On the nature of supernovae Ib and Ic

Abstract

Using non-LTE time-dependent radiative-transfer calculations, we study the\nimpact of mixing and non-thermal processes associated with radioactive decay on\nSN IIb/Ib/Ic light curves (LCs) and spectra. Starting with short-period binary\nmodels of \\leq5Msun He-rich stars (18-25Msun on the main-sequence), we produce\n1.2B ejecta which we artificially mix to alter the chemical stratification.\nWhile the total 56Ni mass influences the LC peak, the spatial distribution of\n56Ni, controlled by mixing processes, impacts both the multi-band LCs and\nspectra. With enhanced mixing, our synthetic LCs start their post-breakout\nre-brightening phase earlier, follow a more gradual rise to peak, appear\nredder, and fade faster after peak due to enhanced gamma-ray escape.\nNon-thermal electrons, crucial for the production of HeI lines, deposit a\ndominant fraction of their energy as heat. Because energy deposition is\ngenerally local well after the LC peak, the broad HeI lines characteristic of\nmaximum-light SN IIb/Ib spectra require mixing that places 56Ni and helium\nnuclei to within a gamma-ray mean-free-path. This requirement indicates that\nSNe IIb and Ib most likely arise from the explosion of stripped-envelope\nmassive stars (main-sequence masses \\leq25Msun) that have evolved through\nmass-transfer in a binary system, rather than from more massive single WR\nstars. In contrast, the lack of HeI lines in SNe Ic may result from a variety\nof causes: A genuine helium deficiency; strongly-asymmetric mixing; weak\nmixing; or a more massive, perhaps single, progenitor characterized by a larger\noxygen-rich core. Our models, subject to different mixing magnitudes, can\nproduce a variety of SN types, including IIb, IIc, Ib, and Ic. As it is poorly\nconstrained by explosion models, mixing challenges our ability to infer the\nprogenitor and explosion properties of SNe IIb/Ib/Ic.\n