NE III charge-exchange lines in the planetary nebula NGC 3918.

Abstract

The detection of Ne III triplet and quintet lines near 2600 Å in the planetary nebula NGC 3918 is reported. They are excited mainly by charge-transfer reactions between Ne³⁺ and H⁰. Empirical reaction rates for triplet and quintet exit channels, deduced from a photoionization model of the nebula and the line strengths, are within a factor 2 of the theoretical rates. The relative strengths of O III singlet and triplet charge-transfer lines in NGC 3918 are also compared with theoretical calculations.