SIMBAD references

We investigate the radial variation of electron densities in planetary nebulae, using values of n_e deriving from the [SII] λ6717/λ6730 line ratio. As a result, we are able to show that there is a sharp discontinuity in densities of order 1.4dex close to nebular radii R=0.1pc. It is proposed, as a consequence, that most nebulae contain two primary [SII] emission zones, with densities differing by a factor ∼10². The intensity of emission from the denser component increases by an order of magnitude where nebulae pass from radiation to density-bound expansion regimes, resulting in a corresponding discontinuous jump in [SII]/Hβ line ratios. The origins of these changes are not entirely clear, although one mechanism is investigated whereby the superwind outflows shock interact with exterior AGB envelopes. Finally, the derived trends in n_e(R) are used to determine distances for a further 262 nebulae. The resulting distance scale appears to be comparable to that of Daub (1982ApJ...260..612D) and Cahn et al. (1992A&AS...94..399C).