Late-time Ejecta-CSM Interaction in the Type II-P SN 2017eaw

In evolving core-collapse supernovae (CCSNe) interaction between the outwardly expanding forward shock and clumpy ejecta with dense circumstellar material associated with pre-supernova mass-loss can be used to explore the progenitor's mass-loss history decades to centuries prior to the explosion. We present late-time nebular-phase spectroscopy of SN 2017eaw, the tenth supernova observed in NGC 6946. SN 2017eaw is a normal Type II-P with an estimated 11-13 Msun red supergiant progenitor. We observed significant changes in the spectral features and emission lines profiles between days +545 and +900. At +900 days the Hα emission is flat-topped and box-like with velocities between ~-8000 and +7500 km/s. This emission is indicative of interaction between the expanding ejecta and a shell of circumstellar material, expelled ~1700 yr before explosion. SN 2017eaw is the fourth SN II-P observed with a flat-topped Halpha emission 2-3 years after explosion. SN 2017eaw provides an example of the importance of late-time monitoring of bright SNe and how pre-supernova mass-loss affects SNe evolution.