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... also, despite your protests, the [SII] is very clear and clean in the "brain coral" section. Well done.
Of interest is that there is almost (but not quite) no [SII] in the WR nebula, despite the high excitation state.
Hershel's Ring does show some [SII] though it is weak. Thor's Helmet shows some [SII] but is stronger in [NII], and importantly, it is much further out, and therefore possibly pre-existing material.
I would love to have a tame astrophysicist explain why there isn't any. Gas too dense for the forbidden transition, as the required excited state destroyed by thermal collisions? Area cleaned of old stuff by predominantly fresh young vital hydrogen and helium from the outer atmosphere of the WR star? Both of these? Or, as Ellery Queen would say, someone else entirely?
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