Heterodyne Spectroscopy of the 63 μm O I Line in M42

We have used a laser heterodyne spectrometer to resolve the emission lineprofile of the 63 micron 3P1 - 3P2 fine-structure transition of O I at twolocations in M42. Comparison of the peak antenna temperature with that of the158 micron C II fine-structure line shows that the gas kinetic temperature inthe photodissociation region near theta1C is 175 - 220 K, the density isgreater than 2x10 ^5 cm-3, and the hydrogen column density is about 1.5x10 ^22cm-2. A somewhat lower temperature and column density are found in the IRc2region, most likely reflecting the smaller UV flux. The observed width of the OI line is 6.8 km/s (FWHM) at theta1C, which is slightly broadened over theintrinsic linewidth by optical depth effects. No significant other differencesbetween the O I and C II line profiles are seen, which shows that the narrowemission 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 emissionfrom shock-excited gas at IRc2, but the C II data show no such effect, asexpected from non-ionizing shock models.Comment: 11 pages including 2 postscript figures, uses aaspp4.st