Are oxygen and neon enriched in PNe and is the current solar Ne/O abundance ratio underestimated?

Abstract

Abstract A thorough critical literature survey has been carried out for reliable measurements of oxygen and neon abundances of planetary nebulae (PNe) and H ii regions. By contrasting the results of PNe and of H ii 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 H ii 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 α-elements are significantly enriched at low metallicity (initial oxygen abundance ≲8.0) but not at metallicity higher than the Small Magellanic Cloud (SMC). The fact that Ne/O ratios measured in PNe are almost the same as those in H ii regions, regardless of the metallicity, suggest 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.