The numerical simulation of astrospheric evolution in different interstellar environments

Abstract

The interaction between the interstellar medium (ISM) and a stellar wind leads to the formation of an astrosphere. In this study, the evolution of astrospheres is simulated with a two-dimensional spherical hydrodynamic (HD) and magnetohydrodynamic (MHD) model, and a three-dimensional cartesian MHD model. The two-dimensional HD and MHD model of Fahr et al. (2000), Scherer & Ferreira (2005) and Ferreira & de Jager (2008) is first used to show the effect of radiative cooling on simulations and also to illustrate the effect that changing parameters such as the mass-loss rate, ISM density and the ISM magnetic field have on the astrospheric evolution. It was shown that radiative cooling has an effect on the overall astrosphere size, and the termination shock (TS), astropause (AP) and bow shock (BS) positions and compression ratios (defined as the difference in density between the shocked and unshocked uid). The compression ratios are also in uenced by the ISM density, along with the rate of cooling. Changing the mass-loss rate in uences the ram pressure, affecting the size of the astrosphere. The ISM magnetic pressure is dependent on the ISM magnetic field magnitude, and increases as the ISM magnetic field increases. Comparing different ISM magnetic field magnitude scenarios with each other shows that a higher ISM magnetic pressure in uences the outer shell of the astrosphere and allows the outer astrosheath (OAS) to decompress faster. For the second part of this study, the three-dimensional cartesian MHD model of Pen et al. (2003) is used to simulate the astrospheric expansion for different grid cell sizes and to show the effect that different ISM wind speeds and different out ow speeds have on the evolution of M-Dwarf astrospheres. The choice of grid cell size is important as it in uences the compression ratios of the TS and BS. Also computed were the astrospheres of LHS 1140 and Proxima Centauri. Comparing these simulations to results obtained by Herbst et al. (2020) showed that there are some differences between the astrospheres of LHS 1140 and Proxima Centauri, which can be attributed to the different models, grid geometries and parameters used.