Density gradients and internal dust in the Orion nebula
  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. 
L. Binette, D. Gonzalez-Gomez, Y. D. Mayya