Low-temperature recombination coefficients and nova-shell abundances

Capture–collision–cascade models developed for calculating recombination spectra of H I, He I and CNO ions from a neutral to a doubly ionized state are described. The method of solution closely follows the treatment of Brocklehurst. The models for complex atoms use LS coupling mainly, although for small principal quantum numbers other coupling schemes have also been used. The results of computations at low temperatures, i.e. between 312.5 and 2500 K, are presented. Forbidden-line fluxes, formed by recombination, are also listed. The H I results supplement the work of Martin and Brocklehurst. The He I results use newer data than the calculations of Brocklehurst and highlight an error in his work. Many references to atomic data post-dating the compendium of Mendoza are given for CNO atoms. The data are then used to check the line identifications in the spectra of the cool nova shells of DQ Her, T Aur and CP Pup. This work confirms that these nova shells are metal enriched, but the enhancements are less extreme then suggested in the original analyses. Many of the CNO lines can be explained as being formed by recombination in a cool gas, although temperature and density inhomogeneities cannot be excluded. The [N II] |λ λ6584, 6550| and [O II ] |λ 3727| lines do not appear to be due to recombination and are not fully understood.