New effective recombination coefficients for nebular N ii lines

Abstract

Aims. In nebular astrophysics, there has been a long-standing dichotomy in plasma diagnostics between abundance determinations using the traditional method based on collisionally excited lines (CELs), on the one hand, and (optical) recombination lines/continuum, on the other. A number of mechanisms have been proposed to explain the dichotomy. Deep spectroscopy and recombination line analysis of emission line nebulae (planetary nebulae and H ii regions) in the past decade have pointed to the existence of another previously unknown component of cold, H-deficient material as the culprit. Better constraints are needed on the physical conditions (electron temperature and density), chemical composition, mass, and spatial distribution of the postulated H-deficient inclusions in order to unravel their astrophysical origins. This requires knowledge of the relevant atomic parameters, most importantly the effective recombination coefficients of abundant heavy element ions such as C ii, O ii, N ii, and Ne ii, appropriate for the physical conditions prevailing in those cold inclusions (e.g. T e ≤ 1000 K). Methods. Here we report new ab initio calculations of the effective recombination coefficients for the 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 term of the recombining ion ( 2 P • 1/2 and 2 P • 3/2 in the case of N iii), 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 N ii with n ≤ 11 and l ≤ 4 have been obtained using the closecoupling 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 • 1/2 and 2 P • 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 N ii optical recombination lines (ORLs). Results. 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 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.

Statistical study of OB stars in NGC 6334 and NGC 6357

“…From our photometric determination of R V , we found for NGC 6357 and NGC 6334 a mean distance of 1.93±0.36 kpc and 1.72 ± 0.26 kpc, respectively, and a mean A V of 5.93 ± 0.49 mag (Bohigas et al 2004, give for Pismis 24 A V = 6.37 mag) and 4.52 ± 0.68 mag, respectively. Fang et al (2011), from isochrone fitting of O stars in Pismis 24, found a distance of 1.7 ± 0.2 kpc and a median extinction of 5.3 mag. We can then conclude that NGC 6334 and NGC 6357 are at an average distance of 1.75 kpc (weighted mean).…”

Atomic processes in planetary nebulae

“…Similar diagnostics have also been developed based on the N ii recombination spectrum. In the case of N ii, the intensity ratio of components λ5679 (J = 3-2) and λ5666 (J = 2-1) of Multiplet V 3 3p 3 D -3s 3 P o is a sensitive density diagnostic, whereas the ratio of the λ5679 line and the 4f G[9/2] 5 -3d 3 F o 4 λ4041 line, again the strongest from the 4f-3d configuration, is sensitive to temperature (Fang, Storey & Liu 2011).…”

“…New ab initio calculations of the effective recombination coefficients, valid down to very low temperatures and taking into account the dependence of the level populations of the ground states of the recombining ion, are now available for the recombination spectrum of O ii (Bastin & Storey 2006;Storey P. J., in preparation) and N ii (Fang, Storey & Liu 2011). The four panels on the left of Fig.…”

New effective recombination coefficients for nebular N ii lines(Corrigendum)

“…As a consequence, the effective recombination coefficients for the N ii lines that are directly related to the above five states were incorrect. We have corrected the labeling of the N ii energy levels and re-calculated the effective recombination coefficients for the N ii lines at the same electron temperature and density ranges as in Fang et al (2011). The newly calculated effective recombination coefficients are given in Tables 3-6.…”

“…">Effective recombination coefficientsErrors in the computed effective recombination coefficients for nebular N ii lines in an earlier publication (Fang et al 2011) have been discovered and are here corrected. Those errors were not due to the basic atomic physics (i.e.…”