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. Te ≤ 1000 K).