Opened 8 years ago

#200 new enhancement

update N II recombintaion rates

Reported by: Gary J. Ferland Owned by: nobody
Priority: major Milestone: C13 branch
Component: atomic/molecular data base Version: trunk
Keywords: Cc:


New effective recombination coefficients for nebular Nii lines ?

  1. Fang1, P. J. Storey2 and X.-W. Liu1;3

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 Hii 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, 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 e ective recombination coecients of abundant heavy element ions such as Cii, Oii, Nii, and Ne ii, appropriate for the physical conditions prevailing in those cold inclusions (e.g. Te 1; 000 K ). Methods. Here we report new ab initio calculations of the e ective recombination coecients for the Nii recombination spectrum. We have taken into account the density dependence of the coecients 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 Nii with n 11 and l 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 coecients, including radiative and dielectronic recombination. Also new is including the e ects of dielectronic recombination via high-n resonances lying between the 2Po 1=2 and 2Po 3=2 levels. The new calculations are valid for temperatures down to an unprecedentedly low level (approximately 100 K). The newly calculated e ective recombination coecients allow us to construct plasma diagnostics based on the measured strengths of the Nii optical recombination lines (ORLs). Results. The derived e ective recombination coecients are fitted with analytic formulae as a function of electron temperature for di erent electron densities. The dependence of the emissivities of the strongest transitions of Nii 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.

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