Thomas H. Greif, Volker Springel, Volker Bromm
We investigate the operation of the chemo-thermal instability in primordial star-forming clouds with a suite of three-dimensional, moving-mesh simulations. In line with previous studies, we find that the gas at the center of high-redshift minihaloes becomes chemo-thermally unstable as three-body reactions convert the atomic hydrogen into a fully molecular gas. The competition between the increasing rate at which the gas cools and the increasing optical depth to H2 line emission creates a characteristic dip in the cooling time over the free-fall time on a scale of about 1000 au. As the effective equation of state drops from about 1.1 to well below unity, the free-fall time decreases to below the sound-crossing time, and density perturbations induced by transonic turbulence can grow. This allows the cloud to fragment on a scale of a few tens of au during the initial free-fall phase. In two of the nine haloes investigated, Jeans-unstable clumps condense out of the parent cloud, which will likely collapse before they are accreted by the primary clump. In the other haloes, fragmentation at such an early stage is less likely. However, given that previous simulations have shown that the infall velocity decreases substantially once the gas becomes rotationally supported, the amount of time available for perturbations to develop may be much greater than is evident from the limited period of time simulated here.
View original:
http://arxiv.org/abs/1305.0823
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