|MadSci Network: Astronomy|
The causes behind the oscillations in RR Lyrae and other variables are well understood, but are a bit subtle. The key thing is that the helium ionization zone is a partial ionization zone. Gases of more than one ionization state can exist together. There are typically two ionization zones in most stars. The hydrogen partial ionization zone contains neutral and ionized hydrogen (i.e., protons). The helium ionization zone, which is mainly responsible for the oscillations, contains both the first and second ionization states of helium (i.e., helium with one and zero electrons, respectively). The opacity in the partial ionization zone depends on two factors: the temperature and the density. Depending on the temperature of the star, the ionization zones can exist right at the surface, at some intermediate depth, or way down deep in the interior. For pulsating stars, these zones contain enough material to act as a piston for the oscillations but are not so deep that they can't rise against the outer layers. You are absolutely right that ionized gasses typically have lower opacity than neutral ones. Normally hotter gasses have lower opacity, too. Suppose there is some sort of compression in the gas to start things off.... Then what happens is that much of the energy available from the compression goes into ionizing the gas (actually increasing the fraction that is ionized) rather than strongly raising the overall temperature. The rise in density increases the opacity to a greater effect than the reduction of opacity from the higher ionization. Then the higher opacity produces an outward pressure on the partially ionized layers, and the stuff rises up. During the expansion, energy is released. The temperature does not go down much, but there is still a big release of energy as the electrons recombine with the atoms to produce more atoms in the next lower ionization state. As the stuff rises, it expands and the density goes down and with it the opacity. But the effect of the density changes is larger than the effect of the ionization changes, and the opacity declines. This is called the Zhevakin opacity mechanism.
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