2008ApJ...675..380S

We show that the inclination to the line of sight of bipolar planetary nebulae strongly affects some of their observed properties. We model these objects as having 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-infrared and optical photometric properties as well as the detected luminosity of the objects. As the inclination angle increases (the viewing angle is closer to the equatorial plane) the objects become redder, their apparent luminosity decreases, and their projected expansion velocity becomes smaller. We compute two-dimensional models of stars embedded in dusty disklike structures 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 disklike structure parameters within a physically meaningful range. We conclude that a only a smooth pole to equator density gradient agrees with the observed phenomena.