Quote:
Originally Posted by Jon
There's also something going on here to do with a recombination front, which I think I just about understand :-) Along the lines of - recombination takes place, a 3->2 transition happens emitting an Ha photon, but then the Hydrogen immediately absorbs another photon of the same energy level bumping it back up to 3, because of the density of the Ha at this point in the nova evolution (unlike a nebula, for instance, where the much less dense Hydrogen will be more likely to continue to cascade down to 1).
Something like that.
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The above makes sense. Although I would expect the same to hold true for the other Balmer lines, so I can't see how this would explain the different intensities of the Balmer series.
I imagine (without any grounding) that it would be more likely to be simply temperature dependent, where the visible ejecta is at a temperature that favours excitation to n=3 and hence the 3->2 H alpha transition is dominant. In much the same way that the temperature of an A class star puts a lot of the hydrogen into n=2 and thus favours absorption in the Balmer lines. That could also account for the decrease in H beta, as the temperature decreases, less and less of the hydrogen is excited to n=4.
Unrelated to the discussion of the relative intensities of the Balmer lines, I don't have a grasp on what mechanism could drive this recombination front outwards through the ejecta. It must be doing this if we ascribe to the idea that the absorption systems are from the ejecta, that there is a velocity gradient embedded in it, and that this recombination front is the cause of the changing observed velocity of the absorption system.
I would imagine the 'sweet spot' for recombination to be highly temperature dependent, and therefore would expect it to migrate inwards to lower velocity over time, not outwards, as the ejecta expands and cools.
Clearly there is some fundamental part of this that I don't get
