Primordial star formation: relative impact of H₂ three-body rates and initial conditions

¹ Institut für Astrophysik, Georg-August Universität, Friedrich-Hund-Platz 1, 37073 Göttingen, Germany
e-mail: sbovino@astro.physik.uni-goettingen.de
² Department of Chemistry, “Sapienza” University of Rome P.le A. Moro 5, 00185 Rome, Italy

Received: 29 July 2013
Accepted: 25 October 2013

Abstract

Context. Population III stars are the first stars in the Universe to form at z = 20–30 out of a pure hydrogen and helium gas in minihalos of 10⁵−10⁶M_⊙. Cooling and fragmentation is thus regulated via molecular hydrogen. At densities above 10⁸ cm^-3, the three-body H₂ formation rates are particularly important for making the gas fully molecular. These rates were considered to be uncertain by at least a few orders of magnitude.

Aims. We explore the impact of recently derived accurate three-body H₂ formation for three different minihalos, and compare them with the results obtained with three-body rates employed in previous other studies.

Methods. The calculations were performed with the cosmological hydrodynamics code enzo (release 2.2) coupled with the chemistry package krome (including a network for primordial chemistry), which was previously shown to be accurate in high-resolution simulations.

Results. While the new rates can shift the point where the gas becomes fully molecular, leading to a different thermal evolution, there is no trivial trend in the way this occurs. While one might naively expect the results to follow the rate coefficients trend, the behavior can vary depending on the dark-matter halo that is explored.

Conclusions. We conclude that employing the correct three-body rates is about equally important as the use of appropriate initial conditions, and that the resulting thermal evolution needs to be calculated for every halo individually.