What velocity characterizes the ejecta during the initial Free Expansion phase of an SNR?
Answer
Exceeding 10,000 km/s because the ejecta mass greatly exceeds encountered interstellar gas mass.
The Free Expansion phase is defined by the remnant expanding unimpeded because the mass of the ejected stellar material far outweighs the mass of the surrounding interstellar medium (ISM) encountered initially. This lack of interaction allows the ejecta to maintain extremely high velocities, specified as often exceeding 10,000 km/s immediately following the explosion. This initial phase is relatively brief, typically lasting only a few hundred years, after which the expanding material begins to significantly interact with and sweep up the ISM, transitioning into the next dynamical stage.
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