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Astron. Astrophys. 342, L37-L40 (1999)

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Letter to the Editor

Physical conditions in the partially ionized zone of the Orion nebula *

César Esteban 1, Manuel Peimbert 2 and Silvia Torres-Peimbert 2

1 Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Canary Islands, Spain (cel@ll.iac.es)
2 Instituto de Astronomía, UNAM Apdo. Postal 70-264, Mexico D.F. 04510, Mexico (peimbert@astroscu.unam.mx, silvia@astroscu.unam.mx)

Received 30 November 1998 / Accepted 21 December 1998


We report the first reliable measurement of the nebular [O I ] [FORMULA]5577 emission line in the Orion nebula. Diagnostic diagrams based on the [O I ] [FORMULA][FORMULA]5577, 6300, and [N I ] [FORMULA][FORMULA]3467, 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 (2[FORMULA]103 - 4[FORMULA]104 cm-3) and with electron temperatures in the 8900 to 12400 K 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.

Key words: ISM: H ii regions – ISM: individual objects: Orion nebula – ISM: structure

* Based on data obtained at the Observatorio Astronómico Nacional, SPM, B.C., México

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999