450 Days of Type II SN 2013ej in Optical and Near-Infrared

TL;DR

This study provides an extensive 450-day optical and near-infrared observation of the Type IIL supernova 2013ej, revealing its luminosity, spectral evolution, and progenitor characteristics, challenging previous assumptions about Type IIL progenitors.

Contribution

It offers detailed photometric and spectroscopic data of SN 2013ej, and constrains its progenitor mass to 12-15 solar masses, questioning the link between supernova type and progenitor mass.

Findings

SN 2013ej has a high peak luminosity but low nickel production.

The supernova's nebular lines suggest a progenitor mass of 12-15 M_sun.

Late-time decline is faster than expected from cobalt decay.

Abstract

We present optical and near-infrared photometric and spectroscopic observations of SN 2013ej, in galaxy M74, from 1 to 450 days after the explosion. SN 2013ej is a hydrogen-rich supernova, classified as a Type IIL due to its relatively fast decline following the initial peak. It has a relatively high peak luminosity (absolute magnitude M$_\rm{V}$ = -17.6) but a small $^{56}$Ni production of ~0.023 M$_\odot$. Its photospheric evolution is similar to other Type II SNe, with shallow absorption in the H{\alpha} profile typical for a Type IIL. During transition to the radioactive decay tail at ~100 days, we find the SN to grow bluer in B - V colour, in contrast to some other Type II supernovae. At late times, the bolometric light curve declined faster than expected from $^{56}$Co decay and we observed unusually broad and asymmetric nebular emission lines. Based on comparison of nebular emission lines most sensitive to the progenitor core mass, we find our observations are best matched to synthesized spectral models with a M$_\rm{ZAMS}$ = 12 - 15 M$_\odot$ progenitor. The derived mass range is similar to but not higher than the mass estimated for Type IIP progenitors. This is against the idea that Type IIL are from more massive stars. Observations are consistent with the SN having a progenitor with a relatively low-mass envelope.