The structures of PN shells and halos are studied in order to find clues to the history of mass ejection in the AGB stage. Data are presented which show that the emission measure profiles of shells of PN share several common observational properties. Constraints are placed on the form of the mass ejection that formed the shell, the external pressure that forces its postejection hydrodynamic evolution to the present, and the evolutionary timescales. Hydrodynamical simulations conducted to model the emission measure distribution of the shells and halos of PN show that mass is ejected in pulses whose duration is short compared to the AGB-PN evolutionary timescale, and that the linear form of the emission measure occurs about 5000 yr after the superwind ejection ceases.
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Frank et al. (1990) studied this question.