| Stars with masses greater than eight times the Sun's explode as supernovae when they fail to extract energy from nuclear fusion. Some of these supernova progenitors shed their outermost hydrogen envelope and acquire high rates of rotation at their cores before they explode. During a supernova collapse phase these stars form a dense disk of material around a degenerate core, which in turn powers energetic jets of material that propagate through the stellar atmosphere outwards. These jets soon acquire bulk speeds close to the speed of light.;We perform a detailed analysis of numerical, hydrodynamic simulations of these jets to determine their stability to pressure waves that grow as Kelvin-Helmholtz modes from the interface between the jet and the stellar medium. We have discovered that the pressure profile of the star makes these jets unstable to such modes just before the jet breaks out compared to the region closest to the center. We have also discovered that only the most energetic jets predicted in this theory support such modes over the time it takes for the jet to reach the stellar surface.;When we follow the jets in the numerical simulations as they break out, we calculate that they become relativistic winds that carry within them enhancements in their pressure and velocity, which can turn into internal shocks. These shocks could accelerate charged particles that cool through gamma ray emission and thus convert the kinetic energy of the wind into radiation that we can detect as a long gamma-ray burst. However, we have found that the fastest moving core of this wind is unlikely to produce substantial internal shocks. Shocks are most likely to happen at an angle from the center of a conical wind. We present a test of this model for such emission geometry and its corresponding rate for detected gamma ray bursts above a given gamma-ray flux. |
