Heterodyne Spectroscopy of the 63 μm O I Line in M42

We have used a laser heterodyne spectrometer to resolve the emission-line profile of the 63 μm 3 P 1 - 3 P 2 fine-structure transition of O I at two locations in M42. Comparison of the peak antenna temperature with that of the 158 μm C II fine-structure line shows that the gas kinetic temperature in the photodissociation region (PDR) near θ 1 C is 175-220 K, the density is greater than 2 × 10 5 cm -3 , and the hydrogen column density is about 1.5 × 10 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 -1 (FWHM) at θ 1 C, which is slightly broadened over the intrinsic line width by optical depth effects. No other significant 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 nonionizing shock models.