A stronger He II 4686 in the spectrum of the planetary nebula NGC 6572
On a recent spectrogram of the planetary nebula NGC 6572 the authors have found He II 4686 clearly in emission. They review previous descriptions of the spectrum and conclude that He II 4686 is stronger than 20 years ago. They briefly discuss the possible implications of this result, and suggest a careful monitoring of several planetary nebulae.
Mendez, R. H. et al.
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12
1988