FAR-INFRARED STUDIES OF THE ORION NEBULA

Two far-infrared studies on the Orion Nebula M42 are presented. The observation of the C-13+ F = 1 --> 0 hyperfine component in M42 shows that the optical depth of the [C-12 II] at 158 mum is about one, consistent with values for temperature and density of the emitting material derived in earlier studies of photodissociation regions. The observations of the 63.2 and 145.5 mum [OI] and 158 mum [CII] fine structure lines around the Orion HII region and the quiescent northern ridge might allow to derive a relative atomic oxygen abundance [O]/[H] in the range of 7 x 10(-5) to 3 x 10(-4) using the knowledge on the [CII] emission of PDRs.