Heterodyne Spectroscopy of the 63 $\mu$m O I Line in M42
  We have used a laser heterodyne spectrometer to resolve the emission line profile of the 63 micron 3P1 - 3P2 fine-structure transition of O I at two locations in M42. Comparison of the peak antenna temperature with that of the 158 micron C II fine-structure line shows that the gas kinetic temperature in the photodissociation region near theta1C is 175 - 220 K, the density is greater than 2x10 ^5 cm-3, and the hydrogen column density is about 1.5x10 ^22 cm-2. A somewhat lower temperature and column density are found in the IRc2 region, most likely reflecting the smaller UV flux. The observed width of the O I line is 6.8 km/s (FWHM) at theta1C, which is slightly broadened over the intrinsic linewidth by optical depth effects. No significant other differences between the O I and C II line profiles are seen, which shows that the narrow emission from both neutral atomic oxygen and ionized carbon comes from the PDR. The O I data do not rule out the possibility of weak broad-velocity emission from shock-excited gas at IRc2, but the C II data show no such effect, as expected from non-ionizing shock models. 
R. T. Boreiko and A. L. Betz (University of Colorado)