  A thorough critical literature survey has been carried out for reliable
measurements of oxygen and neon abundances of planetary nebulae (PNe) and HII
regions. By contrasting the results of PNe and of HII regions, we aim to
address the issues of the evolution of oxygen and neon in the interstellar
medium (ISM) and in the late evolutionary phases of low- and intermediate-mass
stars (LIMS), as well as the currently hotly disputed solar Ne/O abundance
ratio. Through the comparisons, we find that neon abundance and Ne/O ratio
increase with increasing oxygen abundance in both types of nebulae, with
positive correlation coefficients larger than 0.75. The correlations suggest
different enrichment mechanisms for oxygen and neon in the ISM, in the sense
that the growth of neon is delayed compared to oxygen. The differences of
abundances between PNe and HII regions, are mainly attributed to the results of
nucleosynthesis and dredge-up processes that occurred in the progenitor stars
of PNe. We find that both these alpha-elements are significantly enriched at
low metallicity (initial oxygen abundance <= 8.0) but not at metallicity higher
than the SMC. The fact that Ne/O ratios measured in PNe are almost the same as
those in HII regions, regardless of the metallicity, suggests a very similar
production mechanism of neon and oxygen in intermediate mass stars (IMS) of low
initial metallicities and in more massive stars, a conjecture that requires
verification by further theoretical studies. This result also strongly suggests
that both the solar neon abundance and the Ne/O ratio should be revised upwards
by ~0.22 dex from the Asplund, Grevesse & Sauval values or by ~0.14 dex from
the Grevesse & Sauval values.
