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Why can measurements of oxygen abundance differ in the same planetary nebula?

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Planetary nebula Southern Ring Nebula as captured by the James Webb Space Telescope in the near-infrared (left) and the mid-infrared (right). The dimmer star at the center of this scene has been sending out rings of gas and dust for thousands of years in all directions. The Southern Ring Nebula is approximately 2,500 light-years away.

In planetary nebulae, measurements of oxygen abundance often differ depending on whether they are derived from collisionally excited lines or recombination lines.12

The primary method, which relies on bright lines produced by collisions between ions and energetic electrons, typically yields lower abundance values than the method using faint emission lines that follow the recombination process. Observations show that abundances derived from recombination lines are often about 1.8 times higher than those obtained from collisionally excited lines.1

While the exact cause of this discrepancy remains a subject of scientific debate, a common interpretation is that strong temperature fluctuations—estimated at roughly 20 percent of the average—exist within the nebulae. In warmer regions, collisionally excited lines are produced more efficiently, leading to an overestimation of the line strength relative to the actual number of heavy ions present. Other proposed, though unproven, explanations include the presence of chemical inhomogeneities within the nebulae. Currently, no definitive consensus exists, and astronomers continue to use these methods despite the persistent discrepancies.1