The Effects of Thomson Scattering and Chemical Mixing on Early-time Light Curves of Double-peaked Type IIb Supernovae

Previous numerical simulations of double-peaked Type IIb supernova (SN IIb) light curves have demonstrated that the radius and mass of the hydrogen-rich envelope of the progenitor star can significantly influence the brightness and timescale of the early-time light curve around the first peak. In this study, we investigate how Thomson scattering and chemical mixing in the SN ejecta affect the optical light curves during the early stages of the SNe IIb using radiation hydrodynamics simulations. By comparing the results from two different numerical codes (i.e., STELLA and SNEC), we find that the optical brightness of the first peak can be reduced by more than a factor of 3 as a result of the effect of Thomson scattering that causes the thermalization depth to be located below the Rosseland mean photosphere, compared to the corresponding case where this effect is ignored. We also observe a short-lived plateau-like feature lasting for a few days in the early-time optical light curves of our models, in contrast to typical observed SNe IIb that show a quasi-linear decrease in optical magnitudes after the first peak. A significant degree of chemical mixing between the hydrogen-rich envelope and the helium core in SN ejecta is required to reconcile this discrepancy between the model prediction and observation. Meanwhile, to properly reproduce the first peak, a significant mixing of 56Ni into the hydrogen-rich outermost layers should be restricted. Our findings indicate that inferring the SN IIb progenitor structure from a simplified approach that ignores these two factors may introduce substantial uncertainty.