On the nature of supernovae Ib and Ic
  Using non-LTE time-dependent radiative-transfer calculations, we study the
impact of mixing and non-thermal processes associated with radioactive decay on
SN IIb/Ib/Ic light curves (LCs) and spectra. Starting with short-period binary
models of \leq5Msun He-rich stars (18-25Msun on the main-sequence), we produce
1.2B ejecta which we artificially mix to alter the chemical stratification.
While the total 56Ni mass influences the LC peak, the spatial distribution of
56Ni, controlled by mixing processes, impacts both the multi-band LCs and
spectra. With enhanced mixing, our synthetic LCs start their post-breakout
re-brightening phase earlier, follow a more gradual rise to peak, appear
redder, and fade faster after peak due to enhanced gamma-ray escape.
Non-thermal electrons, crucial for the production of HeI lines, deposit a
dominant fraction of their energy as heat. Because energy deposition is
generally local well after the LC peak, the broad HeI lines characteristic of
maximum-light SN IIb/Ib spectra require mixing that places 56Ni and helium
nuclei to within a gamma-ray mean-free-path. This requirement indicates that
SNe IIb and Ib most likely arise from the explosion of stripped-envelope
massive stars (main-sequence masses \leq25Msun) that have evolved through
mass-transfer in a binary system, rather than from more massive single WR
stars. In contrast, the lack of HeI lines in SNe Ic may result from a variety
of causes: A genuine helium deficiency; strongly-asymmetric mixing; weak
mixing; or a more massive, perhaps single, progenitor characterized by a larger
oxygen-rich core. Our models, subject to different mixing magnitudes, can
produce a variety of SN types, including IIb, IIc, Ib, and Ic. As it is poorly
constrained by explosion models, mixing challenges our ability to infer the
progenitor and explosion properties of SNe IIb/Ib/Ic.
