The Formation of the First Stars. I. The Primordial Star‐forming Cloud

Abstract

To constrain the nature of the very first stars, we investigate the collapse and fragmentation of primordial, metal-free gas clouds. We explore the physics of primordial star formation by means of threedimensional simulations of the dark matter and gas components, using smoothed particle hydrodynamics, under a wide range of initial conditions, including the initial spin, the total mass of the halo, the redshift of virialization, the power spectrum of the DM fluctuations, the presence of HD cooling, and the number of particles employed in the simulation. We find characteristic values for the temperature, T ∼ a few 100 K, and the density, n ∼ 10 3 − 10 4 cm −3 , characterising the gas at the end of the initial free-fall phase. These values are rather insensitive to the initial conditions. The corresponding Jeans mass is M J ∼ 10 3 M ⊙ . The existence of these characteristic values has a robust explanation in the microphysics of H 2 cooling, connected to the minimum temperature that can be reached with the H 2 coolant, and to the critical density at which the transition takes place between levels being populated according to NLTE, and according to LTE.In all cases, the gas dissipatively settles into an irregular, central configuration which has a filamentary and knotty appearance. The fluid regions with the highest densities are the first to undergo runaway collapse due to gravitational instability, and to form clumps with initial masses ∼ 10 3 M ⊙ , close to the characteristic Jeans scale. These results suggest that the first stars might have been quite massive, possibly even very massive with M ⋆ ∼ > 100 M ⊙ .After a gas element has undergone runaway collapse, and has reached densities in excess of 10 8 cm −3 , a sink particle is created. This procedure allows us to follow the evolution of the overall system beyond the point where the first nonlinear region would otherwise force the calculation to a halt. These later evolutionary stages, during which the clumps grow in mass due to accretion and merging with other clumps, are quite sensitive to the initial conditions. The key process in building up very massive clumps, with masses up to a few times 10 4 M ⊙ , is merging between clumps. Since the merging rate sensitively depends on the density of the gas, halos with the highest degree of central concentration are able to assemble the most massive clumps. Among these are halos with a low spin (λ ≃ 0.01), and with DM fluctuations imprinted according to a white-noise spectrum.

Formation of supermassive black holes

“…Simulations of the collapse of primordial molecular clouds (Bromm et al, 1999(Bromm et al, , 2002Abel et al, 2000;Yoshida et al, 2006;Gao et al, 2007) suggest that the first generation of stars contained many 'very massive stars' (VMSs) with m > 100 M . This is because of the slow subsonic contraction -a regime set up by the main gas coolant, molecular hydrogen -further fragmentation into sub-components is not seen (although it is not clear if this is a numerical effect, rather than due to the gas physics, see Glover et al, 2008).…”

“…If star formation proceeds in small mini-halos (T vir < 10 4 K), triggered by H 2 cooling (Bromm et al, 1999(Bromm et al, , 2002Abel et al, 2000), then by the time more massive halos are built-up, they will have been enriched with some traces of metals, brought in by their progenitors. Fragmentation and formation of low mass stars starts as soon as gas is polluted by metals created in the first PopIII stars (Omukai et al, 2008).…”

Low-energy Population III supernovae and the origin of extremely metal-poor stars

“…The formation of the first stars marked the end of the cosmic Dark Ages and began the transformation of the universe from a cold, dark featureless void into the hot, transparent cosmic web of galaxies we observe today (Abel et al 2002;Bromm et al 2002;Whalen et al 2004;Kitayama et al 2004;Alvarez et al 2006;Abel et al 2007). Population III (or Pop III) stars were also the first great nucleosynthetic engines of the universe, expelling large quantities of the heavy EACOA Fellow, E-mail: ken.chen@nao.ac.jp elements needed for the later formation of planets and life (e.g., Bromm et al 2003;Kitayama & Yoshida 2005;Greif et al 2007;Karlsson et al 2013).…”

Assessing inflow rates in atomic cooling haloes: implications for direct collapse black holes

“…One potential pathway is the collapse of a primordial, i.e., metal-free, "normal" star into a stellar-mass black hole. Earlier numerical simulations performed to study the formation of the first generation of stars (Abel et al 2002;Bromm et al 2002; ⋆ Corresponding author: latif@iap.fr Yoshida et al 2006) suggested that the masses of the first stars are of the order of a few hundred solar. However, recent simulations found that the protostellar disk forming in the minihalo fragments into multiple clumps (Clark et al 2011;Greif et al 2012;Stacy et al 2012;Latif et al 2013b; and leads to the formation of multiple stars.…”