Modelling O-star astrospheres with different relative speeds between the ISM and the star: 2D and 3D MHD model comparison

Context. State of the art simulations of astrospheres are modelled using three-dimensional (3D) magnetohydrodynamics (MHD). An astrospheric interaction of a stellar wind (SW) with its surrounding interstellar medium (ISM) can only generate a bow shock if the speed of the interstellar inflow is higher than the fast magnetosonic speed. Aims. The differences of astrospheres at differing speeds of the ISM inflow are investigated, and the necessity of the third dimension in modelling is evaluated. Methods. The model astrosphere of the runaway O-star lambda Cephei is computed in both two- and three-dimensional MHD at four different ISM inflow speeds, one of which is barely faster (superfast) and one of which is slower (subfast) than the fast magnetosonic speed. Results. The two-dimensional (2D) and 3D models of astrospheres with ISM inflow speeds considerably higher than the fast magnetosonic speed are in good agreement. However, in 2D models, where no realistic SW magnetic field can be modelled, the downwind structures of the astrospheres vacillate. Models where hydrodynamic effects are not clearly dominant over the magnetic field show asymmetries, thus necessitating a 3D approach. The physical times of simulations of astrospheres with slow ISM inflows can swiftly exceed the lifetime of the corresponding star. A hitherto unobserved structure has been found downwind of the astrotail in the subfast 3D model.

Automated summary

This study used two-dimensional and three-dimensional magnetohydrodynamics simulations to model the interaction between stellar winds and the surrounding interstellar medium. The results showed that when the inflow speed of the interstellar medium is higher than the fast magnetosonic speed, the results of the two-dimensional and three-dimensional models are consistent. However, the two-dimensional models cannot accurately simulate the stellar wind magnetic field and show asymmetry when the magnetic field is not dominant. Therefore, a three-dimensional approach is necessary. Additionally, simulations of astrospheres with slower interstellar medium inflows can exceed the lifetime of the corresponding star.