Numerical simulations of the evolution of astrospheres in different interstellar conditions

Abstract

Astrospheres are large-scale structures arising from the interaction between the stellar wind and the interstellar medium (ISM). This study investigates the effect of various parameters, such as the simulation time, mass loss rate, ISM density, ISM temperature, and ISM magnetic eld, on the evolution of an astrosphere using the two-dimensional magnetohydrodynamic model of Fahr and Kausch (2000). The emphasis is on calculating the density pro le, bow shock (BS) compression ratio, speed, and BS width as the stellar wind (SW) expands into the ISM. Results show that later simulation times lead to a more developed astrosphere with a thin outer shell forming, the thickness of which depends on the radiative cooling process. The mass loss rate affects the size of the astrosphere, with higher rates resulting in larger astrospheric structures. The ISM density also influences the size of the astrospheric cavity and the radiative cooling process. The ISM magnetic eld can either expand or compress the outer structure of the astrosphere, depending on its orientation. The study also investigates the astrosphere of λ Cephei. λ Cephei is a bright runaway star. Simulations of Baalmann et al. (2021) show the position of the termination shock (TS), astropause (AP), and bow shock (BS) of λ Cephei. A comparison between the simulations by Baalmann et al. (2021) and the Pen et al. (2003) model used in this work shows similar results. Due to the Pen et al. (2003) model excluding the radiative cooling process and being solved on a Cartesian grid, the bow shock width is somewhat different. The study also investigated variations in the relative velocity and ISM magnetic eld strength on λ Cephei's astrosphere, where, when the relative velocity was decreased from 80 km/s to 40 km/s. The size of the astrosphere increased, with the largest di erence seen in the bow shock regions. The e ect of the ISM magnetic eld on the astrosphere's form was also investigated. When the ISM magnetic eld increased, the structure of the astrosphere changed, and due to the high ISM magnetic eld, the astrosphere became asymmetrical.