Identification of a strong emission line at 2.8935 μm in the spectrum of the Orion Nebula

A strong emission line at 2.8935μm discovered by Rubin et al. in an ISO SWS02 spectrum of the Orion Nebula is identified as the 4p³P-4s³S^o multiplet of OI. Line formation is due to de-excitation cascades following UV-pumping of high ³S^o and ³D^o terms and occurs in the OI zone immediately behind the hydrogen ionization front. This cascade mechanism also accounts for permitted OI triplet lines in the optical spectrum of the Nebula, as shown by Grandi. An escape probability treatment of the OI cascades accounts for the strength of the λ2.89-μm line and suggests interesting diagnostic possibilities for the optical lines.