SIMBAD references

An observational study of chemical abundances in the galactic planetary nebulae NGC1535, NGC2438, NGC2440, NGC3132, NGC3242, NGC6302, and NGC7009 based on long-slit spectra of high signal-to-noise ratio in the 3100 to 6900 Å range is presented. We determined the N, O, Ne, S, and Cl abundances from collisionally excited lines and the He and O⁺⁺ abundances from recombination lines. The O⁺⁺/H⁺ estimates derived from recombination lines are about a factor of four and two higher than those derived from forbidden lines for NGC7009 and NGC3242, respectively. Spatial profiles of O⁺⁺/H⁺ abundance from Oii permitted lines and from [OIII] forbidden lines were obtained for the planetary nebula NGC7009. The differences between O⁺⁺/H⁺ derived from recombination and from forbidden lines present smooth variations along the nebular surface of NGC7009, with the differences decreasing from the center to the edges of the nebula. If these abundance differences are explained by the presence of electron temperature fluctuations, quantified by the parameter t², a value of about t²=0.09 is required for NGC3242 and NGC7009.