5.4.2. Abundance inhomogeneities Many studies have suggested
that structures seen in planetary nebulae (extended haloes,
condensations) have different composition from the main nebular
body, indicating that they are formed of material arising in
distinct mass loss episodes characterized by different chemical
compositions of the stellar winds. However, these differences in
chemical composition may be spurious, due inadequacies of the
adopted abundance determination scheme. For example, the knots
and other small scale structures seen in PNe are possibly the
result of instabilities or magnetic field shaping, and their
spectroscopic signature could be due to a difference in the
excitation conditions and not in the chemical composition. In
the following, some examples of such studies are presented,
adopting the view of their authors. a) Extended haloes NGC 6720,
the "ring nebula" is surrounded by two haloes: an inner one,
with petal-like morphology, and an outer one, perfectly
circular, as seen in the pictures of