  The ionization structure of the Orion nebula can be described as a skin-like
ionization structure on the surface of a dense cloud. We propose that a steep
density stratification, increasing as a powerlaw ($n \propto x^{-2}$) function
of distance x from the ionization front, exhibits properties which agree with
our long-slit spectrum of the Orion nebula. For instance, there exist a unicity
relation between both the Hbeta surface brightness or the ionization front
[SII] density, and the scale L of the powerlaw, 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 simultaneous acceptable fit of both the
[SII] density and then Hbeta suface 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 38000 K.
Collisional deexcitation appears to be responsible for half of the gradient
observed in the [NII] 5755/6583 temperature sensitive ratio. We propose that
the empirical relationship found by Wen & O'dell between density and stellar
distance may possibly be caused by a power-law density stratification.

Astrophysics