The evolution of the first stellar systems
The formation of first stars is a numerically challenging problem. The corresponding length scale spans more than ten orders of magnitude. It also requires a simulation that starts with the initial cosmological perturbation and continues all the way until the formation of the first stars. Although an ab initio simulation is always favored, the large separation in both length scale and time scale makes it barely possible to study the small scale physics and their subsequent feedback in the ab initio runs. In this thesis, we focus on the small scale physics during the the accretion phase. We first provide an analytic understanding of the thermodynamics and the fragmentation condition of the primordial accretion flow. Unsurprisingly, fragmentation is more likely to take place at r >~ 10 au due to a lower optical depth and thus a fast cooling. In addition, a chemo-thermal instability may take place during the accretion phase but with a low growth rate compared to the dynamical time scale. With the analytic model, we then run a controlled numerical experiment at the accretion phase to study the turbulent dynamo processes (i.e., the energy exchange between turbulence and magnetic field). A numerical algorithm is developed to solve for the Poisson equation in the cylindrical coordinate. It allows us to do the experiment in an angular momentum conserved scheme. We demonstrated that kinetic helicity exists in the primordial accretion flow. It leads to a growth of a large scale coherent magnetic field. In addition, the dynamo number is on the order of 100, which indicates a fast growth rate of the B-field during the initial linear growth phase. We further show that, at the saturation stage, the B-field could become equipartition with the turbulent kinetic energy. This indicates that a dynamically important large scale B-field may have already existed in the primordial stellar systems. It would have an important role in impacting the evolution of primordial stellar systems.
First Star, Early Universe, Star Formation
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign