The masses of the progenitors of planetary nebulae

Bipolar, elliptical, and circular planetary nebulae appear to be possessed of differing Galactic scale heights, a feature which is usually assumed to arise from the differing distributions and masses of their progenitor stars. It would appear that more massive progenitors are responsible for bipolar sources, whilst the least massive progenitors give rise to circular sources. This, clearly, is an important result, since it implies that shell formation mechanisms must be closely intertwined with progenitor type. We point out, however, that previous estimates of nebular scale height are at best somewhat sketchy and may even be wrong. At least two prior analyses, for instance, appear to compare reddened values of nebular scale height with extinction-corrected stellar scale heights. We have therefore attempted, in the following, to undertake a dereddening of nebular scale heights and perform a fresh comparison of these results with those for Galactic stars of differing spectral types. We find that circular and elliptical sources are likely to originate from early F-type stars and bipolar sources from stars of spectral type A7 or earlier. The results appear to be insensitive to uncertainties in the distance scale.