The Origin of the Ha-like Structure in Nebular Spectra of Type IIb Supernovae

We investigate the origin of the Hα-like structure seen in late-phase nebular spectra of type IIb supernovae (SNe IIb) at ~200 days after the explosion. We compare the luminosities of the emission lines of the nebular spectra with the light curve peak magnitudes to reveal their power sources. In this work, we analyze 7 SNe IIb, as well as 2 SNe Ib (SN 2007Y and iPTF 13bvn), which show the Hα-like emission in their nebular spectra. The luminosity of the Hα-like emission shows a tight correlation with the light curve peak magnitude, sharing the same behavior as other nebular lines. This result indicates that the Hα-like emission is powered by the radiative decay of 56Co. The line flux is then expected to roughly follow the mass of the emitting layer. The variation expected from the diversity of the H-rich envelope mass among SNe IIb (reaching nearly to an order of magnitude) is however not seen, suggesting that it is most likely contributed predominantly by [N ii], not by Hα. While further analysis is limited by the available sample size, we find an indication that SNe IIb with a double-peak light curve, which is interpreted as an outcome of the more extended and massive hydrogen envelope, tend to show greater luminosity of the Hα-like feature than those with a single-peak light curve. This behavior indicates a possible additional contribution from Hα. Additionally, we also find a correlation between the [Ca ii]/[O i] ratio and the post-maximum decline rate, indicating that the [Ca ii]/[O i] can be used as a diagnostic for the progenitor mass.