New effective recombination coefficients for nebular N II lines

TL;DR

This paper presents new ab initio calculations of effective recombination coefficients for nebular N II lines, incorporating density dependence and dielectronic recombination effects, enabling improved plasma diagnostics in nebulae.

Contribution

The study introduces the first density-dependent recombination coefficients for N II, derived from detailed R-matrix calculations, enhancing nebular plasma diagnostics.

Findings

Recombination coefficients valid down to 100 K.

Analytic fits provided for temperature and density dependence.

Method demonstrated for simultaneous electron temperature and density determination.

Abstract

Here we report new ${\it ab initio}$ calculations of the effective recombination coefficients for the \ion{N}{ii} recombination spectrum. We have taken into account the density dependence of the coefficients arising from the relative populations of the fine-structure levels of the ground state of the recombining ion, an elaboration that has not been attempted before for this ion, and it opens up the possibility of electron density determination via recombination line analysis. Photoionization cross-sections, bound state energies, and the oscillator strengths of \ion{N}{ii} with $n \leq 11$ and $l \leq 4$ have been obtained using the close-coupling R-matrix method in the intermediate coupling scheme. Photoionization data were computed that accurately map out the near-threshold resonances and were used to derive recombination coefficients, including radiative and dielectronic recombination. Also new is including the effects of dielectronic recombination via high-$n$ resonances lying between the $^2$P$^{\rm o}$\,$_{1/2}$ and $^2$P$^{\rm o}$\,$_{3/2}$ levels. The new calculations are valid for temperatures down to an unprecedentedly low level (approximately 100 K). The newly calculated effective recombination coefficients allow us to construct plasma diagnostics based on the measured strengths of the \ion{N}{ii} optical recombination lines (ORLs). The derived effective recombination coefficients are fitted with analytic formulae as a function of electron temperature for different electron densities. The dependence of the emissivities of the strongest transitions of \ion{N}{ii} on electron density and temperature is illustrated. Potential applications of the current data to electron density and temperature diagnostics for photoionized gaseous nebulae are discussed. We also present a method of determining electron temperature and density simultaneously.