Very deep spectroscopy of the bright Saturn nebula NGC 7009: The optical recombination spectrum and new effective recombination coefficients

Abstract

We present very deep spectroscopic observations of the rich optical recombination line (ORL) spectrum of the bright Saturn nebula NGC 7009. Detailed spectral analyses, combined with systematic emission line identifications, in the whole optical range (3040–11 000 Å) give more than 1000 emission lines, over 80% of which are ORLs. New calculations of the N ii effective recombination coefficients in intermediate coupling scheme are carried out. These N ii atomic data, together with the most recent O ii and Ne ii effective recombination coefficients, are utilized in the spectral analyses of NGC 7009. Departure from LS coupling is noticed. Agreement is found between the observed and predicted relative intensities of ORLs. The elemental abundances of C, N, O, and Ne deduced from ORLs are systematically higher than those from collisionally excited lines (CELs) by a factor of 5–7.