1999A&A...342L..37E

We report the first reliable measurement of the nebular [O I] λ5577 emission line in the Orion nebula. Diagnostic diagrams based on the [O I] λλ5577, 6300, and [N I] λλ3467, 5198, 5200 emission lines indicate that the bulk of the nebular emission in the partially ionized zone of the Orion nebula is produced in regions with low to moderate electron densities (2x10³-4x10⁴cm^–3) and with electron temperatures in the 8900 to 12400K range. This result implies that models based on emission originating in high-density partially ionized zones are not applicable to explain the observed [Fe II] spectrum in the optical region.