Can the Kappa-distributed electron energies account for the intensity ratios of O II lines in photoionized gaseous nebulae?

Abstract

A vexing puzzle in the study of planetary nebulae and \ion{H}{2} regions is that the plasma diagnostic results based on collisionally excited lines systematically differ from those based on recombination lines. A fairly speculative interpretation is the presence of nonthermal electrons with the so-called $κ$ energy distributions, yet there is little observational evidence to verify or disprove this hypothesis. In this paper, we examine the influence of $κ$-distributed electrons on the emissivities of \ion{O}{2} recombination lines using an approximate method, where the rate coefficients for a $κ$ distribution are computed by summing Maxwellian-Boltzmann rate coefficients with appropriate weights. The results show that if invoking $κ$-distributed electrons, the temperatures derived from the [\ion{O}{3}] $(\lambda4959+\lambda5007)/\lambda4363$ ratios could coincide with those estimated from the \ion{O}{2} $\lambda4649/\lambda4089$ ratios. However, the estimated temperatures and $κ$ values are not in agreement with those obtained through comparing the [\ion{O}{3}] $(\lambda4959+\lambda5007)/\lambda4363$ ratios and the hydrogen recombination spectra, suggesting that the electron energy is unlikely to follow the $κ$-distributions over a global scale of the nebular regions. Nevertheless, based on this observation alone, we cannot definitely rule out the presence of $κ$-distributed electrons in some microstructures within nebulae.