Spitzer Observations of Shock-excited Hydrogen Deuteride in IC 443C, HH 7, and HH 54: Probing the Gas-phase Deuterium Abundance in the Dense Interstellar Medium

We present and analyze observations—carried out using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope—of the R(3) and R(4) pure rotational lines of hydrogen deuteride (HD) detected from shock-heated material associated with the supernova remnant IC 443C and with the Herbig-Haro objects HH 7 and HH 54. Assuming a continuous temperature distribution for gas observed along the sight lines, we have constrained the gas density to be in the ~10³-10⁴ cm^-3 range, using both spectroscopic data for H₂, HD, and CO from IRS and from the Infrared Space Observatory (ISO), as well as photometric data from Spitzer's Infrared Array Camera. The derived HD abundance relative to H₂ is quite sensitive to the assumed excitation conditions in the emitting gas. Assuming that HD accounts for all gas-phase deuterium in the emitting material, and using all the available spectroscopic data to constrain the excitation conditions, we obtained gas-phase deuterium abundances [D/H]_gas of 0.95^+0.54 _-0.27 × 10^-5 and 0.87^+0.31 _-0.27 × 10^-5 (statistical errors only) for IC 443C and HH 54, respectively. The uncertainties in the HD abundance are dominated by systematic effects related to the poorly known excitation conditions, and more accurate estimates of the HD abundance in shocked molecular clouds will require measurements of the emission in additional HD rotational transitions.