NEAR-INFRARED SPECTRA OF THE ORION BAR

We have used the LONGSP spectrometer on the 1.5-m TIRGO telescope to o btain long slit spectra in the J, H, and K wavelength bands towards tw o positions along the Orion bar. These data have been supplemented wit h 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 var iety of transitions of hydrogen, helium, OI, FeII, FeIII, and H-2. Fro m our molecular hydrogen data, we conclude that densities are moderate (3 - 6 x 10(4) cm(-3)) in the layer responsible for the molecular hyd rogen emission and give no evidence for the presence of dense neutral clumps. We also find that the molecular hydrogen bar is likely to be t ilted by similar to 10 degrees relative to the line of sight. We discu ss the relative merits of several models of the structure of the bar a nd 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 mu m OI line to estimate the FUV radiation field incident at the ionization front and fmd values of 1 - 3 x 10(4) greater than the average interstellar field. From [FeII] line measurements, we conclud e that the electron density in the ionized layer associated with the i onization 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 Stromgren spheres.