Core-collapse explosions of Wolf–Rayet stars and the connection to Type IIb/Ib/Ic supernovae

Abstract

We present non-Local Thermodynamic Equilibrium (LTE) time-dependent radiative-transfer simulations of supernova (SN) IIb/Ib/Ic spectra and light curves, based on ∼1051 erg pistondriven ejecta, with and without 56Ni, produced from single and binary Wolf–Rayet (WR) stars evolved at solar and sub-solar metallicities. Our bolometric light curves show a 10- d long post-breakout plateau with a luminosity of 1–5 × 107 L , visually brighter by 10 mag than the progenitor WR star. In our 56Ni-rich models, with ∼3M ejecta masses, this plateau precedes a 20 to 30 d long re-brightening phase initiated by the outwarddiffusing heat wave powered by radioactive decay at depth. A larger ejecta mass or a deeper 56Ni location increases the heat diffusion time and acts to both delay and broaden the light-curve peak. Discriminating between the two effects requires spectroscopic modelling. In low ejecta-mass models with moderate mixing, γ -ray leakage starts as early as ∼50 d after explosion and causes the nebular luminosity to steeply decline by ∼0.02 mag d−1. Such signatures, which are observed in standard SNe IIb/Ib/Ic, are consistent with low-mass progenitors derived from a binary-star population. We propose that the majority of stars with an initial mass 20M yield SNe II-P if ‘effectively’ single, SNe IIb/ Ib/Ic if part of a close binary system, and SN-less black holes if more massive. Our ejecta, with outer hydrogen mass fractions as low as 0.01 and a total hydrogen mass of 0.001M , yield the characteristic SN IIb spectral morphology at early times. However at later times, ∼15 d after the explosion, only Hα may remain as a weak absorption feature. Our binary models, characterized by helium surface mass fractions of 0.85, systematically show He I lines during the post-breakout plateau, irrespective of the 56Ni abundance. Synthetic spectra show a strong sensitivity to metallicity, which offers the possibility to constrain it directly from SN spectroscopic modelling.