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 $\kappa$ energy distributions, yet there is little observational evidence to verify or disprove this hypothesis. In this paper, we examine the influence of $\kappa$-distributed electrons on the emissivities of \ion{O}{2} recombination lines using an approximate method, where the rate coefficients for a $\kappa$ distribution are computed by summing Maxwellian-Boltzmann rate coefficients with appropriate weights. The results show that if invoking $\kappa$-distributed electrons, the temperatures derived from the [\ion{O}{3}] $(\lambda 4959+\lambda 5007)/\lambda 4363$ ratios could coincide with those estimated from the \ion{O}{2} $\lambda 4649/\lambda 4089$ ratios. However, the estimated temperatures and $\kappa$ values are not in agreement with those obtained through comparing the [\ion{O}{3}] $(\lambda 4959+\lambda 5007)/\lambda 4363$ ratios and the hydrogen recombination spectra, suggesting that the electron energy is unlikely to follow the $\kappa$-distributions over a global scale of the nebular regions. Nevertheless, based on this observation alone, we cannot definitely rule out the presence of $\kappa$-distributed electrons in some microstructures within nebulae.

arXiv comment18 pages, 8 figures, Accepted for publication in ApJ

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