Seminario DAS // Mapping plasma temperatures in the Lagoon Nebula reveals the origin of a long-standing chemical abundance discrepancy

Speaker: Amrita Singh // PhD student.
Affiliation: Departamento de Astronomía-FCFM // Universidad de Chile.

Abstract: Heavy-element chemical abundances measured in ionized nebulae define the absolute chemical abundance scale used throughout astrophysics, underpinning studies of stellar nucleosynthesis, stellar feedback, galaxy evolution, and the baryon cycle. However, the reliability with which these interstellar chemical abundances can be measured has long been questioned because, for a given object, the abundance of an ion (doubly ionized oxygen, for instance) inferred from spectroscopic emission lines produced after the collisions of free electrons with ions (collisionally excited lines, CELs) is systematically lower, by factors of up to 3 than those inferred from emission lines produced by electron-ion recombination (recombination lines, RLs). This inconsistency, known as the “abundance discrepancy problem”, has plagued nebular astrophysics for more than 80 years, and remains one of the oldest unresolved problems. Despite decades of observational and theoretical effort, it has not been possible to directly connect this discrepancy to the local physical conditions in the emitting plasma.

In this talk, I will present the first spatially resolved map of the abundance discrepancy, allowing us to directly connect it with the local physical conditions of the gas, including electron density, ionization state, and electron temperature (Singh et al. 2026a). For the first time, we find that the abundance discrepancy directly traces electron temperature fluctuations in the emitting plasma, providing the strongest observational evidence to date for its physical origin. Our results demonstrate that the discrepancy is the observational signature of unresolved thermal structure within the nebula. We further develop a novel methodology to quantify both the underlying average electron temperature and the magnitude of the thermal fluctuations around it. Accounting for this thermal structure nearly eliminates the abundance discrepancy, providing a physical explanation for this long-standing problem (Singh et al. 2026b, submitted to ApJ).