Electron Temperatures and Free-Electron Energy Distributions of Nebulae from C II Dielectronic Recombination Lines

TL;DR

This study uses theoretical and observational data of C II dielectronic recombination lines to determine electron temperatures and explore electron energy distributions in planetary nebulae, testing for deviations from Maxwell-Boltzmann distributions.

Contribution

It introduces a method combining theoretical line lists and observational data to simultaneously determine electron temperatures and assess electron energy distributions in nebulae.

Findings

Data best match Maxwell-Boltzmann distributions.

Uncertainties prevent excluding kappa-distributions.

Method can detect departures from Maxwellian distributions.

Abstract

A recently generated theoretical line list of C II dielectronic recombination lines together with observational data gathered from the literature is used to investigate the electron temperature in a range of astronomical objects, mainly planetary nebulae. The electron temperature is obtained by a least-squares optimisation using all the reliable observed lines in each object. In addition, the subset of lines arising directly from autoionising states is used to directly determine the free-electron energy distribution which is then compared with various theoretical possibilities. The method described here can potentially determine whether there are departures from Maxwell-Boltzmann distributions in some nebulae, as has been recently proposed. Using published observations of the three planetary nebulae where the relevant lines are recorded, we find that the data are best matched by Maxwell-Boltzmann distributions but that the uncertainties are sufficiently large at present that kappa-distributions or two-component nebular models are not excluded.