Why do collisionally excited emission lines and recombination lines give different abundance estimates in H II regions?

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H II region Centre of the Orion Nebula (M42). Astronomers have identified some 700 young stars in this 2.5-light-year-wide area. They have also detected over 150 protoplanetary disks, or proplyds, which are believed to be embryonic solar systems that will eventually form planets. These stars and proplyds generate most of the nebula's light. This picture is a mosaic combining 45 images taken by the Hubble Space Telescope.

In H II regions, abundance estimates derived from collisionally excited lines and recombination lines consistently differ, with recombination lines typically yielding abundances about 1.8 times higher than those from collisionally excited lines.1

The primary explanation for this discrepancy is the hypothesis of strong temperature fluctuations within the nebulae. Because collisionally excited lines are highly sensitive to temperature, they are strongly overproduced in warmer regions; consequently, fewer heavy ions are required to explain the observed line strengths. In contrast, hydrogen lines and recombination lines are relatively unaffected by these fluctuations.1

While these temperature variations—estimated to be around 20 percent of the average—are widely cited as the cause, their physical origin remains unexplained, with turbulence and magnetic fields suggested as potential drivers. Alternative theories, such as chemical inhomogeneities within the nebulae, have been proposed, but no consensus has been reached on how such variations could be maintained.1