Luminous Type II Short-Plateau Supernovae 2006Y, 2006ai, and 2016egz: A Transitional Class from Stripped Massive Red Supergiants

Abstract

The diversity of Type II supernovae (SNe II) is thought to be driven mainly by differences in their progenitor's hydrogen-rich (H-rich) envelope mass, with SNe IIP having long plateaus ($\sim 100$ days) and the most massive H-rich envelopes. However, it is an ongoing mystery why SNe II with short plateaus (tens of days) are rarely seen. Here, we present optical/near-infrared photometric and spectroscopic observations of luminous Type II short-plateau SNe 2006Y, 2006ai, and 2016egz. Their plateaus of about $50$-$70$ days and luminous optical peaks ($\lesssim -18.4$ mag) indicate significant pre-explosion mass loss resulting in partially stripped H-rich envelopes and early circumstellar material (CSM) interaction. We compute a large grid of MESA+STELLA single-star progenitor and light-curve models with various progenitor zero-age main-sequence (ZAMS) masses, mass-loss efficiencies, explosion energies, ${}^{56}$Ni masses, and CSM densities. Our model grid shows a continuous population of SNe IIP-IIL-IIb-like light-curve morphology in descending order of H-rich envelope mass. With large ${}^{56}$Ni masses ($\gtrsim 0.05M_{\odot }$), short-plateau SNe II lie in a confined parameter space as a transitional class between SNe IIL and IIb. For SNe 2006Y, 2006ai, and 2016egz, our findings suggest high-mass red supergiant (RSG) progenitors ($M_{\mathrm{ZAMS}}\simeq 18$-$22M_{\odot }$) with small H-rich envelope masses ($M_{{\mathrm{H}}_{\mathrm{env}}}\simeq 1.7M_{\odot }$) that have experienced enhanced mass loss ($\dot{M}\simeq 10^{-2}{M}_{\odot }{\mathrm{yr}}^{-1}$) for the last few decades before the explosion. If high-mass RSGs result in rare short-plateau SNe II, then these events might ease some of the apparent underrepresentation of higher-luminosity RSGs in observed SN II progenitor samples.

Updated to match the published version in ApJ