What is the primary mechanism causing the onset of the pressure-driven snowplow (radiative) phase?
Answer
The shell cools sufficiently (down to around $10^6$ K) allowing atomic recombination and efficient energy radiation.
The radiative phase starts when the shell has cooled enough (around $10^6$ K) that atomic electrons can recombine with heavy ions, enabling the shell to radiate energy away much more efficiently, leading to cooling and densification.
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