Chemical Stratification in a Long Gamma-Ray Burst Cocoon and Early-time Spectral Signatures of Supernovae Associated with Gamma-Ray Bursts

We present the results of 3D hydrodynamic simulations gamma-ray burst (GRB) jet emanating from a massive star with particular focus on formation high-velocity quasi-spherical ejecta and jet-induced chemical mixing. Recent early-time optical observations supernovae associated GRBs (e.g., GRB 171205A/SN 2017iuk) indicate considerable amount heavy metals in outer layers ejecta. Using our simulations, we show that density structure implied by can be naturally reproduced powerful penetrating progenitor star. consider three representative models stripped star, standard jet, weak choked an extended circumstellar medium, to clarify differences dynamical evolution properties ejected materials. The successfully penetrates creates component (cocoon). mixing significantly contaminates cocoon elements have been otherwise embedded inner layer fail produce ultra-relativistic but different properties. discuss impact jet-star interactions expected electromagnetic signatures long how probe dynamics observations.