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The origin of the diverse light-curve shapes of Type II supernovae (SNe), andwhether they come from similar or distinct progenitors, has been activelydiscussed for decades. Here we report spectropolarimetry of two fast decliningType II (Type IIL) SNe: SN 2013ej and SN 2017ahn. SN 2013ej exhibited highcontinuum polarization from very soon after the explosion to the radioactivetail phase with time-variable polarization angles. The origin of thispolarimetric behavior can be interpreted as the combination of two differentaspherical structures, namely an aspherical interaction of the SN ejecta withcircumstellar matter (CSM) and an inherently aspherical explosion. Asphericalexplosions are a common feature of slowly declining Type II (Type IIP) SNe. Bycontrast, SN 2017ahn showed low polarization not only in the photospheric phasebut also in the radioactive tail phase. This low polarization in the tailphase, which has never before been observed in other Type IIP/L SNe, suggeststhat the explosion of SN 2017ahn was nearly spherical. These observations implythat Type IIL SNe have, at least, two different origins: they result from starsthat have different explosion properties and/or different mass-loss processes.This fact might indicate that 13ej-like Type IIL SNe originate from a similarprogenitor to those of Type IIP SNe accompanied by an aspherical CSMinteraction, while 17ahn-like Type IIL SNe come from a more massive progenitorwith less hydrogen in its envelope.

Evidence for multiple origins of fast declining Type II supernovae from spectropolarimetry of SN 2013ej and SN 2017ahn