The origin of the H$\alpha$-like structure in nebular spectra of type IIb supernovae

Abstract

We investigate the origin of the H$\alpha$-like structure seen in late-phase nebular spectra of type IIb supernovae (SNe IIb) at $\sim 200$ days after the explosion. We compare the luminosities of emission lines in 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) that show the H$\alpha$-like emission in their nebular spectra. The luminosity of the H$\alpha$-like emission shows a tight correlation with the light curve peak magnitude, sharing the same behavior with other nebular lines. This result indicates that the H$\alpha$-like emission is powered by the radiative decay of $^{56}$Co. 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 H$\alpha$. While further analysis is limited by the available sample size, we find a hint 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 excess in the luminosity of the H$\alpha$-like feature than those with a single-peak light curve. This behavior indicates possible additional contribution from H$\alpha$. 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 diagnostics for the progenitor mass.