Numerical modeling of the evolution of stellar wind cavities and supernova remnants
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North-West University, Potchefstroom Campus
Abstract
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.
Description
PhD (Physics), North-West University, Potchefstroom Campus, 2017