Astron. Astrophys. 364, 301-317 (2000)

The structure of the Orion bar ^*

C.M. Walmsley, A. Natta, E. Oliva and L. Testi

Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received 21 April 2000 / Accepted 15 August 2000

Abstract

We have obtained NIR spectral line maps of a limited region around the Orion Bar ionization front using the red and blue grisms of SOFI on the ESO NTT telescope. The wavelength range covered was from 0.95 to 2.3 . In addition to many well studied features, we detected two lines at 1.07 and 1.17 due to recombination to CI which have excitations above 60000 cm^-1. We also observe several features at around 1 micron of atomic nitrogen (at still higher excitations) which we believe are caused by fluorescence of UV lines at approximately 955 Å in gas within the ionization front. We give a brief analysis of this process showing that one can infer the relative UV intensities incident upon the ionization front on the basis of the NIR spectra. Analogously, the spatial distribution of the fluorescent OI 1.317 line allows one to infer the geometry of the ionization front. We consider various geometries and conclude that the Orion Bar is most likely to be a cylinder or filament in the plane of the sky. Maps in the 9840 Å CI line were also obtained and compared with the corresponding intensity distribution in C91 from Wyrowski et al. (1997). These are emitted from the same general area but show differences in detail which are likely partly to be due to patchy extinction. The CI NIR line correlates well with molecular hydrogen emission away from the high density high radiation field layers of the Bar. The measured CI line ratios (in particular the ratio of the 0.984 and 1.07 lines) suggest higher temperatures in the C⁺ layer than allowed from the measured radio line width. The explanation for this discrepancy is not clear but it points up the need for observations in the NIR with higher spectral resolution and in the radio with higher spatial resolution.

Key words: ISM: clouds ISM: molecules radio lines: ISM

* Based on observations obtained at the European Southern Observatory, Chile

Send offprint requests to: M. Walmsley (walmsley@arcetri.astro.it)

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Contents

• 1. Introduction
• 2. Observations
  • 2.1. SOFI observations with the NTT
• 3. Results
  • 3.1. Observations with slits perpendicular to the bar
  • 3.2. Comparison of maps in different tracers
• 4. Data analysis
  • 4.1. Hydrogen lines
  • 4.2. Helium
  • 4.3. Iron lines
  • 4.4. Molecular hyhdrogen lines
• 5. Oxygen and Nitrogen fluorescent lines
  • 5.1. Excitation processes
  • 5.2. Illumination models
• 6. Carbon recombination lines in the Bar
• 7. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
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