lines of HD, at rest wavelengths of 28.502 and 23.034 mu m, respectively, obtained using the Infrared Spectrograph on board the Spitzer Space Telescope. The HD R(4)/R(3) line intensity ratio promises to be a valuable probe of the gas pressure in regions where it can be observed. The derived HD/H2 abundance ratios are (1.19(-0.24)(+0.35)) x 10(-5), (1.80(-0.32)(+0.54)) x 10(-5), and (1.41(-0.33)(+0.46)) x 10(-5), respectively (68.3% confidence limits, based upon statistical errors alone) for IC 443 (clump C), HH 54, and HH 7. If HD is the only significant reservoir of gas-phase deuterium in these sources, the inferred HD/H-2 ratios are all consistent with a gas-phase elemental abundance [n(D)/n(H)](gas) similar to 7.5 x 10(-6), a factor of 2-3 below the values obtained previously from observations of atomic deuterium in the Local Bubble and the Galactic halo. However, similarly low gas-phase deuterium abundances have been inferred previously for molecular gas clouds in the Orion region and in atomic clouds along sight lines within the Galactic disk to stars more distant than 500 pc from the Sun.