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David C. Nicholls led a 2012 investigation of discrepant nebular temperature measurements. The paper, with Michael A. Dopita and Ralph S. Sutherland, explores a Kappa distribution for electron energies instead of imposing a Maxwell-Boltzmann distribution.

Different spectral diagnostics weight electron energies differently. Consequently, a departure from the assumed distribution can change inferred temperatures and abundances in different ways. The study presents the nonthermal distribution as a physical explanation to test, rather than treating agreement obtained under a changed assumption as a direct measurement of the distribution itself.^[ref]

Michael Dopita modeled the physical processes producing emission from ionized astronomical gas. With David C. Nicholls and Ralph S. Sutherland, he investigated why different spectral lines can give discrepant electron temperatures and chemical abundances in H II regions and planetary nebulae.

Their proposed explanation changed the assumed electron kinetic energy distribution from a Maxwell-Boltzmann distribution to a Kappa distribution, which has a more populated high-energy tail. Because different atomic transitions sample different electron energies, this change affects their inferred temperatures differently. The paper presented a testable interpretation of the diagnostic discrepancy.^[ref]

Ralph S. Sutherland coauthored a study of probability distributions of electron kinetic energy in H II regions and planetary nebulae. With David C. Nicholls and Michael A. Dopita, he investigated whether deviations from a Maxwell-Boltzmann distribution could account for disagreements between different inferred temperatures and chemical abundances.

The authors explored a Kappa distribution with an enhanced high-energy population. The methodological point is that a temperature diagnostic depends on the assumed particle distribution; fitting different spectral lines can probe that assumption rather than merely repeat the same measurement.^[ref]

By David C. Nicholls, Michael A. Dopita and Ralph S. Sutherland. arXiv:1204.3880v2 (2012). Section 4, equation 18 and the following paragraph give a low-density Maxwellian oxygen-line approximation and its collision-strength qualification.