On the non-thermal kappa-distributed electrons in planetary nebulae and HII regions: the kappa index and its correlations with other nebular properties

TL;DR

This study investigates whether non-thermal kappa-distributed electrons exist in planetary nebulae and HII regions by analyzing spectral line data, but finds no supporting evidence for their presence.

Contribution

It introduces a method to estimate the kappa index in nebulae and applies it to a sample, concluding that non-thermal electrons are unlikely in these environments.

Findings

Average kappa indices are 27 for PNe and 32 for HII regions.

No significant correlation between kappa values and nebular properties.

Results do not support the presence of non-thermal electrons in the studied nebulae.

Abstract

Recently, a suspicion arose that the free electrons in planetary nebulae (PNe) and HII regions might have non-thermal energy distributions. In this scenario, a kappa index is introduced to characterize the electron energy distributions, with smaller kappa values indicating larger deviations from Maxwell-Boltzmann distributions. Assuming that this is the case, we determine the kappa values for a sample of PNe and HII regions by comparing the intensities of [OIII] collisionally excited lines and the hydrogen Balmer jump. We find the average kappa indices of PNe and HII regions to be 27 and 32, respectively. Correlations between the resultant kappa values and various physical properties of the nebulae are examined to explore the potential origin of non-thermal electrons in photoionized gaseous nebulae. However, no positive result is obtained. Thus the current analysis does not lend to support to the idea that kappa-distributed electrons are present in PNe and HII regions.