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| Dissertation / PhD Thesis | FZJ-2014-04688 |
2014
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-89336-979-9
Please use a persistent id in citations: http://hdl.handle.net/2128/7933 urn:nbn:de:0001-2014072202
Abstract: A large fraction of stars (≈ 50% of the field population) are not single but part of a binary or multiple system. These binary systems form from the gas and dust in molecular clouds largely building clusters that are initially still embedded in the star-forming gas. Here the question arises whether the properties and frequency of binaries change during this gas-embedded phase. It is known that the gravitational interactions between stars in a cluster environment can destroy long-period binaries (> 10$^{5}$ days). However, not only can the interaction between the stars themselves change the binary properties but also those between binary systems and the surrounding gas. There, the binary potential torques the nearby gas, producing an outgoing acoustic wave. This wave transports angular momentum from the binary to the gas, resulting in a decay of the binary orbit. This effect is the central focus of the thesis presented here. First, an analytic approximation for the gas-induces orbital decay by Stahler (2010) was applied to a binary population and the results compared to observations. It was found that the process of orbital decay significantly changes the properties of short period binaries (< 10$^{5}$ days). The resulting period distribution resembles the one observed for solar-mass stars, but fails to do so for other mass ranges. The analytic approximation treats only the effect on binary systems with circular orbits and the wave generation is not calculated explicitly. Since, most binary systems have eccentric orbits, a 3D hydrodynamic simulation was developed to avoid these restrictions. It calculates the gravitational binary - gas interaction, the wave generation, and the resulting orbital decay. An extensive parameter study was performed to investigate the dependency of the orbital decay on the binary and gas properties. It was found that the gas density, embedded time span and mass-ratio show a similar scaling as predicted by the analytic approximation. By contrast, all binary and gas properties which influence the wave generation show different dependencies. In particular, it is shown that eccentric orbits lead to a faster orbital decay than their circular counterparts. Eventually, all these effects were combined in a fit formula. Applying this fit-formula to a binary population, the resulting period distribution shows a better matching mass dependency, but still does not resemble the observed period distributions. The cluster model chosen here is only one example and it is still unknown which cluster types contribute to the field population. Furthermore, future observations of young binary systems and their environment could restrict the parameter space presented here. Having detailed knowledge of the binary’s environment, the method developed in this thesis can be used to deduce what impact the gas-induced orbital decay has on a binary population.
Keyword(s): Dissertation
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