First star formation

4.3 First star formation

In CDM cosmogonies, the fluctuation spectrum at small wavelengths has a logarithmic dependence at mass scales smaller than 10⁸ solar masses, which indicates that all small scale fluctuations in this model collapse nearly simultaneously in time. This leads to very complex dynamics during the formation of these first structures. Furthermore, the cooling in these fluctuations is dominated by the rotational/vibrational modes of hydrogen molecules that were able to form using the free electrons left over from recombination and those produced by strong shock waves as catalysts. The first structures to collapse may be capable of producing pop III stars and have a substantial influence on the subsequent thermal evolution of the intergalactic medium, as suggested by Figure 2

, due to the radiation emitted by the first generation stars as well as supernova driven winds. To know the subsequent fate of the Universe and which structures will survive or be destroyed by the UV background, it is first necessary to know when and how the first stars formed.

Ostriker and Gnedin [127 ] have carried out high resolution numerical simulations of the reheating and reionization of the Universe due to star formation bursts triggered by molecular hydrogen cooling. Accounting for the chemistry of the primeval hydrogen/helium plasma, self-shielding of the gas, radiative cooling, and a phenomenological model of star formation, they find that two distinct star populations form: the first generation pop III from $H2$ cooling prior to reheating at redshift $z ≥ 14$ ; and the second generation pop II at $z < 10$ when the virial temperature of the gas clumps reaches 10⁴ K and hydrogen line cooling becomes efficient. Star formation slows in the intermittent epoch due to the depletion of $H2$ by photo-destruction and reheating. In addition, the objects which formed pop III stars also initiate pop II sequences when their virial temperatures reach 10⁴ K through continued mass accretion.

In resolving the details of a single star forming region in a CDM Universe, Abel et al. [2 , 3 ] implemented a non-equilibrium radiative cooling and chemistry model [1 , 21 ] together with the hydrodynamics and dark matter equations, evolving nine separate atomic and molecular species ( $H$ , $+ H$ , $He$ , $+ He$ , $++ He$ , $− H$ , $+ H 2$ , $H2$ , and $− e$ , according to the reactive network described in Section 6.4.1) on nested and adaptively refined numerical grids. They follow the collapse and fragmentation of primordial clouds over many decades in mass and spatial dynamical range, finding a core of mass $∼ 200M ⊙$ forms from a halo of about $∼ 105 M ⊙$ (where a significant number fraction of hydrogen molecules are created) after less than one percent of the halo gas cools by molecular line emission. Bromm et al. [48 ] use a different Smoothed Particle Hydrodynamics (SPH) technique and a six species model ( $H$ , $H+$ , $H −$ , $H+2$ , $H2$ , and $e−$ ) to investigate the initial mass function of the first generation pop III stars. They evolve an isolated $3σ$ peak of mass $2 × 106M ⊙$ which collapses at redshift $z ∼ 30$ and forms clumps of mass $102– 103M ⊙$ which then grow by accretion and merging, suggesting that the very first stars were massive and in agreement with [3 ]. Update

The implications of an early era of massive star populations on the thermal and chemical state of the intergalactic medium was investigated by Yoshida et al. [164 ]. They considered the effects of feedback and radiation transfer in early structure formation simulations to show that a significant fraction of the IGM can be ionized and polluted by metals from the first stars to form and become supernovae by $z ∼ 15$ , thus affecting subsequent stellar populations. They also argue that observed elemental abundances in the intracluster medium are not affected by metals originating from the first stars.