Enormous explosion energy of Type IIP SN 2017gmr with bipolar Ni-56 ejecta

TL;DR

This study reveals that the peculiar Type IIP supernova 2017gmr had an extremely energetic explosion with bipolar Ni-56 ejecta, suggesting a magneto-rotational explosion mechanism and explaining its unusual light curve and spectral features.

Contribution

The paper provides hydrodynamic models indicating an enormous explosion energy and bipolar Ni-56 ejecta structure, highlighting the magneto-rotational explosion as the likely mechanism.

Findings

Explosion energy of about 10^52 erg

Bipolar Ni-56 ejecta structure observed

High-velocity features linked to circumstellar shell

Abstract

The unusual Type IIP SN 2017gmr is revisited in order to pinpoint the origin of its anomalous features, including the peculiar light curve after about 100 days. The hydrodynamic modelling suggests the enormous explosion energy of about 10^52 erg. We find that the light curve with the prolonged plateau/tail transition can be reproduced either in the model with a high hydrogen abundance in the inner ejecta and a large amount of radioactive Ni-56, or in the model with an additional central energy source associated with the fallback/magnetar interaction in the propeller regime. The asymmetry of the late H-alpha emission and the reported linear polarization are reproduced by the model of the bipolar Ni-56 ejecta. The similar bipolar structure of the oxygen distribution is responsible for the two-horn structure of the [O I] 6360, 6364 A emission. The bipolar Ni-56 structure along with the high explosion energy are indicative of the magneto-rotational explosion. We identify narrow high-velocity absorption features in H-alpha and He I 10830 A lines with their origin in the fragmented cold dense shell formed due to the outer ejecta deceleration in a confined circumstellar shell.