Electron densities in planetary nebulae, and the unusual characteristics of the [S II] emission zone
Received 19 June 1998 / Accepted 15 September 1998
We investigate the radial variation of electron densities in planetary nebulae, using values of deriving from the [S II] 6717/6730 line ratio. As a result, we are able to show that there is a sharp discontinuity in densities of order 1.4 dex close to nebular radii pc. It is proposed, as a consequence, that most nebulae contain two primary [S II] 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 [S II ]/H 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.
Key words: ISM: planetary nebulae: general surveys
Send offprint requests to: J.P. Phillips
© European Southern Observatory (ESO) 1998
Online publication: November 9, 1998