We discuss recent claims that the free electrons in ionized nebulae may not have a significantly Maxwellian velocity distribution. Supra-thermal electrons, electrons with much more energy than is encountered at electron temperatures found in nebulae, may solve the t2/ADF puzzle, the observations that abundances obtained from recombination and collisionally excited lines do not agree, and that different temperature indicators give different results. These non-Maxwellian electrons can be designated by the kappa formalism. We show that the distance over which heating rates change are much longer than the distance supra-thermal electrons can travel, and that the timescale to thermalize these electrons are much shorter than the heating or cooling timescales. These estimates show that supra-thermal electrons will have disappeared into the Maxwellian velocity distribution long before they affect the collisionally-excited forbidden and recombination lines, so the electron velocity distribution will be closely thermal.
GJF acknowledges support by NSF (1108928, 1109061, and 1412155), NASA (10- ATP10-0053, 10-ADAP10-0073, NNX12AH73G, and ATP13-0153), and STScI (HST-AR-13245, GO- 12560, HST-GO-12309, GO-13310.002-A, and HST- AR-13914). WJH acknowledges financial sup- port from DGAPA-UNAM through grant PAPIIT- IN11215. CRO’s participation was supported in part by HST program GO 12543. MP received partial support from CONACyT grant 241732.
Ferland, Gary J.; Henney, W. J.; O'Dell, C. R.; and Peimbert, M., "The Abundance Discrepancy Factor and t2 in Nebulae: Are Non-Thermal Electrons the Culprits?" (2016). Physics and Astronomy Faculty Publications. 475.