Optical Color of Type Ib and Ic Supernovae and Implications for Their Progenitors

Type Ib and Ic supernovae (SNe Ib/Ic) originate from hydrogen-deficient massive star progenitors, of which the exact properties are still much debated. Using SN data in the literature, we investigate the optical B - V color of SNe Ib/Ic at the V-band peak and show that SNe Ib are systematically bluer than SNe Ic. We construct SN models from helium-rich and helium-poor progenitors of various masses using the radiation hydrodynamics code STELLA and discuss how the B - V color at the V-band peak is affected by the Ni-56 to ejecta mass ratio, Ni-56 mixing, and the presence/absence of a helium envelope. We argue that the dichotomy in the amount of helium in the progenitors plays the primary role in making the observed systematic color difference at the optical peak, in favor of the most commonly invoked SN scenario that SNe Ib and SNe Ic progenitors are helium rich and helium poor, respectively.