SIMBAD references

Planetary nebula axial ratios have been used to investigate the intrinsic structures of sources having elliptical or circular morphologies, using methods previously applied to elliptical galaxies. The results appear to indicate that most such nebulae have intrinsic axial ratios β in the range 0.92->0.75, and that gradients of surface brightness with axial ratio are most consistent with oblate shell structures. It remains possible however, given the uncertainties in the analysis, that many circular or ellipsoidal planetary nebulae also have prolate configurations. Whilst there may be evidence for an evolution of axial ratio with nebular radius, there appears to be little variation with Galactic latitude. This would suggest that nebular asphericities are not, in the main, dependent upon interaction with the interstellar medium, and that the variation of axial ratio with progenitor mass is also quite low.