Chemical abundances of planetary nebulae from optical recombination lines - II. The neon abundance of NGC 7009

Abstract

We present high-quality observations of Ne ii optical recombination lines (ORLs) for the bright Saturn Nebula NGC 7009. The measured line fluxes are used to determine Ne2+/H+ abundance ratios. The results derived from individual multiplets of the 3s–3p and 3p–3d configurations agree reasonably well, although values derived from the 3d–4f transitions, for which only preliminary effective recombination coefficients are available, tend to be higher by a factor of 2 than those derived from the 3–3 transitions – a pattern also seen in other nebulae analysed by us previously. The ORL Ne2+/H+ abundance ratios of NGC 7009 are found to be higher by a factor of 4 than those derived from the optical collisionally excited lines [Ne iii] λλ3868, 3967 and from the infrared fine-structure lines [Ne iii] 15.5 and 36 μm, similar to the patterns found for C, N and O, analysed previously by Liu et al. The result is in line with the general conclusion that while the ratios of heavy-element abundances, derived from ORLs on the one hand and from CELs on the other hand, vary from target to target and cover a wide range from unity to more than an order of magnitude, the discrepancy factor for the individual elements, C, N, O and Ne, is found to be approximately the same magnitude for a given nebula, a result which may have a fundamental implication for understanding the underlying physical cause(s) of the large discrepancies between heavy-element abundances derived from these two types of emission line. The result also indicates that while the absolute abundances of heavy elements relative to hydrogen remain uncertain, the abundance ratios of heavy elements, such as C/O, N/O and Ne/O, are probably secure, provided that the same type of emission line, i.e. ORLs or CELs, is used to determine the abundances of both heavy elements involved in the ratio. For NGC 7009, the total neon abundances derived from the CELs and ORLs, on a logarithmic scale where H=12.0, are 8.24±0.08 and 8.84±0.25, respectively. The latter is about a factor of 5.5 higher than the solar neon abundance.