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

We present non-Local Thermodynamic Equilibrium (LTE) time-dependent radiative-transfer simulations of supernova (SN) IIb/Ib/Ic spectra and light curves, based on ∼10⁵¹erg piston-driven 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 × 10⁷L_⊙, visually brighter by ≳10mag 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 outward-diffusing 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 ∼50d after explosion and causes the nebular luminosity to steeply decline by ∼0.02mag 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, ∼15d 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 Hei 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.