2025ApJ...988L..53M

Circumstellar interaction of supernova (SN) ejecta is an essential process in its evolution, and observations of SNe have found the signature of circumstellar interaction in both the early and late evolutionary phases of SNe. In this Letter, we show that if the SN forward shock plunges into a tenuous stellar wind from a dense circumstellar medium (CSM) in the vicinity of the progenitor (i.e., confined CSM), the subsequent time evolutions of the SN-CSM interaction system deviate from the prediction of a self-similar solution. In this case, after all of the confined CSM is swept up by the SN forward shock (roughly 10 days after the explosion), the propagation of the shocked shell will be driven by the freely expanding ram pressure of the confined CSM component, instead of the SN ejecta. Meanwhile, the forward shock decelerates faster than the prediction of thin-shell approximation once the confined CSM component reaches homologous expansion. This lasts until the reverse shock in the confined CSM component reaches the head of the SN ejecta, leading to the restoration of the system into the evolutionary model without confined CSM, where the SN ejecta drives the expansion of the system. We also show that this peculiar evolution will be reflected in observational signatures originating from SN-CSM interaction, taking rapid decline and rebrightening of radio emission as examples. Our results shed light on the importance of taking into account the effect of initial SN-CSM interaction even when we focus on observational properties of SNe a few years after the explosion.