On nitrogen abundances of planetary nebulae

Abstract

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 is equal to approximately N(+)/O(+) may introduce systematic errors greater than 25%. A good compromise is to set N/He = (N2(+) + N(+))/He(+). The N/H abundance deduced in this manner is in reasonable (about 25%) agreement with that deduced from the standard analysis.