Interacting supernovae

Modern photometric surveys of the sky suggest that many, perhaps most supernovae (SNe) associated with the explosion of massive stars are influenced at an appreciable level by their interaction with circumstellar material (CSM). The photometric and spectroscopic diversity of these transients point to a wide range of CSM properties in terms of mass, extent, composition, and location relative to the exploding star, suggesting progenitors that cover from standard to the most extreme mass loss rates. Surveys at high-cadence catch massive stars at shock breakout and inform us on the immediate mass loss history before core collapse. In contrast, long-term monitoring of these SNe cover the transition to the birth of a SN remnant and document the progenitor mass loss that took place centuries to millennia before explosion. Interacting SNe are therefore not just extraordinary astrophysical laboratories to study radiation-dominated shocks and probe the distant Universe, they also open the path to novel and fundamental studies on stellar evolution, stellar stability, or mass loss in single and binary massive stars.