Numerical modelling of Astrospheres

Abstract

A stellar wind creates a cavity in the interstellar medium, which is known as an astrophere. In this work, radiative cooling is included in a standard 2D magneto-hydrodynamic numerical model to simulate the evolution of an astrosphere. First, a parameter study is done to determine the effects that an interstellar medium magnetic field, the mass loss rate, the out flow velocity and the interstellar medium density have on astrospheric evolution. It is found, depending of course on the model parameters, that the cooling creates a thin dense shell surrounding the astrosphere during the early stages of the astrospheric evolution. The cooling also increases the compression ratio of the bow shock, while the added interstellar medium magnetic eld reduces the compression. The astrospheres surrounding luminous blue variable stars HD 99953 and AG Carinae are also computed. These stars show variability in their out flow parameters at the inner boundary. It is found that the variations in the out flow parameters have no effect on the astropause or the bow shock of the astrosphere. For AG Carinae, the variations in the mass-loss rate and out flow velocity at the inner boundary are larger compared to HD 99953. These larger variations of AG Carinae also have no in uence beyond the termination shock as with HD 99953. Luminous blue variable stars also undergo eruptive events with large increases in mass loss over a short period of time. These eruptions only slightly in uence the bow shock. Eruptions that increase the out flow velocity and mass loss create an astrosphere that has a larger size and during the early stages of evolution the eruptions cause the termination shock to oscillate between the astropause and the inner boundary. These eruptions also increase the distance to the astropause slightly; however, at later stages, there are no visible signs of the influence of these eruptions.