Orientation effects in Bipolar nebulae
  We show that the inclination to the line of sight of bipolar nebulae strongly affects some of their observed properties. We model these objects as having a spherically symmetric Planetary Nebula and a dusty equatorial density enhancement that produces extinction that varies with the viewing angle. Our sample of 29 nebulae taken from the literature shows a clear correlation between the inclination angle and the near-IR and optical photometric properties as well as the apparent luminosity of the objects. As the inclination angle increases --the viewing angle is closer to the equatorial plane-- the objects become redder, their average apparent luminosity decreases, and their average projected expansion velocity becomes smaller.   We compute two-dimensional models of stars embedded in dusty disks of various shapes and compositions and show that the observed data can be reproduced by disk-star combinations with reasonable parameters. To compare with the observational data, we generate sets of model data by randomly varying the star and disk parameters within a physically meaningful range.   We conclude that a only a smooth pole to equator density gradient agrees with the observed phenomena and that thin, equatorially concentrated disks can be discarded. 
Hugo E. Schwarz, Hektor Monteiro, & Ryan Peterson