Fallback in bipolar Planetary Nebulae ?
  The subclass of bipolar Planetary Nebulae (PN) exhibits well-defined
low-power outflows and some show shock-related equatorial spiderweb structures
and hourglass structures surrounding these outflows. These structures are
distinctly different from the phenomena associated with spherical and
elliptical PN and suggest a non-standard way to simultaneously energise both
kind of structures. This paper presents evidence from the published literature
on bipolar PN Hb\,12 and other sources in support of an alternative scenario
for energizing these structures by means of accretion from material shells
deposited during earlier post-AGB and pre-PN evolutionary stages. In addition
to energizing the bipolar outflow, a sub-Eddington accretion scenario could
hydrodynamically explain the spiderweb and outer hourglass structures as
oblique shockwaves for guiding the accreting material into the equatorial
region of the source. Estimates of the accretion rate resulting from
fallback-related spherical accretion could indeed help to drive a low-power
outflow and contribute to the total luminosity of these sources.
