Detection of Late-Time Circumstellar Interaction of SN 2017eaw in NGC 6946

TL;DR

This paper presents late-time spectral observations of SN 2017eaw, revealing strong circumstellar interaction occurring hundreds of days after explosion, which provides insights into pre-supernova mass-loss events.

Contribution

It reports the detection of late-time circumstellar interaction in SN 2017eaw through nebular spectra taken at +545 and +900 days, extending previous observations and highlighting the importance of long-term monitoring.

Findings

Spectral changes at +900 days indicate strong circumstellar interaction.

Halpha emission shows flat-topped, box-like profile with high-velocity edges.

Interaction suggests a mass-loss shell expelled ~1700 years before explosion.

Abstract

SN 2017eaw, the tenth supernova observed in NGC 6946, was a normal Type II-P supernova with an estimated 11 - 13 Msun 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 spectra 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 Halpha emission is flat-topped and box-like with sharp blue and red profile velocities of ~ -8000 and +7500 km/s. These late-time spectral changes indicate strong circumstellar interaction with a mass-loss shell, expelled ~ 1700 years before explosion. SN 2017eaw's +900 day spectrum is similar to those seen for SN 2004et and SN 2013ej observed 2 - 3 years after explosion. We discuss the importance of late-time monitoring of bright SNe II-P and the nature of pre-supernova mass-loss events for SN II-P evolution.