Near Infrared Spectra of the Orion Bar
A. Marconi (1,3), L. Testi (1,4), A. Natta (2), C.M. Walmsley (2)
(1) Dipartimento di Astronomia e Scienza dello Spazio,
Università di Firenze, Largo E. Fermi, 5, I-50125 Firenze, Italy
(2) Osservatorio Astrofisico di Arcetri,
Largo Enrico Fermi 5, I-50125 Firenze, Italy
(3) Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218
(4) Division of Physics, Mathematics and Astronomy, Caltech,
MS 105-24, Pasadena, CA 91125, USA
We have used the LONGSP spectrometer on the 1.5-m TIRGO telescope to
obtain long slit spectra in the J, H, and K wavelength bands
towards two positions along the Orion bar. These data have
been supplemented with images made using the ARNICA camera mounted
on TIRGO as well as with an ESO NTT observation carried out by
Dr A. Moorwood. We detect a variety of transitions of hydrogen,
helium, OI, FeII, FeIII, and H2. From our molecular hydrogen
data, we conclude that densities are moderate (3-6x10^4
cm-3) in the layer responsible for the
molecular hydrogen emission and
give no evidence for the presence of dense neutral clumps. We
also find that the molecular hydrogen bar is likely to be
tilted by ~10 degrees relative to the line of sight.
We discuss the relative merits of several models of the structure of
the bar and conclude that it may be split into two
structures separated by 0.2-0.3 parsec along the line of sight.
It also seems likely to us that in both structures,
density increases along a line
perpendicular to the ionization front which penetrates into
the neutral gas.
We have used the 1.317 um OI line to estimate the FUV
radiation field incident at the ionization front and find
values of 1-3x10^4 greater than
the average interstellar field.
From [FeII] line measurements,
we conclude that the electron density
in the ionized layer associated with the ionization front is of
order 10^4 cm-3.
Finally, our analysis of the helium and
hydrogen recombination lines implies essential coincidence of the
helium and hydrogen Stroemgren spheres.