Temperature Variations and N/O in the Orion Nebula from HST Observations

Authors

R. H. Rubin, NASA/Ames Research Center
P. G. Martin, University of Toronto, Canada
R. J. Dufour, Rice University
Gary J. Ferland, University of KentuckyFollow
J. A. Baldwin, Cerro Tololo Inter-American Observatory, Chile
J. J. Hester, Arizona State University
D. K. Walter, South Carolina State University

Abstract

Using the Goddard High Resolution Spectrograph (GHRS) and the Faint Object Spectrograph (FOS) on the Hubble Space Telescope, we measured the flux of the N II] (2s2p^{3 5}S₂ --> 2s²2p^{2 3}P_2,1) lines at λ_vac = 2143.45, 2139.68 Å in the Orion Nebula--the first detection of these lines in an H II region. In order to assess the N⁺/O⁺ ratio, we also measured the flux of the [O II] (2p³²P^o_1/2,3/2-->2p³⁴S^o_3/2) lines at λ_vac = 2471.05, 2471.12 Å. In addition, with the FOS, other emission lines were measured in the same aperture in order to assess the average electron temperature and mean-square temperature variation (t²) in the N⁺ region, as well as the N⁺/O⁺ ratio. When we require that the empirically determined values be equal for (N⁺/O⁺)_uv (obtained from the N II] 2142 and [O II] 2471 lines) and (N⁺/O⁺)_opt (obtained from the [N II] 6585 and [O II] 3728 lines), we obtain the following. For the (N⁺, O⁺) zone, the average electron density is ~7000 cm^-3, the average electron temperature is 9500 K, t² = 0.032, and N⁺/O⁺ = 0.14. By comparing our FOS observations to predicted fluxes, utilizing our two previous photoionization models, we are able to derive the N/O ratio. There is fairly good agreement between (N/O)_uv and (N/O)_opt as derived from the two models with a range between 0.13 and 0.18. This range also encompasses our model-derived values for (N/O)_ir (0.17-0.18), which fit the observed far-infrared line ratio [N III] 57 μm/[O III] 52 μm. The empirically derived N⁺/O⁺ value requires a correction for the possibility that the N⁺ and O⁺ regions are not identical. Our overall results place the gas-phase Orion N/O ratio in the range 0.13-0.18, which is somewhat higher than solar. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at STScI, which is operated by AURA, Inc., under NASA contract NAS 5-26555.

Document Type

Article

Publication Date

3-10-1998

Notes/Citation Information

Published in The Astrophysical Journal, v. 495, no. 2, p. 891-904.

© 1998. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

The copyright holder has granted permission for posting the article here.

Digital Object Identifier (DOI)

http://dx.doi.org/10.1086/305326

Repository Citation

Rubin, R. H.; Martin, P. G.; Dufour, R. J.; Ferland, Gary J.; Baldwin, J. A.; Hester, J. J.; and Walter, D. K., "Temperature Variations and N/O in the Orion Nebula from HST Observations" (1998). Physics and Astronomy Faculty Publications. 118.
https://uknowledge.uky.edu/physastron_facpub/118