Xuan Fang (Department of Astronomy, School of Physics, Peking University, China)
New ab-initio calculations of the effective recombination coefficients for the N II and O II recombination spectrum were carried out in the intermediate coupling scheme by Fang, Storey & Liu and Storey, respectively. Both calculations have taken into account the density dependence of effective recombination coefficients arising from the relative populations of the fine-structure levels of the ground term of the recombining ions. This opens up the possibility of electron density determination via recombination lines analysis. Also new is including the effects of dielectronic recombination via high-n resonances lying between the ground-term thresholds of the recombining ions. The two calculations are valid down to very low electron temperatures (~100 K for N II and ~390 K for O II). The new effective recombination coefficients allows plasma diagnostics based on the N II and O II optical recombination lines (ORLs). Very deep spectroscopy of planetary nebula NGC 7009 yields rich ORLs of heavy element ions. Accurate measurements of the most prominent ORLs of C II, N II, O II and Ne II were obtained through Gaussian profile fits. Electron temperatures and densities of NGC 7009 were determined from the N II and O II ORLs, using the new effective recombination coefficients, and both temperatures are close to 1000 K, which indicates these ORLs originate from very cold region. Plasma diagnostics give an electron temperature pattern in NGC 7009, Te(CELs) ≥ Te(H I BJ) ≥ Te(He I) ≥ Te(N II & O II ORLs), which has been predicted by the two-abundance nebular model of Liu et al. Ionic abundances of C, N, O and Ne were derived from ORLs with the electron temperatures determined from ORLs assumed. For the same ion, ORLs from different multiplets yield close abundance values, indicating that our current recombination theories of N II and O II are reliable. The elemental abundances of C, N, O and Ne relative to hydrogen derived from ORLs are higher than those given by the collisional excited lines (CELs) by a factor of 5 to 7, confirming the long-standing abundance discrepancy in PNe. The deep spectroscopy of NGC 7009 unveils the temperature structure in this PN, proves the existence of the cold, metal-rich component, and helps to assess the new atomic data.