(Oxford University Press (OUP), 1981) Wilkes, Belinda; Ferland, George; Hanes, D.; Truran, J. W.
Observations of N II recombination lines in the planetary nebula NGC 3242 are presented which allow the first temperature independent measurement of the nitrogen abundance of a nebula. Model nebula calculations are utilized to identify a reliable N/H indicator. The standard technique employing strong forbidden lines and the approximation that N/O ≃ N+/O+ may introduce systematic errors greater than 25 per cent. A good compromise is to set N/He = (N2+ + N+)/He+. The N/H abundance deduced in this manner is in reasonable (≃ 25 per cent) agreement with that deduced from the standard analysis.
