Electron Energy Distributions in HII Regions and Planetary Nebulae: kappa-Distributions Do Not Apply

TL;DR

This paper demonstrates that electron energy distributions in H II regions and planetary nebulae are nearly Maxwellian, invalidating the use of kappa-distributions and attributing observed line ratio deviations to temperature variations.

Contribution

It provides evidence that electron energy distributions are almost Maxwellian, challenging previous suggestions of nonthermal kappa-distributions in these regions.

Findings

Electron distributions are close to Maxwellian up to ~13 eV in H II regions.

Electron distributions are close to Maxwellian up to ~16 eV in planetary nebulae.

Deviations in observed line ratios are due to temperature variations, not nonthermal distributions.

Abstract

Some authors have proposed that electron energy distributions in H II regions and planetary nebulae may be significantly nonthermal, and kappa-distributions have been suggested as being appropriate. Here it is demonstrated that the electron energy distribution function is extremely close to a Maxwellian up to electron kinetic energies ~13 eV in HII regions, and up to ~16eV in planetary nebulae: kappa-distributions are inappropriate. The small departures from a Maxwellian have negligible effects on line ratios. When observed line ratios in H II regions deviate from models with a single electron temperature, it must arise from spatial variations in electron temperature, rather than local deviations from a Maxwellian.