Numerical modeling of the evolution of stellar wind cavities and supernova remnants
Van der Schyff, Augusts Eric
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An astrosphere is a low density cavity that results from an outflowing supersonic wind. Of particular interest in this study is the effect of radiative cooling on the computed evolution of astrospheres created by O and B type stars. These stars are selected because their relatively large cavities results in effective radiative cooling. For this purpose, an existing hydrodynamic numerical model is adapted to include the effects of radiative cooling and magnetic pressure. Numerical calculations are performed, and the results from computations including radiative cooling and those without are compared throughout this work. Radiative cooling is found to have a significant impact on the evolution of astrospheres. It is found that the choice of a cooling function as a parameter in the model is not trivial and can impact the evolution of the computed astrosphere. The interstellar magnetic field is similarly found to be important and results in radiative cooling being less efficient if the magnetic pressure is comparable to the thermal pressure. Also shown is that relative motion results in a more bullet shaped cavity, and the inclusion of radiative cooling results in more compression at the bow shock than cor- responding results without radiative cooling. It is also found that for stars with relative motion the magnetic pressure results in radiative cooling less efficient when this pressure is compa- rable to the thermal pressure. Supernova remnant evolution is also studied for a case with a pre-existing cavity and then compared to the supernova remnant evolution in a uniform and undisturbed interstellar medium. Radiative cooling is found to impact supernova rem- nant evolution at later stages of evolution. The supernova remnant evolution in a pre-exciting cavity is found to result in reflected and transmitted shocks when the forward shock of the supernova interacts with the outer structures of the pre-existing cavity. This is not the case for evolution in the undisturbed interstellar medium. Lastly, the transport of galactic cosmic rays into these simulated astrospheres is studied using a newly developed stochastic differential equation approach. Radiative cooling is assumed when the original cavity is computed and the modulation of these particles is studied. The effect on the modulation of cosmic rays is shown as this cavity expands into the interstellar medium. The transport of galactic cosmic rays in these simulated astrospheres is found to be dependent on the stellar mean free path, the energy of the particles and the shape of the cavity which can be influenced by radiative cooling.