THE OPTICAL-SPECTRUM OF NGC-7009 .1. A LOW-EXCITATION BRIGHT RING REGION ON THE MAJOR AXIS

Recent observations suggest that NGC 7009 has a very complicated struc ture. Detailed high-spectral-resolution observations of the spectrum o f NGC 7009 have been made for two regions in the bright ring on the ma jor and minor axes, for the wavelength region 3650 to 10 050 Angstrom, using the Hamilton echelle spectrograph of the Lick Observatory. In t his article we present only the results for the major axis. We have an alysed the nebular spectra with aid of theoretical photoionization mod els and come to the following conclusions: (i) it appears that such a photoionization model can fit the emission reasonably well in the brig ht ring; (ii) it is unlikely that the knots lie along an axis that is highly inclined with respect to the observer, an idea which was introd uced from kinematical studies, wherein the knots are considered as a b yproduct of highly collimated outflows. NGC 7009 is oxygen-rich becaus e the O/C ratio exceeds 1. The relative abundances of C, O and K appea r to be smaller than in the Sun; abundances of other 'metals' seem to be comparable with the solar values. With the help of the model, a com parison of the Hamilton data with the previous image-tube scanner (ITS ) data suggests that a variation in nitrogen abundance may exist in NG C 7009, a result in harmony with that obtained by previous workers.