# Simulations in Astrophysics

## Binary Systems

Most stars are a member of a binary or even higher order system. In different stellar environments differences in the stellar binary populations have been observed. We look at very young binary systems still embedded in their natal gas. Using hydrodynamic simulations, we investigate the influences of this surrounding gas onto the period of such binary systems.

The video below shows in the first panel the orbital position of the binary system, in the second panel a 2d cut through the equatorial plane and in the third panel a 1d radial cut at phi = 0 & pi/2. In the first row we see how a circular binary system with equal stellar masses induces two symmetrical spiral arms in the surrounding gas. In the second row the binary system has a mass-ratio of 1:2 and therefore one of the spiral arms is nearly vanished. The binary system in the third row is on an elliptical orbit (eccentricity = 0.2) which results in an asymmetric acoustic wave.

This induced waves transport angular momentum from the binary system to the gas and leads to a reduction of the orbit. For more details see:

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## Smoothed Particle Hydrodynamics

Stars form by gravitational collapse of molecular clouds. Due to angular momentum conservation, discs consisting of gas and dust form during this process around the young stars. Due to the potential of later formation of planets out of the disc-material, they are called protoplanetary discs.

Especially discs with masses higher than 10% of the central stars mass behave in ﬁrst approximation like self-gravitating, viscous ﬂuids. The exact interplay of the gravitational force with viscosity and other processes is still unknown.

To contribute to the understanding of these processes, a code for the simulation of protoplanetary discs with highly parallel supercomputers is developed. Therefore, the highly scalable plasma-code PEPC was modiﬁed for the fast computation of gravitational forces and extended with the Smoothed Particle Hydrodynamics (SPH) method for ﬂuid computation.

The resulting code is expected to simulate up to 10^{9 }particles on highly parallel machines like JSC's supercomputer Jugene.

To demonstrate, that the code produces correct physical results, several test problems were investigated. The ability of the new code to resolve sound and shock waves sufficiently and the correct cooperation of gravitational and fluid part have already been shown.