The abundance discrepancy in ionized nebulae: which are the correct abundances?

TL;DR

This paper reviews the longstanding abundance discrepancy problem in ionized nebulae, exploring differences in heavy-element abundance measurements from different spectral lines and discussing recent insights into temperature inhomogeneities and their implications.

Contribution

It provides a comprehensive overview of the current understanding of the abundance discrepancy in H II regions and planetary nebulae, highlighting recent developments and unresolved issues.

Findings

Temperature inhomogeneities affect abundance measurements in H II regions.

Planetary nebulae show different abundance discrepancy trends.

The origin of the discrepancy may differ between nebula types.

Abstract

Ionized nebulae are key to understanding the chemical composition and evolution of the Universe. Among these nebulae, H~{\sc ii} regions and planetary nebulae are particularly important as they provide insights into the present and past chemical composition of the interstellar medium, along with the nucleosynthetic processes involved in the chemical evolution of the gas. However, the heavy-element abundances derived from collisional excited lines (CELs) and recombination lines (RLs) do not align. This longstanding abundance-discrepancy problem calls into question our absolute abundance determinations. Which of the lines (if any) provides the correct heavy-element abundances? Recently, it has been shown that there are temperature inhomogeneities concentrated within the highly ionized gas of the H~{\sc ii} regions, causing the reported discrepancy. However, planetary nebulae do not exhibit the same trends as the H~{\sc ii} regions, suggesting a different origin for the abundance discrepancy. In this proceedings, we briefly discuss the state-of-the-art of the abundance discrepancy problem in both H~{\sc ii} regions and planetary nebulae.