Abstract
The ionization structure of the Orion nebula can be described as a thin ionized skin on the surface of a dense cloud. We propose that a steep density stratification, described by a power law ( n ∝ x-2, where x is the distance from the ionization front), exhibits properties which agree with our long-slit spectrum of the Orion nebula. For instance, there exists a unicity relation between both the H β surface brightness or the ionization front [S II] density, and the scale L of the power law, where L is the distance between the ionization front and the onset of the density near the exciting star. Internal dust is required to obtain a simultaneously acceptable fit to both the [S II] density and the H β surface brightness observations. Nebular models containing small dust grains provide a better fit than large grains. The line ratio gradients observed along the slit are qualitatively reproduced by our density stratified models, assuming a stellar temperature of 38,000 K. Collisional deexcitation appears to be responsible for half of the gradient observed in the [N II] 5755/6583 temperature sensitive ratio. We propose that the empirical relationship found by Wen & O'Dell (1995) between density and stellar distance may possibly be due to a power-law density stratification.
