Allard Jan van Marle

Using a combined PIC-MHD code to simulate particle acceleration in astrophysical shocks

In order to model the magnetic field amplification and particle acceleration that takes place in astrophysical shocks, we need a code that can efficiently model the large-scale structure of the shock, while still taking the kinetic aspect of non-thermal particles into account. Starting from the proven MPI-AMRVAC magnetohydrodynamics code we have created a code that combines the kinetic treatment of the Particle-in-Cell (PIC) method for non-thermal particles with the large-scale effects of grid-based hydrodynamics (MHD) to model the thermal plasma, including the use of adaptive mesh refinement. Using this code we simulate astrophysical shocks, varying the angle between the magnetic field and the shock to test our code against existing results and study both the evolution of the shock and the behaviour of non-thermal particles. We find that the combined PIC-MHD method can accurately recover the results that were previously obtained with pure PIC codes. Furthermore, the efficiency of the code allows us to explore the available parameter space to a larger degree than has been done in previous work. Our results suggest that efficient particle acceleration can take place in near-oblique shocks where the magnetic field makes a large angle with the direction of the flow.

Dust distribution in circumstellar shells

We present numerical simulations of the hydrodynamical interactions that produce circumstellar shells. These simulations include several scenarios, such as wind-wind interaction and wind-ISM collisions. In our calculations we have taken into account the presence of dust in the stellar wind. Our results show that, while small dust grains tend to be strongly coupled to the gas, large dust grains are only weakly coupled. As a result, the distribution of the large dust grains is not representative of the gas distribution. Combining these results with observations may give us a new way of validating hydrodynamical models of the circumstellar medium. 1. Intoduction The winds of cool stars contain a large amount of dust. This dust is crucial in driving the wind of the star and, at larger distances, it moves along with the flow of the wind (Lamers & Cassinelli 1999). When the gas collides with the surrounding medium (inter- stellar medium, a previously ejected wind, wind from an other star, etc.) it slows down to form a shell, creating a velocity difference between gas and dust. Whether the dust will follow the change in velocity depends on the strength of the coupling between gas and dust, which in turn depends on such parameters as gas density, velocity difference, tem- perature and the size of the dust grains. The larger the dust grains, the more momentum they have and the more force will be required to slow them down. In this paper we use numerical models of a stellar wind colliding with its surrounding medium to investigate the behavior of the dust grains in the resulting shell.

Numerical simulations of the circumstellar medium of massive binaries

We have made 3-D models of the collision of binary star winds and followed their interaction over multiple orbits. This allows us to explore how the wind-wind interaction shapes the circumstellar environment. Specifically, we can model the highly radiative shock that occurs where the winds collide. We find that the shell that is created at the collision front between the two winds can be highly unstable, depending on the characteristics of the stellar winds.