Late-time Circumstellar Interaction of SN 2017eaw in NGC 6946

SN 2017eaw, the tenth supernova observed in NGC 6946, was a normal Type II-P supernova with an estimated 11-13 M_⊙ red supergiant progenitor. Here we present nebular-phase spectra of SN 2017eaw at +545 and +900 days post-max, extending approximately 50-400 days past the epochs of previously published spectra. While the +545 day spectrum is similar to spectra taken between days +400 and +493, the +900 day spectrum shows dramatic changes both in spectral features and emission-line profiles. The Hα emission is flat-topped and boxlike with sharp blue and red profile velocities of ≃-8000 and +7500 km s^-1. These late-time spectral changes indicate strong circumstellar interaction with a mass-loss shell, expelled ∼1700 yr before explosion. SN 2017eaw's +900 day spectrum is similar to those seen for SN 2004et and SN 2013ej observed 2-3 yr after explosion. We discuss the importance of late-time monitoring of bright SNe II-P and the nature of presupernova mass-loss events for SN II-P evolution.